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    //! # Configs
    //!
    //! There isn't just one type of Base64; that would be too simple. You need to choose a character
    //! set (standard, URL-safe, etc) and padding suffix (yes/no).
    //! The `Config` struct encapsulates this info. There are some common configs included: `STANDARD`,
    //! `URL_SAFE`, etc. You can also make your own `Config` if needed.
    //!
    //! The functions that don't have `config` in the name (e.g. `encode()` and `decode()`) use the
    //! `STANDARD` config .
    //!
    //! The functions that write to a slice (the ones that end in `_slice`) are generally the fastest
    //! because they don't need to resize anything. If it fits in your workflow and you care about
    //! performance, keep using the same buffer (growing as need be) and use the `_slice` methods for
    //! the best performance.
    //!
    //! # Encoding
    //!
    //! Several different encoding functions are available to you depending on your desire for
    //! convenience vs performance.
    //!
    //! | Function                | Output                       | Allocates                      |
    //! | ----------------------- | ---------------------------- | ------------------------------ |
    //! | `encode`                | Returns a new `String`       | Always                         |
    //! | `encode_config`         | Returns a new `String`       | Always                         |
    //! | `encode_config_buf`     | Appends to provided `String` | Only if `String` needs to grow |
    //! | `encode_config_slice`   | Writes to provided `&[u8]`   | Never                          |
    //!
    //! All of the encoding functions that take a `Config` will pad as per the config.
    //!
    //! # Decoding
    //!
    //! Just as for encoding, there are different decoding functions available.
    //!
    //! | Function                | Output                        | Allocates                      |
    //! | ----------------------- | ----------------------------- | ------------------------------ |
    //! | `decode`                | Returns a new `Vec<u8>`       | Always                         |
    //! | `decode_config`         | Returns a new `Vec<u8>`       | Always                         |
    //! | `decode_config_buf`     | Appends to provided `Vec<u8>` | Only if `Vec` needs to grow    |
    //! | `decode_config_slice`   | Writes to provided `&[u8]`    | Never                          |
    //!
    //! Unlike encoding, where all possible input is valid, decoding can fail (see `DecodeError`).
    //!
    //! Input can be invalid because it has invalid characters or invalid padding. (No padding at all is
    //! valid, but excess padding is not.) Whitespace in the input is invalid.
    //!
    //! # `Read` and `Write`
    //!
    //! To map a `Read` of b64 bytes to the decoded bytes, wrap a reader (file, network socket, etc)
    //! with `base64::read::DecoderReader`. To write raw bytes and have them b64 encoded on the fly,
    //! wrap a writer with `base64::write::EncoderWriter`. There is some performance overhead (15% or
    //! so) because of the necessary buffer shuffling -- still fast enough that almost nobody cares.
    //! Also, these implementations do not heap allocate.
    //!
    //! # Panics
    //!
    //! If length calculations result in overflowing `usize`, a panic will result.
    //!
    //! The `_slice` flavors of encode or decode will panic if the provided output slice is too small,

    #![cfg_attr(feature = "cargo-clippy", allow(clippy::cast_lossless))]
    #![deny(
        missing_docs,
        trivial_casts,
        trivial_numeric_casts,
        unused_extern_crates,
        unused_import_braces,
        unused_results,
        variant_size_differences,
        warnings
    )]
    #![forbid(unsafe_code)]
    #![cfg_attr(not(any(feature = "std", test)), no_std)]

    #[cfg(all(feature = "alloc", not(any(feature = "std", test))))]
    extern crate alloc;
    #[cfg(any(feature = "std", test))]
    extern crate std as alloc;

    mod chunked_encoder;
    pub mod display;
    #[cfg(any(feature = "std", test))]
    pub mod read;
    mod tables;
    #[cfg(any(feature = "std", test))]
    pub mod write;

    mod encode;
    pub use crate::encode::encode_config_slice;
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub use crate::encode::{encode, encode_config, encode_config_buf};

    mod decode;
    #[cfg(any(feature = "alloc", feature = "std", test))]
    pub use crate::decode::{decode, decode_config, decode_config_buf};
    pub use crate::decode::{decode_config_slice, DecodeError};

    #[cfg(test)]
    mod tests;

