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    use crate::byte_str::ByteStr;
    use bytes::{Bytes, BytesMut};

    use std::borrow::Borrow;
    use std::error::Error;
    use std::convert::{TryFrom};
    use std::hash::{Hash, Hasher};
    use std::str::FromStr;
    use std::{fmt, mem};

    /// Represents an HTTP header field name
    ///
    /// Header field names identify the header. Header sets may include multiple
    /// headers with the same name. The HTTP specification defines a number of
    /// standard headers, but HTTP messages may include non-standard header names as
    /// well as long as they adhere to the specification.
    ///
    /// `HeaderName` is used as the [`HeaderMap`] key. Constants are available for
    /// all standard header names in the [`header`] module.
    ///
    /// # Representation
    ///
    /// `HeaderName` represents standard header names using an `enum`, as such they
    /// will not require an allocation for storage. All custom header names are
    /// lower cased upon conversion to a `HeaderName` value. This avoids the
    /// overhead of dynamically doing lower case conversion during the hash code
    /// computation and the comparison operation.
    ///
    /// [`HeaderMap`]: struct.HeaderMap.html
    /// [`header`]: index.html
    #[derive(Clone, Eq, PartialEq, Hash)]
    pub struct HeaderName {
        inner: Repr<Custom>,
    }

    // Almost a full `HeaderName`
    #[derive(Debug, Hash)]
    pub struct HdrName<'a> {
        inner: Repr<MaybeLower<'a>>,
    }

    #[derive(Debug, Clone, Eq, PartialEq, Hash)]
    enum Repr<T> {
        Standard(StandardHeader),
        Custom(T),
    }

    // Used to hijack the Hash impl
    #[derive(Debug, Clone, Eq, PartialEq)]
    struct Custom(ByteStr);

    #[derive(Debug, Clone)]
    struct MaybeLower<'a> {
        buf: &'a [u8],
        lower: bool,
    }

    /// A possible error when converting a `HeaderName` from another type.
    pub struct InvalidHeaderName {
        _priv: (),
    }

    macro_rules! standard_headers {
        (
            $(
                $(#[$docs:meta])*
                ($konst:ident, $upcase:ident, $name:expr);
            )+
        ) => {
            #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
            enum StandardHeader {
                $(
                    $konst,
                )+
            }

            $(
                $(#[$docs])*
                pub const $upcase: HeaderName = HeaderName {
                    inner: Repr::Standard(StandardHeader::$konst),
                };
            )+

            impl StandardHeader {
                #[inline]
                fn as_str(&self) -> &'static str {
                    match *self {
                        $(
                        StandardHeader::$konst => $name,
                        )+
                    }
                }
            }

            #[cfg(test)]
            const TEST_HEADERS: &'static [(StandardHeader, &'static str)] = &[
                $(
                (StandardHeader::$konst, $name),
                )+
            ];

            #[test]
            fn test_parse_standard_headers() {
                for &(std, name) in TEST_HEADERS {
                    // Test lower case
                    assert_eq!(HeaderName::from_bytes(name.as_bytes()).unwrap(), HeaderName::from(std));

                    // Test upper case
                    let upper = name.to_uppercase().to_string();
                    assert_eq!(HeaderName::from_bytes(upper.as_bytes()).unwrap(), HeaderName::from(std));
                }
            }

            #[test]
            fn test_standard_headers_into_bytes() {
                for &(std, name) in TEST_HEADERS {
                    let std = HeaderName::from(std);
                    // Test lower case
                    let name_bytes = name.as_bytes();
                    let bytes: Bytes =
                        HeaderName::from_bytes(name_bytes).unwrap().inner.into();
                    assert_eq!(bytes, name_bytes);
                    assert_eq!(HeaderName::from_bytes(name_bytes).unwrap(), std);

                    // Test upper case
                    let upper = name.to_uppercase().to_string();
                    let bytes: Bytes =
                        HeaderName::from_bytes(upper.as_bytes()).unwrap().inner.into();
                    assert_eq!(bytes, name.as_bytes());
                    assert_eq!(HeaderName::from_bytes(upper.as_bytes()).unwrap(),
                               std);

                }

            }
        }
    }

    // Generate constants for all standard HTTP headers. This includes a static hash
    // code for the "fast hash" path. The hash code for static headers *do not* have
    // to match the text representation of those headers. This is because header
    // strings are always converted to the static values (when they match) before
    // being hashed. This means that it is impossible to compare the static hash
    // code of CONTENT_LENGTH with "content-length".
    standard_headers! {
        /// Advertises which content types the client is able to understand.
        ///
        /// The Accept request HTTP header advertises which content types, expressed
        /// as MIME types, the client is able to understand. Using content
        /// negotiation, the server then selects one of the proposals, uses it and
        /// informs the client of its choice with the Content-Type response header.
        /// Browsers set adequate values for this header depending of the context
        /// where the request is done: when fetching a CSS stylesheet a different
        /// value is set for the request than when fetching an image, video or a
        /// script.
        (Accept, ACCEPT, "accept");

        /// Advertises which character set the client is able to understand.
        ///
        /// The Accept-Charset request HTTP header advertises which character set
        /// the client is able to understand. Using content negotiation, the server
        /// then selects one of the proposals, uses it and informs the client of its
        /// choice within the Content-Type response header. Browsers usually don't
        /// set this header as the default value for each content type is usually
        /// correct and transmitting it would allow easier fingerprinting.
        ///
        /// If the server cannot serve any matching character set, it can
        /// theoretically send back a 406 (Not Acceptable) error code. But, for a
        /// better user experience, this is rarely done and the more common way is
        /// to ignore the Accept-Charset header in this case.
        (AcceptCharset, ACCEPT_CHARSET, "accept-charset");

        /// Advertises which content encoding the client is able to understand.
        ///
        /// The Accept-Encoding request HTTP header advertises which content
        /// encoding, usually a compression algorithm, the client is able to
        /// understand. Using content negotiation, the server selects one of the
        /// proposals, uses it and informs the client of its choice with the
        /// Content-Encoding response header.
        ///
        /// Even if both the client and the server supports the same compression
        /// algorithms, the server may choose not to compress the body of a
        /// response, if the identity value is also acceptable. Two common cases
        /// lead to this:
        ///
        /// * The data to be sent is already compressed and a second compression
        /// won't lead to smaller data to be transmitted. This may the case with
        /// some image formats;
        ///
        /// * The server is overloaded and cannot afford the computational overhead
        /// induced by the compression requirement. Typically, Microsoft recommends
        /// not to compress if a server use more than 80 % of its computational
        /// power.
        ///
        /// As long as the identity value, meaning no encryption, is not explicitly
        /// forbidden, by an identity;q=0 or a *;q=0 without another explicitly set
        /// value for identity, the server must never send back a 406 Not Acceptable
        /// error.
        (AcceptEncoding, ACCEPT_ENCODING, "accept-encoding");

        /// Advertises which languages the client is able to understand.
        ///
        /// The Accept-Language request HTTP header advertises which languages the
        /// client is able to understand, and which locale variant is preferred.
        /// Using content negotiation, the server then selects one of the proposals,
        /// uses it and informs the client of its choice with the Content-Language
        /// response header. Browsers set adequate values for this header according
        /// their user interface language and even if a user can change it, this
        /// happens rarely (and is frown upon as it leads to fingerprinting).
        ///
        /// This header is a hint to be used when the server has no way of
        /// determining the language via another way, like a specific URL, that is
        /// controlled by an explicit user decision. It is recommended that the
        /// server never overrides an explicit decision. The content of the
        /// Accept-Language is often out of the control of the user (like when
        /// traveling and using an Internet Cafe in a different country); the user
        /// may also want to visit a page in another language than the locale of
        /// their user interface.
        ///
        /// If the server cannot serve any matching language, it can theoretically
        /// send back a 406 (Not Acceptable) error code. But, for a better user
        /// experience, this is rarely done and more common way is to ignore the
        /// Accept-Language header in this case.
        (AcceptLanguage, ACCEPT_LANGUAGE, "accept-language");

        /// Marker used by the server to advertise partial request support.
        ///
        /// The Accept-Ranges response HTTP header is a marker used by the server to
        /// advertise its support of partial requests. The value of this field
        /// indicates the unit that can be used to define a range.
        ///
        /// In presence of an Accept-Ranges header, the browser may try to resume an
        /// interrupted download, rather than to start it from the start again.
        (AcceptRanges, ACCEPT_RANGES, "accept-ranges");

        /// Preflight response indicating if the response to the request can be
        /// exposed to the page.
        ///
        /// The Access-Control-Allow-Credentials response header indicates whether
        /// or not the response to the request can be exposed to the page. It can be
        /// exposed when the true value is returned; it can't in other cases.
        ///
        /// Credentials are cookies, authorization headers or TLS client
        /// certificates.
        ///
        /// When used as part of a response to a preflight request, this indicates
        /// whether or not the actual request can be made using credentials. Note
        /// that simple GET requests are not preflighted, and so if a request is
        /// made for a resource with credentials, if this header is not returned
        /// with the resource, the response is ignored by the browser and not
        /// returned to web content.
        ///
        /// The Access-Control-Allow-Credentials header works in conjunction with
        /// the XMLHttpRequest.withCredentials property or with the credentials
        /// option in the Request() constructor of the Fetch API. Credentials must
        /// be set on both sides (the Access-Control-Allow-Credentials header and in
        /// the XHR or Fetch request) in order for the CORS request with credentials
        /// to succeed.
        (AccessControlAllowCredentials, ACCESS_CONTROL_ALLOW_CREDENTIALS, "access-control-allow-credentials");

        /// Preflight response indicating permitted HTTP headers.
        ///
        /// The Access-Control-Allow-Headers response header is used in response to
        /// a preflight request to indicate which HTTP headers will be available via
        /// Access-Control-Expose-Headers when making the actual request.
        ///
        /// The simple headers, Accept, Accept-Language, Content-Language,
        /// Content-Type (but only with a MIME type of its parsed value (ignoring
        /// parameters) of either application/x-www-form-urlencoded,
        /// multipart/form-data, or text/plain), are always available and don't need
        /// to be listed by this header.
        ///
        /// This header is required if the request has an
        /// Access-Control-Request-Headers header.
        (AccessControlAllowHeaders, ACCESS_CONTROL_ALLOW_HEADERS, "access-control-allow-headers");

        /// Preflight header response indicating permitted access methods.
        ///
        /// The Access-Control-Allow-Methods response header specifies the method or
        /// methods allowed when accessing the resource in response to a preflight
        /// request.
        (AccessControlAllowMethods, ACCESS_CONTROL_ALLOW_METHODS, "access-control-allow-methods");

        /// Indicates whether the response can be shared with resources with the
        /// given origin.
        (AccessControlAllowOrigin, ACCESS_CONTROL_ALLOW_ORIGIN, "access-control-allow-origin");

        /// Indicates which headers can be exposed as part of the response by
        /// listing their names.
        (AccessControlExposeHeaders, ACCESS_CONTROL_EXPOSE_HEADERS, "access-control-expose-headers");

        /// Indicates how long the results of a preflight request can be cached.
        (AccessControlMaxAge, ACCESS_CONTROL_MAX_AGE, "access-control-max-age");

        /// Informs the server which HTTP headers will be used when an actual
        /// request is made.
        (AccessControlRequestHeaders, ACCESS_CONTROL_REQUEST_HEADERS, "access-control-request-headers");

        /// Informs the server know which HTTP method will be used when the actual
        /// request is made.
        (AccessControlRequestMethod, ACCESS_CONTROL_REQUEST_METHOD, "access-control-request-method");