    /// Available encoding character sets
    #[derive(Clone, Copy, Debug)]
    pub enum CharacterSet {
        /// The standard character set (uses `+` and `/`).
        ///
        /// See [RFC 3548](https://tools.ietf.org/html/rfc3548#section-3).
        Standard,
        /// The URL safe character set (uses `-` and `_`).
        ///
        /// See [RFC 3548](https://tools.ietf.org/html/rfc3548#section-4).
        UrlSafe,
        /// The `crypt(3)` character set (uses `./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz`).
        ///
        /// Not standardized, but folk wisdom on the net asserts that this alphabet is what crypt uses.
        Crypt,
        /// The bcrypt character set (uses `./ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789`).
        Bcrypt,
        /// The character set used in IMAP-modified UTF-7 (uses `+` and `,`).
        ///
        /// See [RFC 3501](https://tools.ietf.org/html/rfc3501#section-5.1.3)
        ImapMutf7,
        /// The character set used in BinHex 4.0 files.
        ///
        /// See [BinHex 4.0 Definition](http://files.stairways.com/other/binhex-40-specs-info.txt)
        BinHex,
    }

    impl CharacterSet {
        fn encode_table(self) -> &'static [u8; 64] {
            match self {
                CharacterSet::Standard => tables::STANDARD_ENCODE,
                CharacterSet::UrlSafe => tables::URL_SAFE_ENCODE,
                CharacterSet::Crypt => tables::CRYPT_ENCODE,
                CharacterSet::Bcrypt => tables::BCRYPT_ENCODE,
                CharacterSet::ImapMutf7 => tables::IMAP_MUTF7_ENCODE,
                CharacterSet::BinHex => tables::BINHEX_ENCODE,
            }
        }

        fn decode_table(self) -> &'static [u8; 256] {
            match self {
                CharacterSet::Standard => tables::STANDARD_DECODE,
                CharacterSet::UrlSafe => tables::URL_SAFE_DECODE,
                CharacterSet::Crypt => tables::CRYPT_DECODE,
                CharacterSet::Bcrypt => tables::BCRYPT_DECODE,
                CharacterSet::ImapMutf7 => tables::IMAP_MUTF7_DECODE,
                CharacterSet::BinHex => tables::BINHEX_DECODE,
            }
        }
    }

    /// Contains configuration parameters for base64 encoding
    #[derive(Clone, Copy, Debug)]
    pub struct Config {
        /// Character set to use
        char_set: CharacterSet,
        /// True to pad output with `=` characters
        pad: bool,
        /// True to ignore excess nonzero bits in the last few symbols, otherwise an error is returned.
        decode_allow_trailing_bits: bool,
    }

    impl Config {
        /// Create a new `Config`.
        pub const fn new(char_set: CharacterSet, pad: bool) -> Config {
            Config {
                char_set,
                pad,
                decode_allow_trailing_bits: false,
            }
        }

        /// Sets whether to pad output with `=` characters.
        pub const fn pad(self, pad: bool) -> Config {
            Config { pad, ..self }
        }

        /// Sets whether to emit errors for nonzero trailing bits.
        ///
        /// This is useful when implementing
        /// [forgiving-base64 decode](https://infra.spec.whatwg.org/#forgiving-base64-decode).
        pub const fn decode_allow_trailing_bits(self, allow: bool) -> Config {
            Config {
                decode_allow_trailing_bits: allow,
                ..self
            }
        }
    }

    /// Standard character set with padding.
    pub const STANDARD: Config = Config {
        char_set: CharacterSet::Standard,
        pad: true,
        decode_allow_trailing_bits: false,
    };

    /// Standard character set without padding.
    pub const STANDARD_NO_PAD: Config = Config {
        char_set: CharacterSet::Standard,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    /// URL-safe character set with padding
    pub const URL_SAFE: Config = Config {
        char_set: CharacterSet::UrlSafe,
        pad: true,
        decode_allow_trailing_bits: false,
    };

    /// URL-safe character set without padding
    pub const URL_SAFE_NO_PAD: Config = Config {
        char_set: CharacterSet::UrlSafe,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    /// As per `crypt(3)` requirements
    pub const CRYPT: Config = Config {
        char_set: CharacterSet::Crypt,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    /// Bcrypt character set
    pub const BCRYPT: Config = Config {
        char_set: CharacterSet::Bcrypt,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    /// IMAP modified UTF-7 requirements
    pub const IMAP_MUTF7: Config = Config {
        char_set: CharacterSet::ImapMutf7,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    /// BinHex character set
    pub const BINHEX: Config = Config {
        char_set: CharacterSet::BinHex,
        pad: false,
        decode_allow_trailing_bits: false,
    };

    const PAD_BYTE: u8 = b'=';