        /// Indicates the time in seconds the object has been in a proxy cache.
        ///
        /// The Age header is usually close to zero. If it is Age: 0, it was
        /// probably just fetched from the origin server; otherwise It is usually
        /// calculated as a difference between the proxy's current date and the Date
        /// general header included in the HTTP response.
        (Age, AGE, "age");

        /// Lists the set of methods support by a resource.
        ///
        /// This header must be sent if the server responds with a 405 Method Not
        /// Allowed status code to indicate which request methods can be used. An
        /// empty Allow header indicates that the resource allows no request
        /// methods, which might occur temporarily for a given resource, for
        /// example.
        (Allow, ALLOW, "allow");

        /// Advertises the availability of alternate services to clients.
        (AltSvc, ALT_SVC, "alt-svc");

        /// Contains the credentials to authenticate a user agent with a server.
        ///
        /// Usually this header is included after the server has responded with a
        /// 401 Unauthorized status and the WWW-Authenticate header.
        (Authorization, AUTHORIZATION, "authorization");

        /// Specifies directives for caching mechanisms in both requests and
        /// responses.
        ///
        /// Caching directives are unidirectional, meaning that a given directive in
        /// a request is not implying that the same directive is to be given in the
        /// response.
        (CacheControl, CACHE_CONTROL, "cache-control");

        /// Controls whether or not the network connection stays open after the
        /// current transaction finishes.
        ///
        /// If the value sent is keep-alive, the connection is persistent and not
        /// closed, allowing for subsequent requests to the same server to be done.
        ///
        /// Except for the standard hop-by-hop headers (Keep-Alive,
        /// Transfer-Encoding, TE, Connection, Trailer, Upgrade, Proxy-Authorization
        /// and Proxy-Authenticate), any hop-by-hop headers used by the message must
        /// be listed in the Connection header, so that the first proxy knows he has
        /// to consume them and not to forward them further. Standard hop-by-hop
        /// headers can be listed too (it is often the case of Keep-Alive, but this
        /// is not mandatory.
        (Connection, CONNECTION, "connection");

        /// Indicates if the content is expected to be displayed inline.
        ///
        /// In a regular HTTP response, the Content-Disposition response header is a
        /// header indicating if the content is expected to be displayed inline in
        /// the browser, that is, as a Web page or as part of a Web page, or as an
        /// attachment, that is downloaded and saved locally.
        ///
        /// In a multipart/form-data body, the HTTP Content-Disposition general
        /// header is a header that can be used on the subpart of a multipart body
        /// to give information about the field it applies to. The subpart is
        /// delimited by the boundary defined in the Content-Type header. Used on
        /// the body itself, Content-Disposition has no effect.
        ///
        /// The Content-Disposition header is defined in the larger context of MIME
        /// messages for e-mail, but only a subset of the possible parameters apply
        /// to HTTP forms and POST requests. Only the value form-data, as well as
        /// the optional directive name and filename, can be used in the HTTP
        /// context.
        (ContentDisposition, CONTENT_DISPOSITION, "content-disposition");

        /// Used to compress the media-type.
        ///
        /// When present, its value indicates what additional content encoding has
        /// been applied to the entity-body. It lets the client know, how to decode
        /// in order to obtain the media-type referenced by the Content-Type header.
        ///
        /// It is recommended to compress data as much as possible and therefore to
        /// use this field, but some types of resources, like jpeg images, are
        /// already compressed.  Sometimes using additional compression doesn't
        /// reduce payload size and can even make the payload longer.
        (ContentEncoding, CONTENT_ENCODING, "content-encoding");

        /// Used to describe the languages intended for the audience.
        ///
        /// This header allows a user to differentiate according to the users' own
        /// preferred language. For example, if "Content-Language: de-DE" is set, it
        /// says that the document is intended for German language speakers
        /// (however, it doesn't indicate the document is written in German. For
        /// example, it might be written in English as part of a language course for
        /// German speakers).
        ///
        /// If no Content-Language is specified, the default is that the content is
        /// intended for all language audiences. Multiple language tags are also
        /// possible, as well as applying the Content-Language header to various
        /// media types and not only to textual documents.
        (ContentLanguage, CONTENT_LANGUAGE, "content-language");

        /// Indicates the size fo the entity-body.
        ///
        /// The header value must be a decimal indicating the number of octets sent
        /// to the recipient.
        (ContentLength, CONTENT_LENGTH, "content-length");

        /// Indicates an alternate location for the returned data.
        ///
        /// The principal use case is to indicate the URL of the resource
        /// transmitted as the result of content negotiation.
        ///
        /// Location and Content-Location are different: Location indicates the
        /// target of a redirection (or the URL of a newly created document), while
        /// Content-Location indicates the direct URL to use to access the resource,
        /// without the need of further content negotiation. Location is a header
        /// associated with the response, while Content-Location is associated with
        /// the entity returned.
        (ContentLocation, CONTENT_LOCATION, "content-location");

        /// Indicates where in a full body message a partial message belongs.
        (ContentRange, CONTENT_RANGE, "content-range");

        /// Allows controlling resources the user agent is allowed to load for a
        /// given page.
        ///
        /// With a few exceptions, policies mostly involve specifying server origins
        /// and script endpoints. This helps guard against cross-site scripting
        /// attacks (XSS).
        (ContentSecurityPolicy, CONTENT_SECURITY_POLICY, "content-security-policy");

        /// Allows experimenting with policies by monitoring their effects.
        ///
        /// The HTTP Content-Security-Policy-Report-Only response header allows web
        /// developers to experiment with policies by monitoring (but not enforcing)
        /// their effects. These violation reports consist of JSON documents sent
        /// via an HTTP POST request to the specified URI.
        (ContentSecurityPolicyReportOnly, CONTENT_SECURITY_POLICY_REPORT_ONLY, "content-security-policy-report-only");

        /// Used to indicate the media type of the resource.
        ///
        /// In responses, a Content-Type header tells the client what the content
        /// type of the returned content actually is. Browsers will do MIME sniffing
        /// in some cases and will not necessarily follow the value of this header;
        /// to prevent this behavior, the header X-Content-Type-Options can be set
        /// to nosniff.
        ///
        /// In requests, (such as POST or PUT), the client tells the server what
        /// type of data is actually sent.
        (ContentType, CONTENT_TYPE, "content-type");

        /// Contains stored HTTP cookies previously sent by the server with the
        /// Set-Cookie header.
        ///
        /// The Cookie header might be omitted entirely, if the privacy setting of
        /// the browser are set to block them, for example.
        (Cookie, COOKIE, "cookie");

        /// Indicates the client's tracking preference.
        ///
        /// This header lets users indicate whether they would prefer privacy rather
        /// than personalized content.
        (Dnt, DNT, "dnt");

        /// Contains the date and time at which the message was originated.
        (Date, DATE, "date");

        /// Identifier for a specific version of a resource.
        ///
        /// This header allows caches to be more efficient, and saves bandwidth, as
        /// a web server does not need to send a full response if the content has
        /// not changed. On the other side, if the content has changed, etags are
        /// useful to help prevent simultaneous updates of a resource from
        /// overwriting each other ("mid-air collisions").
        ///
        /// If the resource at a given URL changes, a new Etag value must be
        /// generated. Etags are therefore similar to fingerprints and might also be
        /// used for tracking purposes by some servers. A comparison of them allows
        /// to quickly determine whether two representations of a resource are the
        /// same, but they might also be set to persist indefinitely by a tracking
        /// server.
        (Etag, ETAG, "etag");

        /// Indicates expectations that need to be fulfilled by the server in order
        /// to properly handle the request.
        ///
        /// The only expectation defined in the specification is Expect:
        /// 100-continue, to which the server shall respond with:
        ///
        /// * 100 if the information contained in the header is sufficient to cause
        /// an immediate success,
        ///
        /// * 417 (Expectation Failed) if it cannot meet the expectation; or any
        /// other 4xx status otherwise.
        ///
        /// For example, the server may reject a request if its Content-Length is
        /// too large.
        ///
        /// No common browsers send the Expect header, but some other clients such
        /// as cURL do so by default.
        (Expect, EXPECT, "expect");

        /// Contains the date/time after which the response is considered stale.
        ///
        /// Invalid dates, like the value 0, represent a date in the past and mean
        /// that the resource is already expired.
        ///
        /// If there is a Cache-Control header with the "max-age" or "s-max-age"
        /// directive in the response, the Expires header is ignored.
        (Expires, EXPIRES, "expires");

        /// Contains information from the client-facing side of proxy servers that
        /// is altered or lost when a proxy is involved in the path of the request.
        ///
        /// The alternative and de-facto standard versions of this header are the
        /// X-Forwarded-For, X-Forwarded-Host and X-Forwarded-Proto headers.
        ///
        /// This header is used for debugging, statistics, and generating
        /// location-dependent content and by design it exposes privacy sensitive
        /// information, such as the IP address of the client. Therefore the user's
        /// privacy must be kept in mind when deploying this header.
        (Forwarded, FORWARDED, "forwarded");

        /// Contains an Internet email address for a human user who controls the
        /// requesting user agent.
        ///
        /// If you are running a robotic user agent (e.g. a crawler), the From
        /// header should be sent, so you can be contacted if problems occur on
        /// servers, such as if the robot is sending excessive, unwanted, or invalid
        /// requests.
        (From, FROM, "from");

        /// Specifies the domain name of the server and (optionally) the TCP port
        /// number on which the server is listening.
        ///
        /// If no port is given, the default port for the service requested (e.g.,
        /// "80" for an HTTP URL) is implied.
        ///
        /// A Host header field must be sent in all HTTP/1.1 request messages. A 400
        /// (Bad Request) status code will be sent to any HTTP/1.1 request message
        /// that lacks a Host header field or contains more than one.
        (Host, HOST, "host");

        /// Makes a request conditional based on the E-Tag.
        ///
        /// For GET and HEAD methods, the server will send back the requested
        /// resource only if it matches one of the listed ETags. For PUT and other
        /// non-safe methods, it will only upload the resource in this case.
        ///
        /// The comparison with the stored ETag uses the strong comparison
        /// algorithm, meaning two files are considered identical byte to byte only.
        /// This is weakened when the  W/ prefix is used in front of the ETag.
        ///
        /// There are two common use cases:
        ///
        /// * For GET and HEAD methods, used in combination with an Range header, it
        /// can guarantee that the new ranges requested comes from the same resource
        /// than the previous one. If it doesn't match, then a 416 (Range Not
        /// Satisfiable) response is returned.
        ///
        /// * For other methods, and in particular for PUT, If-Match can be used to
        /// prevent the lost update problem. It can check if the modification of a
        /// resource that the user wants to upload will not override another change
        /// that has been done since the original resource was fetched. If the
        /// request cannot be fulfilled, the 412 (Precondition Failed) response is
        /// returned.
        (IfMatch, IF_MATCH, "if-match");

        /// Makes a request conditional based on the modification date.
        ///
        /// The If-Modified-Since request HTTP header makes the request conditional:
        /// the server will send back the requested resource, with a 200 status,
        /// only if it has been last modified after the given date. If the request
        /// has not been modified since, the response will be a 304 without any
        /// body; the Last-Modified header will contain the date of last
        /// modification. Unlike If-Unmodified-Since, If-Modified-Since can only be
        /// used with a GET or HEAD.
        ///
        /// When used in combination with If-None-Match, it is ignored, unless the
        /// server doesn't support If-None-Match.
        ///
        /// The most common use case is to update a cached entity that has no
        /// associated ETag.
        (IfModifiedSince, IF_MODIFIED_SINCE, "if-modified-since");

        /// Makes a request conditional based on the E-Tag.
        ///
        /// The If-None-Match HTTP request header makes the request conditional. For
        /// GET and HEAD methods, the server will send back the requested resource,
        /// with a 200 status, only if it doesn't have an ETag matching the given
        /// ones. For other methods, the request will be processed only if the
        /// eventually existing resource's ETag doesn't match any of the values
        /// listed.
        ///
        /// When the condition fails for GET and HEAD methods, then the server must
        /// return HTTP status code 304 (Not Modified). For methods that apply
        /// server-side changes, the status code 412 (Precondition Failed) is used.
        /// Note that the server generating a 304 response MUST generate any of the
        /// following header fields that would have been sent in a 200 (OK) response
        /// to the same request: Cache-Control, Content-Location, Date, ETag,
        /// Expires, and Vary.
        ///
        /// The comparison with the stored ETag uses the weak comparison algorithm,
        /// meaning two files are considered identical not only if they are
        /// identical byte to byte, but if the content is equivalent. For example,
        /// two pages that would differ only by the date of generation in the footer
        /// would be considered as identical.
        ///
        /// When used in combination with If-Modified-Since, it has precedence (if
        /// the server supports it).
        ///
        /// There are two common use cases:
        ///
        /// * For `GET` and `HEAD` methods, to update a cached entity that has an associated ETag.
        /// * For other methods, and in particular for `PUT`, `If-None-Match` used with
        /// the `*` value can be used to save a file not known to exist,
        /// guaranteeing that another upload didn't happen before, losing the data
        /// of the previous put; this problems is the variation of the lost update
        /// problem.
        (IfNoneMatch, IF_NONE_MATCH, "if-none-match");

        /// Makes a request conditional based on range.
        ///
        /// The If-Range HTTP request header makes a range request conditional: if
        /// the condition is fulfilled, the range request will be issued and the
        /// server sends back a 206 Partial Content answer with the appropriate
        /// body. If the condition is not fulfilled, the full resource is sent back,
        /// with a 200 OK status.
        ///
        /// This header can be used either with a Last-Modified validator, or with
        /// an ETag, but not with both.
        ///
        /// The most common use case is to resume a download, to guarantee that the
        /// stored resource has not been modified since the last fragment has been
        /// received.
        (IfRange, IF_RANGE, "if-range");

        /// Makes the request conditional based on the last modification date.
        ///
        /// The If-Unmodified-Since request HTTP header makes the request
        /// conditional: the server will send back the requested resource, or accept
        /// it in the case of a POST or another non-safe method, only if it has not
        /// been last modified after the given date. If the request has been
        /// modified after the given date, the response will be a 412 (Precondition
        /// Failed) error.
        ///
        /// There are two common use cases:
        ///
        /// * In conjunction non-safe methods, like POST, it can be used to
        /// implement an optimistic concurrency control, like done by some wikis:
        /// editions are rejected if the stored document has been modified since the
        /// original has been retrieved.
        ///
        /// * In conjunction with a range request with a If-Range header, it can be
        /// used to ensure that the new fragment requested comes from an unmodified
        /// document.
        (IfUnmodifiedSince, IF_UNMODIFIED_SINCE, "if-unmodified-since");

        /// Content-Types that are acceptable for the response.
        (LastModified, LAST_MODIFIED, "last-modified");

        /// Allows the server to point an interested client to another resource
        /// containing metadata about the requested resource.
        (Link, LINK, "link");

        /// Indicates the URL to redirect a page to.
        ///
        /// The Location response header indicates the URL to redirect a page to. It
        /// only provides a meaning when served with a 3xx status response.
        ///
        /// The HTTP method used to make the new request to fetch the page pointed
        /// to by Location depends of the original method and of the kind of
        /// redirection:
        ///
        /// * If 303 (See Also) responses always lead to the use of a GET method,
        /// 307 (Temporary Redirect) and 308 (Permanent Redirect) don't change the
        /// method used in the original request;
        ///
        /// * 301 (Permanent Redirect) and 302 (Found) doesn't change the method
        /// most of the time, though older user-agents may (so you basically don't
        /// know).
        ///
        /// All responses with one of these status codes send a Location header.
        ///
        /// Beside redirect response, messages with 201 (Created) status also
        /// include the Location header. It indicates the URL to the newly created
        /// resource.
        ///
        /// Location and Content-Location are different: Location indicates the
        /// target of a redirection (or the URL of a newly created resource), while
        /// Content-Location indicates the direct URL to use to access the resource
        /// when content negotiation happened, without the need of further content
        /// negotiation. Location is a header associated with the response, while
        /// Content-Location is associated with the entity returned.
        (Location, LOCATION, "location");

        /// Indicates the max number of intermediaries the request should be sent
        /// through.
        (MaxForwards, MAX_FORWARDS, "max-forwards");

        /// Indicates where a fetch originates from.
        ///
        /// It doesn't include any path information, but only the server name. It is
        /// sent with CORS requests, as well as with POST requests. It is similar to
        /// the Referer header, but, unlike this header, it doesn't disclose the
        /// whole path.
        (Origin, ORIGIN, "origin");

        /// HTTP/1.0 header usually used for backwards compatibility.
        ///
        /// The Pragma HTTP/1.0 general header is an implementation-specific header
        /// that may have various effects along the request-response chain. It is
        /// used for backwards compatibility with HTTP/1.0 caches where the
        /// Cache-Control HTTP/1.1 header is not yet present.
        (Pragma, PRAGMA, "pragma");

        /// Defines the authentication method that should be used to gain access to
        /// a proxy.
        ///
        /// Unlike `www-authenticate`, the `proxy-authenticate` header field applies
        /// only to the next outbound client on the response chain. This is because
        /// only the client that chose a given proxy is likely to have the
        /// credentials necessary for authentication. However, when multiple proxies
        /// are used within the same administrative domain, such as office and
        /// regional caching proxies within a large corporate network, it is common
        /// for credentials to be generated by the user agent and passed through the
        /// hierarchy until consumed. Hence, in such a configuration, it will appear
        /// as if Proxy-Authenticate is being forwarded because each proxy will send
        /// the same challenge set.
        ///
        /// The `proxy-authenticate` header is sent along with a `407 Proxy
        /// Authentication Required`.
        (ProxyAuthenticate, PROXY_AUTHENTICATE, "proxy-authenticate");

        /// Contains the credentials to authenticate a user agent to a proxy server.
        ///
        /// This header is usually included after the server has responded with a
        /// 407 Proxy Authentication Required status and the Proxy-Authenticate
        /// header.
        (ProxyAuthorization, PROXY_AUTHORIZATION, "proxy-authorization");

        /// Associates a specific cryptographic public key with a certain server.
        ///
        /// This decreases the risk of MITM attacks with forged certificates. If one
        /// or several keys are pinned and none of them are used by the server, the
        /// browser will not accept the response as legitimate, and will not display
        /// it.
        (PublicKeyPins, PUBLIC_KEY_PINS, "public-key-pins");

        /// Sends reports of pinning violation to the report-uri specified in the
        /// header.
        ///
        /// Unlike `Public-Key-Pins`, this header still allows browsers to connect
        /// to the server if the pinning is violated.
        (PublicKeyPinsReportOnly, PUBLIC_KEY_PINS_REPORT_ONLY, "public-key-pins-report-only");

        /// Indicates the part of a document that the server should return.
        ///
        /// Several parts can be requested with one Range header at once, and the
        /// server may send back these ranges in a multipart document. If the server
        /// sends back ranges, it uses the 206 Partial Content for the response. If
        /// the ranges are invalid, the server returns the 416 Range Not Satisfiable
        /// error. The server can also ignore the Range header and return the whole
        /// document with a 200 status code.
        (Range, RANGE, "range");

        /// Contains the address of the previous web page from which a link to the
        /// currently requested page was followed.
        ///
        /// The Referer header allows servers to identify where people are visiting
        /// them from and may use that data for analytics, logging, or optimized
        /// caching, for example.
        (Referer, REFERER, "referer");

        /// Governs which referrer information should be included with requests
        /// made.
        (ReferrerPolicy, REFERRER_POLICY, "referrer-policy");

        /// Informs the web browser that the current page or frame should be
        /// refreshed.
        (Refresh, REFRESH, "refresh");

        /// The Retry-After response HTTP header indicates how long the user agent
        /// should wait before making a follow-up request. There are two main cases
        /// this header is used:
        ///
        /// * When sent with a 503 (Service Unavailable) response, it indicates how
        /// long the service is expected to be unavailable.
        ///
        /// * When sent with a redirect response, such as 301 (Moved Permanently),
        /// it indicates the minimum time that the user agent is asked to wait
        /// before issuing the redirected request.
        (RetryAfter, RETRY_AFTER, "retry-after");

        /// The |Sec-WebSocket-Accept| header field is used in the WebSocket
        /// opening handshake. It is sent from the server to the client to
        /// confirm that the server is willing to initiate the WebSocket
        /// connection.
        (SecWebSocketAccept, SEC_WEBSOCKET_ACCEPT, "sec-websocket-accept");

        /// The |Sec-WebSocket-Extensions| header field is used in the WebSocket
        /// opening handshake. It is initially sent from the client to the
        /// server, and then subsequently sent from the server to the client, to
        /// agree on a set of protocol-level extensions to use for the duration
        /// of the connection.
        (SecWebSocketExtensions, SEC_WEBSOCKET_EXTENSIONS, "sec-websocket-extensions");

        /// The |Sec-WebSocket-Key| header field is used in the WebSocket opening
        /// handshake. It is sent from the client to the server to provide part
        /// of the information used by the server to prove that it received a
        /// valid WebSocket opening handshake. This helps ensure that the server
        /// does not accept connections from non-WebSocket clients (e.g., HTTP
        /// clients) that are being abused to send data to unsuspecting WebSocket
        /// servers.
        (SecWebSocketKey, SEC_WEBSOCKET_KEY, "sec-websocket-key");

        /// The |Sec-WebSocket-Protocol| header field is used in the WebSocket
        /// opening handshake. It is sent from the client to the server and back
        /// from the server to the client to confirm the subprotocol of the
        /// connection.  This enables scripts to both select a subprotocol and be
        /// sure that the server agreed to serve that subprotocol.
        (SecWebSocketProtocol, SEC_WEBSOCKET_PROTOCOL, "sec-websocket-protocol");

        /// The |Sec-WebSocket-Version| header field is used in the WebSocket
        /// opening handshake.  It is sent from the client to the server to
        /// indicate the protocol version of the connection.  This enables
        /// servers to correctly interpret the opening handshake and subsequent
        /// data being sent from the data, and close the connection if the server
        /// cannot interpret that data in a safe manner.
        (SecWebSocketVersion, SEC_WEBSOCKET_VERSION, "sec-websocket-version");

        /// Contains information about the software used by the origin server to
        /// handle the request.
        ///
        /// Overly long and detailed Server values should be avoided as they
        /// potentially reveal internal implementation details that might make it
        /// (slightly) easier for attackers to find and exploit known security
        /// holes.
        (Server, SERVER, "server");

        /// Used to send cookies from the server to the user agent.
        (SetCookie, SET_COOKIE, "set-cookie");

        /// Tells the client to communicate with HTTPS instead of using HTTP.
        (StrictTransportSecurity, STRICT_TRANSPORT_SECURITY, "strict-transport-security");

        /// Informs the server of transfer encodings willing to be accepted as part
        /// of the response.
        ///
        /// See also the Transfer-Encoding response header for more details on
        /// transfer encodings. Note that chunked is always acceptable for HTTP/1.1
        /// recipients and you that don't have to specify "chunked" using the TE
        /// header. However, it is useful for setting if the client is accepting
        /// trailer fields in a chunked transfer coding using the "trailers" value.
        (Te, TE, "te");

        /// Allows the sender to include additional fields at the end of chunked
        /// messages.
        (Trailer, TRAILER, "trailer");

        /// Specifies the form of encoding used to safely transfer the entity to the
        /// client.
        ///
        /// `transfer-encoding` is a hop-by-hop header, that is applying to a
        /// message between two nodes, not to a resource itself. Each segment of a
        /// multi-node connection can use different `transfer-encoding` values. If
        /// you want to compress data over the whole connection, use the end-to-end
        /// header `content-encoding` header instead.
        ///
        /// When present on a response to a `HEAD` request that has no body, it
        /// indicates the value that would have applied to the corresponding `GET`
        /// message.
        (TransferEncoding, TRANSFER_ENCODING, "transfer-encoding");

        /// Contains a string that allows identifying the requesting client's
        /// software.
        (UserAgent, USER_AGENT, "user-agent");

        /// Used as part of the exchange to upgrade the protocol.
        (Upgrade, UPGRADE, "upgrade");

        /// Sends a signal to the server expressing the client’s preference for an
        /// encrypted and authenticated response.
        (UpgradeInsecureRequests, UPGRADE_INSECURE_REQUESTS, "upgrade-insecure-requests");

        /// Determines how to match future requests with cached responses.
        ///
        /// The `vary` HTTP response header determines how to match future request
        /// headers to decide whether a cached response can be used rather than
        /// requesting a fresh one from the origin server. It is used by the server
        /// to indicate which headers it used when selecting a representation of a
        /// resource in a content negotiation algorithm.
        ///
        /// The `vary` header should be set on a 304 Not Modified response exactly
        /// like it would have been set on an equivalent 200 OK response.
        (Vary, VARY, "vary");

        /// Added by proxies to track routing.
        ///
        /// The `via` general header is added by proxies, both forward and reverse
        /// proxies, and can appear in the request headers and the response headers.
        /// It is used for tracking message forwards, avoiding request loops, and
        /// identifying the protocol capabilities of senders along the
        /// request/response chain.
        (Via, VIA, "via");

        /// General HTTP header contains information about possible problems with
        /// the status of the message.
        ///
        /// More than one `warning` header may appear in a response. Warning header
        /// fields can in general be applied to any message, however some warn-codes
        /// are specific to caches and can only be applied to response messages.
        (Warning, WARNING, "warning");

        /// Defines the authentication method that should be used to gain access to
        /// a resource.
        (WwwAuthenticate, WWW_AUTHENTICATE, "www-authenticate");

        /// Marker used by the server to indicate that the MIME types advertised in
        /// the `content-type` headers should not be changed and be followed.
        ///
        /// This allows to opt-out of MIME type sniffing, or, in other words, it is
        /// a way to say that the webmasters knew what they were doing.
        ///
        /// This header was introduced by Microsoft in IE 8 as a way for webmasters
        /// to block content sniffing that was happening and could transform
        /// non-executable MIME types into executable MIME types. Since then, other
        /// browsers have introduced it, even if their MIME sniffing algorithms were
        /// less aggressive.
        ///
        /// Site security testers usually expect this header to be set.
        (XContentTypeOptions, X_CONTENT_TYPE_OPTIONS, "x-content-type-options");

        /// Controls DNS prefetching.
        ///
        /// The `x-dns-prefetch-control` HTTP response header controls DNS
        /// prefetching, a feature by which browsers proactively perform domain name
        /// resolution on both links that the user may choose to follow as well as
        /// URLs for items referenced by the document, including images, CSS,
        /// JavaScript, and so forth.
        ///
        /// This prefetching is performed in the background, so that the DNS is
        /// likely to have been resolved by the time the referenced items are
        /// needed. This reduces latency when the user clicks a link.
        (XDnsPrefetchControl, X_DNS_PREFETCH_CONTROL, "x-dns-prefetch-control");

        /// Indicates whether or not a browser should be allowed to render a page in
        /// a frame.
        ///
        /// Sites can use this to avoid clickjacking attacks, by ensuring that their
        /// content is not embedded into other sites.
        ///
        /// The added security is only provided if the user accessing the document
        /// is using a browser supporting `x-frame-options`.
        (XFrameOptions, X_FRAME_OPTIONS, "x-frame-options");

        /// Stop pages from loading when an XSS attack is detected.
        ///
        /// The HTTP X-XSS-Protection response header is a feature of Internet
        /// Explorer, Chrome and Safari that stops pages from loading when they
        /// detect reflected cross-site scripting (XSS) attacks. Although these
        /// protections are largely unnecessary in modern browsers when sites
        /// implement a strong Content-Security-Policy that disables the use of
        /// inline JavaScript ('unsafe-inline'), they can still provide protections
        /// for users of older web browsers that don't yet support CSP.
        (XXssProtection, X_XSS_PROTECTION, "x-xss-protection");
    }

    /// Valid header name characters
    ///
    /// ```not_rust
    ///       field-name     = token
    ///       separators     = "(" | ")" | "<" | ">" | "@"
    ///                      | "," | ";" | ":" | "\" | <">
    ///                      | "/" | "[" | "]" | "?" | "="
    ///                      | "{" | "}" | SP | HT
    ///       token          = 1*tchar
    ///       tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
    ///                      / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
    ///                      / DIGIT / ALPHA
    ///                      ; any VCHAR, except delimiters
    /// ```
    const HEADER_CHARS: [u8; 256] = [
        //  0      1      2      3      4      5      6      7      8      9
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //   x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  1x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  2x
            0,     0,     0,  b'!',  b'"',  b'#',  b'$',  b'%',  b'&', b'\'', //  3x
            0,     0,  b'*',  b'+',     0,  b'-',  b'.',     0,  b'0',  b'1', //  4x
         b'2',  b'3',  b'4',  b'5',  b'6',  b'7',  b'8',  b'9',     0,     0, //  5x
            0,     0,     0,     0,     0,  b'a',  b'b',  b'c',  b'd',  b'e', //  6x
         b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm',  b'n',  b'o', //  7x
         b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w',  b'x',  b'y', //  8x
         b'z',     0,     0,     0,  b'^',  b'_',  b'`',  b'a',  b'b',  b'c', //  9x
         b'd',  b'e',  b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm', // 10x
         b'n',  b'o',  b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w', // 11x
         b'x',  b'y',  b'z',     0,  b'|',     0,  b'~',     0,     0,     0, // 12x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 13x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 14x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 15x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 16x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 17x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 18x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 19x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 20x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 21x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 22x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 23x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 24x
            0,     0,     0,     0,     0,     0                              // 25x
    ];

    /// Valid header name characters for HTTP/2.0 and HTTP/3.0
    const HEADER_CHARS_H2: [u8; 256] = [
        //  0      1      2      3      4      5      6      7      8      9
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //   x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  1x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  2x
            0,     0,     0,  b'!',  b'"',  b'#',  b'$',  b'%',  b'&', b'\'', //  3x
            0,     0,  b'*',  b'+',     0,  b'-',  b'.',     0,  b'0',  b'1', //  4x
         b'2',  b'3',  b'4',  b'5',  b'6',  b'7',  b'8',  b'9',     0,     0, //  5x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  6x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  7x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, //  8x
            0,     0,     0,     0,  b'^',  b'_',  b'`',  b'a',  b'b',  b'c', //  9x
         b'd',  b'e',  b'f',  b'g',  b'h',  b'i',  b'j',  b'k',  b'l',  b'm', // 10x
         b'n',  b'o',  b'p',  b'q',  b'r',  b's',  b't',  b'u',  b'v',  b'w', // 11x
         b'x',  b'y',  b'z',     0,  b'|',     0,  b'~',     0,     0,     0, // 12x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 13x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 14x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 15x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 16x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 17x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 18x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 19x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 20x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 21x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 22x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 23x
            0,     0,     0,     0,     0,     0,     0,     0,     0,     0, // 24x
            0,     0,     0,     0,     0,     0                              // 25x
    ];

    #[cfg(any(not(debug_assertions), not(target_arch = "wasm32")))]
    macro_rules! eq {
        (($($cmp:expr,)*) $v:ident[$n:expr] ==) => {
            $($cmp) && *
        };
        (($($cmp:expr,)*) $v:ident[$n:expr] == $a:tt $($rest:tt)*) => {
            eq!(($($cmp,)* $v[$n] == $a,) $v[$n+1] == $($rest)*)
        };
        ($v:ident == $($rest:tt)+) => {
            eq!(() $v[0] == $($rest)+)
        };
        ($v:ident[$n:expr] == $($rest:tt)+) => {
            eq!(() $v[$n] == $($rest)+)
        };
    }

    #[cfg(any(not(debug_assertions), not(target_arch = "wasm32")))]
    /// This version is best under optimized mode, however in a wasm debug compile,
    /// the `eq` macro expands to 1 + 1 + 1 + 1... and wasm explodes when this chain gets too long
    /// See https://github.com/DenisKolodin/yew/issues/478
    fn parse_hdr<'a>(
        data: &'a [u8],
        b: &'a mut [u8; 64],
        table: &[u8; 256],
    ) -> Result<HdrName<'a>, InvalidHeaderName> {
        use self::StandardHeader::*;

        let len = data.len();

        let validate = |buf: &'a [u8], len: usize| {
            let buf = &buf[..len];
            if buf.iter().any(|&b| b == 0) {
                Err(InvalidHeaderName::new())
            } else {
                Ok(HdrName::custom(buf, true))
            }
        };

        macro_rules! to_lower {
            ($d:ident, $src:ident, 1) => { $d[0] = table[$src[0] as usize]; };
            ($d:ident, $src:ident, 2) => { to_lower!($d, $src, 1); $d[1] = table[$src[1] as usize]; };
            ($d:ident, $src:ident, 3) => { to_lower!($d, $src, 2); $d[2] = table[$src[2] as usize]; };
            ($d:ident, $src:ident, 4) => { to_lower!($d, $src, 3); $d[3] = table[$src[3] as usize]; };
            ($d:ident, $src:ident, 5) => { to_lower!($d, $src, 4); $d[4] = table[$src[4] as usize]; };
            ($d:ident, $src:ident, 6) => { to_lower!($d, $src, 5); $d[5] = table[$src[5] as usize]; };
            ($d:ident, $src:ident, 7) => { to_lower!($d, $src, 6); $d[6] = table[$src[6] as usize]; };
            ($d:ident, $src:ident, 8) => { to_lower!($d, $src, 7); $d[7] = table[$src[7] as usize]; };
            ($d:ident, $src:ident, 9) => { to_lower!($d, $src, 8); $d[8] = table[$src[8] as usize]; };
            ($d:ident, $src:ident, 10) => { to_lower!($d, $src, 9); $d[9] = table[$src[9] as usize]; };
            ($d:ident, $src:ident, 11) => { to_lower!($d, $src, 10); $d[10] = table[$src[10] as usize]; };
            ($d:ident, $src:ident, 12) => { to_lower!($d, $src, 11); $d[11] = table[$src[11] as usize]; };
            ($d:ident, $src:ident, 13) => { to_lower!($d, $src, 12); $d[12] = table[$src[12] as usize]; };
            ($d:ident, $src:ident, 14) => { to_lower!($d, $src, 13); $d[13] = table[$src[13] as usize]; };
            ($d:ident, $src:ident, 15) => { to_lower!($d, $src, 14); $d[14] = table[$src[14] as usize]; };
            ($d:ident, $src:ident, 16) => { to_lower!($d, $src, 15); $d[15] = table[$src[15] as usize]; };
            ($d:ident, $src:ident, 17) => { to_lower!($d, $src, 16); $d[16] = table[$src[16] as usize]; };
            ($d:ident, $src:ident, 18) => { to_lower!($d, $src, 17); $d[17] = table[$src[17] as usize]; };
            ($d:ident, $src:ident, 19) => { to_lower!($d, $src, 18); $d[18] = table[$src[18] as usize]; };
            ($d:ident, $src:ident, 20) => { to_lower!($d, $src, 19); $d[19] = table[$src[19] as usize]; };
            ($d:ident, $src:ident, 21) => { to_lower!($d, $src, 20); $d[20] = table[$src[20] as usize]; };
            ($d:ident, $src:ident, 22) => { to_lower!($d, $src, 21); $d[21] = table[$src[21] as usize]; };
            ($d:ident, $src:ident, 23) => { to_lower!($d, $src, 22); $d[22] = table[$src[22] as usize]; };
            ($d:ident, $src:ident, 24) => { to_lower!($d, $src, 23); $d[23] = table[$src[23] as usize]; };
            ($d:ident, $src:ident, 25) => { to_lower!($d, $src, 24); $d[24] = table[$src[24] as usize]; };
            ($d:ident, $src:ident, 26) => { to_lower!($d, $src, 25); $d[25] = table[$src[25] as usize]; };
            ($d:ident, $src:ident, 27) => { to_lower!($d, $src, 26); $d[26] = table[$src[26] as usize]; };
            ($d:ident, $src:ident, 28) => { to_lower!($d, $src, 27); $d[27] = table[$src[27] as usize]; };
            ($d:ident, $src:ident, 29) => { to_lower!($d, $src, 28); $d[28] = table[$src[28] as usize]; };
            ($d:ident, $src:ident, 30) => { to_lower!($d, $src, 29); $d[29] = table[$src[29] as usize]; };
            ($d:ident, $src:ident, 31) => { to_lower!($d, $src, 30); $d[30] = table[$src[30] as usize]; };
            ($d:ident, $src:ident, 32) => { to_lower!($d, $src, 31); $d[31] = table[$src[31] as usize]; };
            ($d:ident, $src:ident, 33) => { to_lower!($d, $src, 32); $d[32] = table[$src[32] as usize]; };
            ($d:ident, $src:ident, 34) => { to_lower!($d, $src, 33); $d[33] = table[$src[33] as usize]; };
            ($d:ident, $src:ident, 35) => { to_lower!($d, $src, 34); $d[34] = table[$src[34] as usize]; };
        }

        match len {
            0 => Err(InvalidHeaderName::new()),
            2 => {
                to_lower!(b, data, 2);

                if eq!(b == b't' b'e') {
                    Ok(Te.into())
                } else {
                    validate(b, len)
                }
            }
            3 => {
                to_lower!(b, data, 3);

                if eq!(b == b'a' b'g' b'e') {
                    Ok(Age.into())
                } else if eq!(b == b'v' b'i' b'a') {
                    Ok(Via.into())
                } else if eq!(b == b'd' b'n' b't') {
                    Ok(Dnt.into())
                } else {
                    validate(b, len)
                }
            }
            4 => {
                to_lower!(b, data, 4);

                if eq!(b == b'd' b'a' b't' b'e') {
                    Ok(Date.into())
                } else if eq!(b == b'e' b't' b'a' b'g') {
                    Ok(Etag.into())
                } else if eq!(b == b'f' b'r' b'o' b'm') {
                    Ok(From.into())
                } else if eq!(b == b'h' b'o' b's' b't') {
                    Ok(Host.into())
                } else if eq!(b == b'l' b'i' b'n' b'k') {
                    Ok(Link.into())
                } else if eq!(b == b'v' b'a' b'r' b'y') {
                    Ok(Vary.into())
                } else {
                    validate(b, len)
                }
            }
            5 => {
                to_lower!(b, data, 5);

                if eq!(b == b'a' b'l' b'l' b'o' b'w') {
                    Ok(Allow.into())
                } else if eq!(b == b'r' b'a' b'n' b'g' b'e') {
                    Ok(Range.into())
                } else {
                    validate(b, len)
                }
            }
            6 => {
                to_lower!(b, data, 6);

                if eq!(b == b'a' b'c' b'c' b'e' b'p' b't') {
                    return Ok(Accept.into());
                } else if eq!(b == b'c' b'o' b'o' b'k' b'i' b'e') {
                    return Ok(Cookie.into());
                } else if eq!(b == b'e' b'x' b'p' b'e' b'c' b't') {
                    return Ok(Expect.into());
                } else if eq!(b == b'o' b'r' b'i' b'g' b'i' b'n') {
                    return Ok(Origin.into());
                } else if eq!(b == b'p' b'r' b'a' b'g' b'm' b'a') {
                    return Ok(Pragma.into());
                } else if b[0] == b's' {
                    if eq!(b[1] == b'e' b'r' b'v' b'e' b'r') {
                        return Ok(Server.into());
                    }
                }

                validate(b, len)
            }
            7 => {
                to_lower!(b, data, 7);

                if eq!(b == b'a' b'l' b't' b'-' b's' b'v' b'c') {
                    Ok(AltSvc.into())
                } else if eq!(b == b'e' b'x' b'p' b'i' b'r' b'e' b's') {
                    Ok(Expires.into())
                } else if eq!(b == b'r' b'e' b'f' b'e' b'r' b'e' b'r') {
                    Ok(Referer.into())
                } else if eq!(b == b'r' b'e' b'f' b'r' b'e' b's' b'h') {
                    Ok(Refresh.into())
                } else if eq!(b == b't' b'r' b'a' b'i' b'l' b'e' b'r') {
                    Ok(Trailer.into())
                } else if eq!(b == b'u' b'p' b'g' b'r' b'a' b'd' b'e') {
                    Ok(Upgrade.into())
                } else if eq!(b == b'w' b'a' b'r' b'n' b'i' b'n' b'g') {
                    Ok(Warning.into())
                } else {
                    validate(b, len)
                }
            }
            8 => {
                to_lower!(b, data, 8);

                if eq!(b == b'i' b'f' b'-') {
                    if eq!(b[3] == b'm' b'a' b't' b'c' b'h') {
                        return Ok(IfMatch.into());
                    } else if eq!(b[3] == b'r' b'a' b'n' b'g' b'e') {
                        return Ok(IfRange.into());
                    }
                } else if eq!(b == b'l' b'o' b'c' b'a' b't' b'i' b'o' b'n') {
                    return Ok(Location.into());
                }

                validate(b, len)
            }
            9 => {
                to_lower!(b, data, 9);

                if eq!(b == b'f' b'o' b'r' b'w' b'a' b'r' b'd' b'e' b'd') {
                    Ok(Forwarded.into())
                } else {
                    validate(b, len)
                }
            }
            10 => {
                to_lower!(b, data, 10);

                if eq!(b == b'c' b'o' b'n' b'n' b'e' b'c' b't' b'i' b'o' b'n') {
                    Ok(Connection.into())
                } else if eq!(b == b's' b'e' b't' b'-' b'c' b'o' b'o' b'k' b'i' b'e') {
                    Ok(SetCookie.into())
                } else if eq!(b == b'u' b's' b'e' b'r' b'-' b'a' b'g' b'e' b'n' b't') {
                    Ok(UserAgent.into())
                } else {
                    validate(b, len)
                }
            }
            11 => {
                to_lower!(b, data, 11);

                if eq!(b == b'r' b'e' b't' b'r' b'y' b'-' b'a' b'f' b't' b'e' b'r') {
                    Ok(RetryAfter.into())
                } else {
                    validate(b, len)
                }
            }
            12 => {
                to_lower!(b, data, 12);

                if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b't' b'y' b'p' b'e') {
                    Ok(ContentType.into())
                } else if eq!(b == b'm' b'a' b'x' b'-' b'f' b'o' b'r' b'w' b'a' b'r' b'd' b's') {
                    Ok(MaxForwards.into())
                } else {
                    validate(b, len)
                }
            }
            13 => {
                to_lower!(b, data, 13);

                if b[0] == b'a' {
                    if eq!(b[1] == b'c' b'c' b'e' b'p' b't' b'-' b'r' b'a' b'n' b'g' b'e' b's') {
                        return Ok(AcceptRanges.into());
                    } else if eq!(b[1] == b'u' b't' b'h' b'o' b'r' b'i' b'z' b'a' b't' b'i' b'o' b'n') {
                        return Ok(Authorization.into());
                    }
                } else if b[0] == b'c' {
                    if eq!(b[1] == b'a' b'c' b'h' b'e' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l') {
                        return Ok(CacheControl.into());
                    } else if eq!(b[1] == b'o' b'n' b't' b'e' b'n' b't' b'-' b'r' b'a' b'n' b'g' b'e' )
                    {
                        return Ok(ContentRange.into());
                    }
                } else if eq!(b == b'i' b'f' b'-' b'n' b'o' b'n' b'e' b'-' b'm' b'a' b't' b'c' b'h') {
                    return Ok(IfNoneMatch.into());
                } else if eq!(b == b'l' b'a' b's' b't' b'-' b'm' b'o' b'd' b'i' b'f' b'i' b'e' b'd') {
                    return Ok(LastModified.into());
                }

                validate(b, len)
            }
            14 => {
                to_lower!(b, data, 14);

                if eq!(b == b'a' b'c' b'c' b'e' b'p' b't' b'-' b'c' b'h' b'a' b'r' b's' b'e' b't') {
                    Ok(AcceptCharset.into())
                } else if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b'l' b'e' b'n' b'g' b't' b'h')
                {
                    Ok(ContentLength.into())
                } else {
                    validate(b, len)
                }
            }
            15 => {
                to_lower!(b, data, 15);

                if eq!(b == b'a' b'c' b'c' b'e' b'p' b't' b'-') { // accept-
                    if eq!(b[7] == b'e' b'n' b'c' b'o' b'd' b'i' b'n' b'g') {
                        return Ok(AcceptEncoding.into())
                    } else if eq!(b[7] == b'l' b'a' b'n' b'g' b'u' b'a' b'g' b'e') {
                        return Ok(AcceptLanguage.into())
                    }
                } else if eq!(b == b'p' b'u' b'b' b'l' b'i' b'c' b'-' b'k' b'e' b'y' b'-' b'p' b'i' b'n' b's') {
                    return Ok(PublicKeyPins.into())
                } else if eq!(b == b'x' b'-' b'f' b'r' b'a' b'm' b'e' b'-' b'o' b'p' b't' b'i' b'o' b'n' b's') {
                    return Ok(XFrameOptions.into())
                }
                else if eq!(b == b'r' b'e' b'f' b'e' b'r' b'r' b'e' b'r' b'-' b'p' b'o' b'l' b'i' b'c' b'y') {
                    return Ok(ReferrerPolicy.into())
                }

                validate(b, len)
            }
            16 => {
                to_lower!(b, data, 16);

                if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-') {
                    if eq!(b[8] == b'l' b'a' b'n' b'g' b'u' b'a' b'g' b'e') {
                        return Ok(ContentLanguage.into())
                    } else if eq!(b[8] == b'l' b'o' b'c' b'a' b't' b'i' b'o' b'n') {
                        return Ok(ContentLocation.into())
                    } else if eq!(b[8] == b'e' b'n' b'c' b'o' b'd' b'i' b'n' b'g') {
                        return Ok(ContentEncoding.into())
                    }
                } else if eq!(b == b'w' b'w' b'w' b'-' b'a' b'u' b't' b'h' b'e' b'n' b't' b'i' b'c' b'a' b't' b'e') {
                    return Ok(WwwAuthenticate.into())
                } else if eq!(b == b'x' b'-' b'x' b's' b's' b'-' b'p' b'r' b'o' b't' b'e' b'c' b't' b'i' b'o' b'n') {
                    return Ok(XXssProtection.into())
                }

                validate(b, len)
            }
            17 => {
                to_lower!(b, data, 17);

                if eq!(b == b't' b'r' b'a' b'n' b's' b'f' b'e' b'r' b'-' b'e' b'n' b'c' b'o' b'd' b'i' b'n' b'g') {
                    Ok(TransferEncoding.into())
                } else if eq!(b == b'i' b'f' b'-' b'm' b'o' b'd' b'i' b'f' b'i' b'e' b'd' b'-' b's' b'i' b'n' b'c' b'e') {
                    Ok(IfModifiedSince.into())
                } else if eq!(b == b's' b'e' b'c' b'-' b'w' b'e' b'b' b's' b'o' b'c' b'k' b'e' b't' b'-' b'k' b'e' b'y') {
                    Ok(SecWebSocketKey.into())
                } else {
                    validate(b, len)
                }
            }
            18 => {
                to_lower!(b, data, 18);

                if eq!(b == b'p' b'r' b'o' b'x' b'y' b'-' b'a' b'u' b't' b'h' b'e' b'n' b't' b'i' b'c' b'a' b't' b'e') {
                    Ok(ProxyAuthenticate.into())
                } else {
                    validate(b, len)
                }
            }
            19 => {
                to_lower!(b, data, 19);

                if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b'd' b'i' b's' b'p' b'o' b's' b'i' b't' b'i' b'o' b'n') {
                    Ok(ContentDisposition.into())
                } else if eq!(b == b'i' b'f' b'-' b'u' b'n' b'm' b'o' b'd' b'i' b'f' b'i' b'e' b'd' b'-' b's' b'i' b'n' b'c' b'e') {
                    Ok(IfUnmodifiedSince.into())
                } else if eq!(b == b'p' b'r' b'o' b'x' b'y' b'-' b'a' b'u' b't' b'h' b'o' b'r' b'i' b'z' b'a' b't' b'i' b'o' b'n') {
                    Ok(ProxyAuthorization.into())
                } else {
                    validate(b, len)
                }
            }
            20 => {
                to_lower!(b, data, 20);

                if eq!(b == b's' b'e' b'c' b'-' b'w' b'e' b'b' b's' b'o' b'c' b'k' b'e' b't' b'-' b'a' b'c' b'c' b'e' b'p' b't') {
                    Ok(SecWebSocketAccept.into())
                } else {
                    validate(b, len)
                }
            }
            21 => {
                to_lower!(b, data, 21);

                if eq!(b == b's' b'e' b'c' b'-' b'w' b'e' b'b' b's' b'o' b'c' b'k' b'e' b't' b'-' b'v' b'e' b'r' b's' b'i' b'o' b'n') {
                    Ok(SecWebSocketVersion.into())
                } else {
                    validate(b, len)
                }
            }
            22 => {
                to_lower!(b, data, 22);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-' b'm' b'a' b'x' b'-' b'a' b'g' b'e') {
                    Ok(AccessControlMaxAge.into())
                } else if eq!(b == b'x' b'-' b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b't' b'y' b'p' b'e' b'-' b'o' b'p' b't' b'i' b'o' b'n' b's') {
                    Ok(XContentTypeOptions.into())
                } else if eq!(b == b'x' b'-' b'd' b'n' b's' b'-' b'p' b'r' b'e' b'f' b'e' b't' b'c' b'h' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l') {
                    Ok(XDnsPrefetchControl.into())
                } else if eq!(b == b's' b'e' b'c' b'-' b'w' b'e' b'b' b's' b'o' b'c' b'k' b'e' b't' b'-' b'p' b'r' b'o' b't' b'o' b'c' b'o' b'l') {
                    Ok(SecWebSocketProtocol.into())
                } else {
                    validate(b, len)
                }
            }
            23 => {
                to_lower!(b, data, 23);

                if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b's' b'e' b'c' b'u' b'r' b'i' b't' b'y' b'-' b'p' b'o' b'l' b'i' b'c' b'y') {
                    Ok(ContentSecurityPolicy.into())
                } else {
                    validate(b, len)
                }
            }
            24 => {
                to_lower!(b, data, 24);

                if eq!(b == b's' b'e' b'c' b'-' b'w' b'e' b'b' b's' b'o' b'c' b'k' b'e' b't' b'-' b'e' b'x' b't' b'e' b'n' b's' b'i' b'o' b'n' b's') {
                    Ok(SecWebSocketExtensions.into())
                } else {
                    validate(b, len)
                }
            }
            25 => {
                to_lower!(b, data, 25);

                if eq!(b == b's' b't' b'r' b'i' b'c' b't' b'-' b't' b'r' b'a' b'n' b's' b'p' b'o' b'r' b't' b'-' b's' b'e' b'c' b'u' b'r' b'i' b't' b'y') {
                    Ok(StrictTransportSecurity.into())
                } else if eq!(b == b'u' b'p' b'g' b'r' b'a' b'd' b'e' b'-' b'i' b'n' b's' b'e' b'c' b'u' b'r' b'e' b'-' b'r' b'e' b'q' b'u' b'e' b's' b't' b's') {
                    Ok(UpgradeInsecureRequests.into())
                } else {
                    validate(b, len)
                }
            }
            27 => {
                to_lower!(b, data, 27);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-' b'a' b'l' b'l' b'o' b'w' b'-' b'o' b'r' b'i' b'g' b'i' b'n') {
                    Ok(AccessControlAllowOrigin.into())
                } else if eq!(b == b'p' b'u' b'b' b'l' b'i' b'c' b'-' b'k' b'e' b'y' b'-' b'p' b'i' b'n' b's' b'-' b'r' b'e' b'p' b'o' b'r' b't' b'-' b'o' b'n' b'l' b'y') {
                    Ok(PublicKeyPinsReportOnly.into())
                } else {
                    validate(b, len)
                }
            }
            28 => {
                to_lower!(b, data, 28);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-' b'a' b'l' b'l' b'o' b'w' b'-') {
                    if eq!(b[21] == b'h' b'e' b'a' b'd' b'e' b'r' b's') {
                        return Ok(AccessControlAllowHeaders.into())
                    } else if eq!(b[21] == b'm' b'e' b't' b'h' b'o' b'd' b's') {
                        return Ok(AccessControlAllowMethods.into())
                    }
                }

                validate(b, len)
            }
            29 => {
                to_lower!(b, data, 29);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-') {
                    if eq!(b[15] == b'e' b'x' b'p' b'o' b's' b'e' b'-' b'h' b'e' b'a' b'd' b'e' b'r' b's') {
                        return Ok(AccessControlExposeHeaders.into())
                    } else if eq!(b[15] == b'r' b'e' b'q' b'u' b'e' b's' b't' b'-' b'm' b'e' b't' b'h' b'o' b'd') {
                        return Ok(AccessControlRequestMethod.into())
                    }
                }

                validate(b, len)
            }
            30 => {
                to_lower!(b, data, 30);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-' b'r' b'e' b'q' b'u' b'e' b's' b't' b'-' b'h' b'e' b'a' b'd' b'e' b'r' b's') {
                    Ok(AccessControlRequestHeaders.into())
                } else {
                    validate(b, len)
                }
            }
            32 => {
                to_lower!(b, data, 32);

                if eq!(b == b'a' b'c' b'c' b'e' b's' b's' b'-' b'c' b'o' b'n' b't' b'r' b'o' b'l' b'-' b'a' b'l' b'l' b'o' b'w' b'-' b'c' b'r' b'e' b'd' b'e' b'n' b't' b'i' b'a' b'l' b's') {
                    Ok(AccessControlAllowCredentials.into())
                } else {
                    validate(b, len)
                }
            }
            35 => {
                to_lower!(b, data, 35);

                if eq!(b == b'c' b'o' b'n' b't' b'e' b'n' b't' b'-' b's' b'e' b'c' b'u' b'r' b'i' b't' b'y' b'-' b'p' b'o' b'l' b'i' b'c' b'y' b'-' b'r' b'e' b'p' b'o' b'r' b't' b'-' b'o' b'n' b'l' b'y') {
                    Ok(ContentSecurityPolicyReportOnly.into())
                } else {
                    validate(b, len)
                }
            }
            len if len < 64 => {
                for i in 0..len {
                    b[i] = table[data[i] as usize];
                }
                validate(b, len)
            }
            len if len <= super::MAX_HEADER_NAME_LEN => {
                Ok(HdrName::custom(data, false))
            }
            _ => Err(InvalidHeaderName::new()),
        }
    }

    #[cfg(all(debug_assertions, target_arch = "wasm32"))]
    /// This version works best in debug mode in wasm
    fn parse_hdr<'a>(
        data: &'a [u8],
        b: &'a mut [u8; 64],
        table: &[u8; 256],
    ) -> Result<HdrName<'a>, InvalidHeaderName> {
        use self::StandardHeader::*;

        let len = data.len();

        let validate = |buf: &'a [u8], len: usize| {
            let buf = &buf[..len];
            if buf.iter().any(|&b| b == 0) {
                Err(InvalidHeaderName::new())
            } else {
                Ok(HdrName::custom(buf, true))
            }
        };

        assert!(
            len < super::MAX_HEADER_NAME_LEN,
            "header name too long -- max length is {}",
            super::MAX_HEADER_NAME_LEN
        );

        match len {
            0 => Err(InvalidHeaderName::new()),
            len if len > 64 => Ok(HdrName::custom(data, false)),
            len => {
                // Read from data into the buffer - transforming using `table` as we go
                data.iter().zip(b.iter_mut()).for_each(|(index, out)| *out = table[*index as usize]);
                match &b[0..len] {
                    b"te" => Ok(Te.into()),
                    b"age" => Ok(Age.into()),
                    b"via" => Ok(Via.into()),
                    b"dnt" => Ok(Dnt.into()),
                    b"date" => Ok(Date.into()),
                    b"etag" => Ok(Etag.into()),
                    b"from" => Ok(From.into()),
                    b"host" => Ok(Host.into()),
                    b"link" => Ok(Link.into()),
                    b"vary" => Ok(Vary.into()),
                    b"allow" => Ok(Allow.into()),
                    b"range" => Ok(Range.into()),
                    b"accept" => Ok(Accept.into()),
                    b"cookie" => Ok(Cookie.into()),
                    b"expect" => Ok(Expect.into()),
                    b"origin" => Ok(Origin.into()),
                    b"pragma" => Ok(Pragma.into()),
                    b"server" => Ok(Server.into()),
                    b"alt-svc" => Ok(AltSvc.into()),
                    b"expires" => Ok(Expires.into()),
                    b"referer" => Ok(Referer.into()),
                    b"refresh" => Ok(Refresh.into()),
                    b"trailer" => Ok(Trailer.into()),
                    b"upgrade" => Ok(Upgrade.into()),
                    b"warning" => Ok(Warning.into()),
                    b"if-match" => Ok(IfMatch.into()),
                    b"if-range" => Ok(IfRange.into()),
                    b"location" => Ok(Location.into()),
                    b"forwarded" => Ok(Forwarded.into()),
                    b"connection" => Ok(Connection.into()),
                    b"set-cookie" => Ok(SetCookie.into()),
                    b"user-agent" => Ok(UserAgent.into()),
                    b"retry-after" => Ok(RetryAfter.into()),
                    b"content-type" => Ok(ContentType.into()),
                    b"max-forwards" => Ok(MaxForwards.into()),
                    b"accept-ranges" => Ok(AcceptRanges.into()),
                    b"authorization" => Ok(Authorization.into()),
                    b"cache-control" => Ok(CacheControl.into()),
                    b"content-range" => Ok(ContentRange.into()),
                    b"if-none-match" => Ok(IfNoneMatch.into()),
                    b"last-modified" => Ok(LastModified.into()),
                    b"accept-charset" => Ok(AcceptCharset.into()),
                    b"content-length" => Ok(ContentLength.into()),
                    b"accept-encoding" => Ok(AcceptEncoding.into()),
                    b"accept-language" => Ok(AcceptLanguage.into()),
                    b"public-key-pins" => Ok(PublicKeyPins.into()),
                    b"x-frame-options" => Ok(XFrameOptions.into()),
                    b"referrer-policy" => Ok(ReferrerPolicy.into()),
                    b"content-language" => Ok(ContentLanguage.into()),
                    b"content-location" => Ok(ContentLocation.into()),
                    b"content-encoding" => Ok(ContentEncoding.into()),
                    b"www-authenticate" => Ok(WwwAuthenticate.into()),
                    b"x-xss-protection" => Ok(XXssProtection.into()),
                    b"transfer-encoding" => Ok(TransferEncoding.into()),
                    b"if-modified-since" => Ok(IfModifiedSince.into()),
                    b"sec-websocket-key" => Ok(SecWebSocketKey.into()),
                    b"proxy-authenticate" => Ok(ProxyAuthenticate.into()),
                    b"content-disposition" => Ok(ContentDisposition.into()),
                    b"if-unmodified-since" => Ok(IfUnmodifiedSince.into()),
                    b"proxy-authorization" => Ok(ProxyAuthorization.into()),
                    b"sec-websocket-accept" => Ok(SecWebSocketAccept.into()),
                    b"sec-websocket-version" => Ok(SecWebSocketVersion.into()),
                    b"access-control-max-age" => Ok(AccessControlMaxAge.into()),
                    b"x-content-type-options" => Ok(XContentTypeOptions.into()),
                    b"x-dns-prefetch-control" => Ok(XDnsPrefetchControl.into()),
                    b"sec-websocket-protocol" => Ok(SecWebSocketProtocol.into()),
                    b"content-security-policy" => Ok(ContentSecurityPolicy.into()),
                    b"sec-websocket-extensions" => Ok(SecWebSocketExtensions.into()),
                    b"strict-transport-security" => Ok(StrictTransportSecurity.into()),
                    b"upgrade-insecure-requests" => Ok(UpgradeInsecureRequests.into()),
                    b"access-control-allow-origin" => Ok(AccessControlAllowOrigin.into()),
                    b"public-key-pins-report-only" => Ok(PublicKeyPinsReportOnly.into()),
                    b"access-control-allow-headers" => Ok(AccessControlAllowHeaders.into()),
                    b"access-control-allow-methods" => Ok(AccessControlAllowMethods.into()),
                    b"access-control-expose-headers" => Ok(AccessControlExposeHeaders.into()),
                    b"access-control-request-method" => Ok(AccessControlRequestMethod.into()),
                    b"access-control-request-headers" => Ok(AccessControlRequestHeaders.into()),
                    b"access-control-allow-credentials" => Ok(AccessControlAllowCredentials.into()),
                    b"content-security-policy-report-only" => {
                        Ok(ContentSecurityPolicyReportOnly.into())
                    }
                    other => validate(other, len),
                }
            }
        }
    }

    impl<'a> From<StandardHeader> for HdrName<'a> {
        fn from(hdr: StandardHeader) -> HdrName<'a> {
            HdrName { inner: Repr::Standard(hdr) }
        }
    }

    impl HeaderName {
        /// Converts a slice of bytes to an HTTP header name.
        ///
        /// This function normalizes the input.
        #[allow(deprecated)]
        pub fn from_bytes(src: &[u8]) -> Result<HeaderName, InvalidHeaderName> {
            #[allow(deprecated)]
            let mut buf = unsafe { mem::uninitialized() };
            match parse_hdr(src, &mut buf, &HEADER_CHARS)?.inner {
                Repr::Standard(std) => Ok(std.into()),
                Repr::Custom(MaybeLower { buf, lower: true }) => {
                    let buf = Bytes::copy_from_slice(buf);
                    let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                    Ok(Custom(val).into())
                }
                Repr::Custom(MaybeLower { buf, lower: false }) => {
                    use bytes::{BufMut};
                    let mut dst = BytesMut::with_capacity(buf.len());

                    for b in buf.iter() {
                        let b = HEADER_CHARS[*b as usize];

                        if b == 0 {
                            return Err(InvalidHeaderName::new());
                        }

                        dst.put_u8(b);
                    }

                    let val = unsafe { ByteStr::from_utf8_unchecked(dst.freeze()) };

                    Ok(Custom(val).into())
                }
            }
        }

        /// Converts a slice of bytes to an HTTP header name.
        ///
        /// This function expects the input to only contain lowercase characters.
        /// This is useful when decoding HTTP/2.0 or HTTP/3.0 headers. Both
        /// require that all headers be represented in lower case.
        ///
        /// # Examples
        ///
        /// ```
        /// # use http::header::*;
        ///
        /// // Parsing a lower case header
        /// let hdr = HeaderName::from_lowercase(b"content-length").unwrap();
        /// assert_eq!(CONTENT_LENGTH, hdr);
        ///
        /// // Parsing a header that contains uppercase characters
        /// assert!(HeaderName::from_lowercase(b"Content-Length").is_err());
        /// ```
        #[allow(deprecated)]
        pub fn from_lowercase(src: &[u8]) -> Result<HeaderName, InvalidHeaderName> {
            #[allow(deprecated)]
            let mut buf = unsafe { mem::uninitialized() };
            match parse_hdr(src, &mut buf, &HEADER_CHARS_H2)?.inner {
                Repr::Standard(std) => Ok(std.into()),
                Repr::Custom(MaybeLower { buf, lower: true }) => {
                    let buf = Bytes::copy_from_slice(buf);
                    let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                    Ok(Custom(val).into())
                }
                Repr::Custom(MaybeLower { buf, lower: false }) => {
                    for &b in buf.iter() {
                        if b != HEADER_CHARS[b as usize] {
                            return Err(InvalidHeaderName::new());
                        }
                    }

                    let buf = Bytes::copy_from_slice(buf);
                    let val = unsafe { ByteStr::from_utf8_unchecked(buf) };
                    Ok(Custom(val).into())
                }
            }
        }

        /// Converts a static string to a HTTP header name.
        ///
        /// This function panics when the static string is a invalid header.
        ///
        /// This function requires the static string to only contain lowercase
        /// characters, numerals and symbols, as per the HTTP/2.0 specification
        /// and header names internal representation within this library.
        ///
        ///
        /// # Examples
        ///
        /// ```
        /// # use http::header::*;
        /// // Parsing a standard header
        /// let hdr = HeaderName::from_static("content-length");
        /// assert_eq!(CONTENT_LENGTH, hdr);
        ///
        /// // Parsing a custom header
        /// let CUSTOM_HEADER: &'static str = "custom-header";
        ///
        /// let a = HeaderName::from_lowercase(b"custom-header").unwrap();
        /// let b = HeaderName::from_static(CUSTOM_HEADER);
        /// assert_eq!(a, b);
        /// ```
        ///
        /// ```should_panic
        /// # use http::header::*;
        /// #
        /// // Parsing a header that contains invalid symbols(s):
        /// HeaderName::from_static("content{}{}length"); // This line panics!
        ///
        /// // Parsing a header that contains invalid uppercase characters.
        /// let a = HeaderName::from_static("foobar");
        /// let b = HeaderName::from_static("FOOBAR"); // This line panics!
        /// ```
        #[allow(deprecated)]
        pub fn from_static(src: &'static str) -> HeaderName {
            let bytes = src.as_bytes();
            #[allow(deprecated)]
            let mut buf = unsafe { mem::uninitialized() };
            match parse_hdr(bytes, &mut buf, &HEADER_CHARS_H2) {
                Ok(hdr_name) => match hdr_name.inner {
                    Repr::Standard(std) => std.into(),
                    Repr::Custom(MaybeLower { buf: _, lower: true }) => {
                        let val = ByteStr::from_static(src);
                        Custom(val).into()
                    },
                    Repr::Custom(MaybeLower { buf: _, lower: false }) => {
                        // With lower false, the string is left unchecked by
                        // parse_hdr and must be validated manually.
                        for &b in bytes.iter() {
                            if HEADER_CHARS_H2[b as usize] == 0 {
                                panic!("invalid header name")
                            }
                        }

                        let val = ByteStr::from_static(src);
                        Custom(val).into()
                    }
                },

                Err(_) => panic!("invalid header name")
            }
        }

        /// Returns a `str` representation of the header.
        ///
        /// The returned string will always be lower case.
        #[inline]
        pub fn as_str(&self) -> &str {
            match self.inner {
                Repr::Standard(v) => v.as_str(),
                Repr::Custom(ref v) => &*v.0,
            }
        }

        pub(super) fn into_bytes(self) -> Bytes {
            self.inner.into()
        }
    }

    impl FromStr for HeaderName {
        type Err = InvalidHeaderName;

        fn from_str(s: &str) -> Result<HeaderName, InvalidHeaderName> {
            HeaderName::from_bytes(s.as_bytes()).map_err(|_| InvalidHeaderName { _priv: () })
        }
    }

    impl AsRef<str> for HeaderName {
        fn as_ref(&self) -> &str {
            self.as_str()
        }
    }

    impl AsRef<[u8]> for HeaderName {
        fn as_ref(&self) -> &[u8] {
            self.as_str().as_bytes()
        }
    }

    impl Borrow<str> for HeaderName {
        fn borrow(&self) -> &str {
            self.as_str()
        }
    }

    impl fmt::Debug for HeaderName {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(self.as_str(), fmt)
        }
    }

    impl fmt::Display for HeaderName {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(self.as_str(), fmt)
        }
    }

    impl InvalidHeaderName {
        fn new() -> InvalidHeaderName {
            InvalidHeaderName { _priv: () }
        }
    }

    impl<'a> From<&'a HeaderName> for HeaderName {
        fn from(src: &'a HeaderName) -> HeaderName {
            src.clone()
        }
    }

    #[doc(hidden)]
    impl<T> From<Repr<T>> for Bytes
    where
        T: Into<Bytes>,
    {
        fn from(repr: Repr<T>) -> Bytes {
            match repr {
                Repr::Standard(header) => Bytes::from_static(header.as_str().as_bytes()),
                Repr::Custom(header) => header.into(),
            }
        }
    }

    impl From<Custom> for Bytes {
        #[inline]
        fn from(Custom(inner): Custom) -> Bytes {
            Bytes::from(inner)
        }
    }

    impl<'a> TryFrom<&'a str> for HeaderName {
        type Error = InvalidHeaderName;
        #[inline]
        fn try_from(s: &'a str) -> Result<Self, Self::Error> {
            Self::from_bytes(s.as_bytes())
        }
    }

    impl<'a> TryFrom<&'a String> for HeaderName {
        type Error = InvalidHeaderName;
        #[inline]
        fn try_from(s: &'a String) -> Result<Self, Self::Error> {
            Self::from_bytes(s.as_bytes())
        }
    }

    impl<'a> TryFrom<&'a [u8]> for HeaderName {
        type Error = InvalidHeaderName;
        #[inline]
        fn try_from(s: &'a [u8]) -> Result<Self, Self::Error> {
            Self::from_bytes(s)
        }
    }

    #[doc(hidden)]
    impl From<StandardHeader> for HeaderName {
        fn from(src: StandardHeader) -> HeaderName {
            HeaderName {
                inner: Repr::Standard(src),
            }
        }
    }

    #[doc(hidden)]
    impl From<Custom> for HeaderName {
        fn from(src: Custom) -> HeaderName {
            HeaderName {
                inner: Repr::Custom(src),
            }
        }
    }

    impl<'a> PartialEq<&'a HeaderName> for HeaderName {
        #[inline]
        fn eq(&self, other: &&'a HeaderName) -> bool {
            *self == **other
        }
    }

    impl<'a> PartialEq<HeaderName> for &'a HeaderName {
        #[inline]
        fn eq(&self, other: &HeaderName) -> bool {
            *other == *self
        }
    }

    impl PartialEq<str> for HeaderName {
        /// Performs a case-insensitive comparison of the string against the header
        /// name
        ///
        /// # Examples
        ///
        /// ```
        /// use http::header::CONTENT_LENGTH;
        ///
        /// assert_eq!(CONTENT_LENGTH, "content-length");
        /// assert_eq!(CONTENT_LENGTH, "Content-Length");
        /// assert_ne!(CONTENT_LENGTH, "content length");
        /// ```
        #[inline]
        fn eq(&self, other: &str) -> bool {
            eq_ignore_ascii_case(self.as_ref(), other.as_bytes())
        }
    }

    impl PartialEq<HeaderName> for str {
        /// Performs a case-insensitive comparison of the string against the header
        /// name
        ///
        /// # Examples
        ///
        /// ```
        /// use http::header::CONTENT_LENGTH;
        ///
        /// assert_eq!(CONTENT_LENGTH, "content-length");
        /// assert_eq!(CONTENT_LENGTH, "Content-Length");
        /// assert_ne!(CONTENT_LENGTH, "content length");
        /// ```
        #[inline]
        fn eq(&self, other: &HeaderName) -> bool {
            *other == *self
        }
    }

    impl<'a> PartialEq<&'a str> for HeaderName {
        /// Performs a case-insensitive comparison of the string against the header
        /// name
        #[inline]
        fn eq(&self, other: &&'a str) -> bool {
            *self == **other
        }
    }

    impl<'a> PartialEq<HeaderName> for &'a str {
        /// Performs a case-insensitive comparison of the string against the header
        /// name
        #[inline]
        fn eq(&self, other: &HeaderName) -> bool {
            *other == *self
        }
    }

    impl fmt::Debug for InvalidHeaderName {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("InvalidHeaderName")
                // skip _priv noise
                .finish()
        }
    }

    impl fmt::Display for InvalidHeaderName {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("invalid HTTP header name")
        }
    }

    impl Error for InvalidHeaderName {}

    // ===== HdrName =====

    impl<'a> HdrName<'a> {
        fn custom(buf: &'a [u8], lower: bool) -> HdrName<'a> {
            HdrName {
                inner: Repr::Custom(MaybeLower {
                    buf: buf,
                    lower: lower,
                }),
            }
        }

        #[allow(deprecated)]
        pub fn from_bytes<F, U>(hdr: &[u8], f: F) -> Result<U, InvalidHeaderName>
            where F: FnOnce(HdrName<'_>) -> U,
        {
            #[allow(deprecated)]
            let mut buf = unsafe { mem::uninitialized() };
            let hdr = parse_hdr(hdr, &mut buf, &HEADER_CHARS)?;
            Ok(f(hdr))
        }

        #[allow(deprecated)]
        pub fn from_static<F, U>(hdr: &'static str, f: F) -> U
        where
            F: FnOnce(HdrName<'_>) -> U,
        {
            #[allow(deprecated)]
            let mut buf = unsafe { mem::uninitialized() };
            let hdr =
                parse_hdr(hdr.as_bytes(), &mut buf, &HEADER_CHARS).expect("static str is invalid name");
            f(hdr)
        }
    }

    #[doc(hidden)]
    impl<'a> From<HdrName<'a>> for HeaderName {
        fn from(src: HdrName<'a>) -> HeaderName {
            match src.inner {
                Repr::Standard(s) => HeaderName {
                    inner: Repr::Standard(s),
                },
                Repr::Custom(maybe_lower) => {
                    if maybe_lower.lower {
                        let buf = Bytes::copy_from_slice(&maybe_lower.buf[..]);
                        let byte_str = unsafe { ByteStr::from_utf8_unchecked(buf) };

                        HeaderName {
                            inner: Repr::Custom(Custom(byte_str)),
                        }
                    } else {
                        use bytes::BufMut;
                        let mut dst = BytesMut::with_capacity(maybe_lower.buf.len());

                        for b in maybe_lower.buf.iter() {
                            dst.put_u8(HEADER_CHARS[*b as usize]);
                        }

                        let buf = unsafe { ByteStr::from_utf8_unchecked(dst.freeze()) };

                        HeaderName {
                            inner: Repr::Custom(Custom(buf)),
                        }
                    }
                }
            }
        }
    }

    #[doc(hidden)]
    impl<'a> PartialEq<HdrName<'a>> for HeaderName {
        #[inline]
        fn eq(&self, other: &HdrName<'a>) -> bool {
            match self.inner {
                Repr::Standard(a) => match other.inner {
                    Repr::Standard(b) => a == b,
                    _ => false,
                },
                Repr::Custom(Custom(ref a)) => match other.inner {
                    Repr::Custom(ref b) => {
                        if b.lower {
                            a.as_bytes() == b.buf
                        } else {
                            eq_ignore_ascii_case(a.as_bytes(), b.buf)
                        }
                    }
                    _ => false,
                },
            }
        }
    }

    // ===== Custom =====

    impl Hash for Custom {
        #[inline]
        fn hash<H: Hasher>(&self, hasher: &mut H) {
            hasher.write(self.0.as_bytes())
        }
    }

    // ===== MaybeLower =====

    impl<'a> Hash for MaybeLower<'a> {
        #[inline]
        fn hash<H: Hasher>(&self, hasher: &mut H) {
            if self.lower {
                hasher.write(self.buf);
            } else {
                for &b in self.buf {
                    hasher.write(&[HEADER_CHARS[b as usize]]);
                }
            }
        }
    }

    // Assumes that the left hand side is already lower case
    #[inline]
    fn eq_ignore_ascii_case(lower: &[u8], s: &[u8]) -> bool {
        if lower.len() != s.len() {
            return false;
        }

        lower.iter().zip(s).all(|(a, b)| {
            *a == HEADER_CHARS[*b as usize]
        })
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use self::StandardHeader::Vary;

        #[test]
        fn test_bounds() {
            fn check_bounds<T: Sync + Send>() {}
            check_bounds::<HeaderName>();
        }

        #[test]
        fn test_parse_invalid_headers() {
            for i in 0..128 {
                let hdr = vec![1u8; i];
                assert!(HeaderName::from_bytes(&hdr).is_err(), "{} invalid header chars did not fail", i);
            }
        }

        #[test]
        fn test_invalid_name_lengths() {
            assert!(
                HeaderName::from_bytes(&[]).is_err(),
                "zero-length header name is an error",
            );
            let mut long = vec![b'a'; super::super::MAX_HEADER_NAME_LEN];
            assert!(
                HeaderName::from_bytes(long.as_slice()).is_ok(),
                "max header name length is ok",
            );
            long.push(b'a');
            assert!(
                HeaderName::from_bytes(long.as_slice()).is_err(),
                "longer than max header name length is an error",
            );
        }

        #[test]
        fn test_from_hdr_name() {
            use self::StandardHeader::Vary;

            let name = HeaderName::from(HdrName {
                inner: Repr::Standard(Vary),
            });

            assert_eq!(name.inner, Repr::Standard(Vary));

            let name = HeaderName::from(HdrName {
                inner: Repr::Custom(MaybeLower {
                    buf: b"hello-world",
                    lower: true,
                }),
            });

            assert_eq!(name.inner, Repr::Custom(Custom(ByteStr::from_static("hello-world"))));

            let name = HeaderName::from(HdrName {
                inner: Repr::Custom(MaybeLower {
                    buf: b"Hello-World",
                    lower: false,
                }),
            });

            assert_eq!(name.inner, Repr::Custom(Custom(ByteStr::from_static("hello-world"))));
        }

        #[test]
        fn test_eq_hdr_name() {
            use self::StandardHeader::Vary;

            let a = HeaderName { inner: Repr::Standard(Vary) };
            let b = HdrName { inner: Repr::Standard(Vary) };

            assert_eq!(a, b);

            let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("vaary"))) };
            assert_ne!(a, b);

            let b = HdrName { inner: Repr::Custom(MaybeLower {
                buf: b"vaary",
                lower: true,
            })};

            assert_eq!(a, b);

            let b = HdrName { inner: Repr::Custom(MaybeLower {
                buf: b"vaary",
                lower: false,
            })};

            assert_eq!(a, b);

            let b = HdrName { inner: Repr::Custom(MaybeLower {
                buf: b"VAARY",
                lower: false,
            })};

            assert_eq!(a, b);

            let a = HeaderName { inner: Repr::Standard(Vary) };
            assert_ne!(a, b);
        }

        #[test]
        fn test_from_static_std() {
            let a = HeaderName { inner: Repr::Standard(Vary) };

            let b = HeaderName::from_static("vary");
            assert_eq!(a, b);

            let b = HeaderName::from_static("vaary");
            assert_ne!(a, b);
        }

        #[test]
        #[should_panic]
        fn test_from_static_std_uppercase() {
            HeaderName::from_static("Vary");
        }

        #[test]
        #[should_panic]
        fn test_from_static_std_symbol() {
            HeaderName::from_static("vary{}");
        }

        // MaybeLower { lower: true }
        #[test]
        fn test_from_static_custom_short() {
            let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("customheader"))) };
            let b = HeaderName::from_static("customheader");
            assert_eq!(a, b);
        }

        #[test]
        #[should_panic]
        fn test_from_static_custom_short_uppercase() {
            HeaderName::from_static("custom header");
        }

        #[test]
        #[should_panic]
        fn test_from_static_custom_short_symbol() {
            HeaderName::from_static("CustomHeader");
        }

        // MaybeLower { lower: false }
        #[test]
        fn test_from_static_custom_long() {
            let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static(
                "longer-than-63--thisheaderislongerthansixtythreecharactersandthushandleddifferent"
            ))) };
            let b = HeaderName::from_static(
                "longer-than-63--thisheaderislongerthansixtythreecharactersandthushandleddifferent"
            );
            assert_eq!(a, b);
        }

        #[test]
        #[should_panic]
        fn test_from_static_custom_long_uppercase() {
            HeaderName::from_static(
                "Longer-Than-63--ThisHeaderIsLongerThanSixtyThreeCharactersAndThusHandledDifferent"
            );
        }

        #[test]
        #[should_panic]
        fn test_from_static_custom_long_symbol() {
            HeaderName::from_static(
                "longer-than-63--thisheader{}{}{}{}islongerthansixtythreecharactersandthushandleddifferent"
            );
        }

        #[test]
        fn test_from_static_custom_single_char() {
            let a = HeaderName { inner: Repr::Custom(Custom(ByteStr::from_static("a"))) };
            let b = HeaderName::from_static("a");
            assert_eq!(a, b);
        }

        #[test]
        #[should_panic]
        fn test_from_static_empty() {
            HeaderName::from_static("");
        }

        #[test]
        fn test_all_tokens() {
            HeaderName::from_static("!#$%&'*+-.^_`|~0123456789abcdefghijklmnopqrstuvwxyz");
        }
    }