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    // Copyright 2013 The rust-url developers.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Punycode ([RFC 3492](http://tools.ietf.org/html/rfc3492)) implementation.
    //!
    //! Since Punycode fundamentally works on unicode code points,
    //! `encode` and `decode` take and return slices and vectors of `char`.
    //! `encode_str` and `decode_to_string` provide convenience wrappers
    //! that convert from and to Rust’s UTF-8 based `str` and `String` types.

    use std::char;
    use std::u32;

    // Bootstring parameters for Punycode
    static BASE: u32 = 36;
    static T_MIN: u32 = 1;
    static T_MAX: u32 = 26;
    static SKEW: u32 = 38;
    static DAMP: u32 = 700;
    static INITIAL_BIAS: u32 = 72;
    static INITIAL_N: u32 = 0x80;
    static DELIMITER: char = '-';

    #[inline]
    fn adapt(mut delta: u32, num_points: u32, first_time: bool) -> u32 {
        delta /= if first_time { DAMP } else { 2 };
        delta += delta / num_points;
        let mut k = 0;
        while delta > ((BASE - T_MIN) * T_MAX) / 2 {
            delta /= BASE - T_MIN;
            k += BASE;
        }
        k + (((BASE - T_MIN + 1) * delta) / (delta + SKEW))
    }

    /// Convert Punycode to an Unicode `String`.
    ///
    /// This is a convenience wrapper around `decode`.
    #[inline]
    pub fn decode_to_string(input: &str) -> Option<String> {
        decode(input).map(|chars| chars.into_iter().collect())
    }

    /// Convert Punycode to Unicode.
    ///
    /// Return None on malformed input or overflow.
    /// Overflow can only happen on inputs that take more than
    /// 63 encoded bytes, the DNS limit on domain name labels.
    pub fn decode(input: &str) -> Option<Vec<char>> {
        Some(Decoder::default().decode(input).ok()?.collect())
    }

    #[derive(Default)]
    pub(crate) struct Decoder {
        insertions: Vec<(usize, char)>,
    }

    impl Decoder {
        /// Split the input iterator and return a Vec with insertions of encoded characters
        pub(crate) fn decode<'a>(&'a mut self, input: &'a str) -> Result<Decode<'a>, ()> {
            self.insertions.clear();
            // Handle "basic" (ASCII) code points.
            // They are encoded as-is before the last delimiter, if any.
            let (base, input) = match input.rfind(DELIMITER) {
                None => ("", input),
                Some(position) => (
                    &input[..position],
                    if position > 0 {
                        &input[position + 1..]
                    } else {
                        input
                    },
                ),
            };

            let base_len = base.len();
            let mut length = base_len as u32;
            let mut code_point = INITIAL_N;
            let mut bias = INITIAL_BIAS;
            let mut i = 0;
            let mut iter = input.bytes();
            loop {
                let previous_i = i;
                let mut weight = 1;
                let mut k = BASE;
                let mut byte = match iter.next() {
                    None => break,
                    Some(byte) => byte,
                };

                // Decode a generalized variable-length integer into delta,
                // which gets added to i.
                loop {
                    let digit = match byte {
                        byte @ b'0'..=b'9' => byte - b'0' + 26,
                        byte @ b'A'..=b'Z' => byte - b'A',
                        byte @ b'a'..=b'z' => byte - b'a',
                        _ => return Err(()),
                    } as u32;
                    if digit > (u32::MAX - i) / weight {
                        return Err(()); // Overflow
                    }
                    i += digit * weight;
                    let t = if k <= bias {
                        T_MIN
                    } else if k >= bias + T_MAX {
                        T_MAX
                    } else {
                        k - bias
                    };
                    if digit < t {
                        break;
                    }
                    if weight > u32::MAX / (BASE - t) {
                        return Err(()); // Overflow
                    }
                    weight *= BASE - t;
                    k += BASE;
                    byte = match iter.next() {
                        None => return Err(()), // End of input before the end of this delta
                        Some(byte) => byte,
                    };
                }

                bias = adapt(i - previous_i, length + 1, previous_i == 0);
                if i / (length + 1) > u32::MAX - code_point {
                    return Err(()); // Overflow
                }

                // i was supposed to wrap around from length+1 to 0,
                // incrementing code_point each time.
                code_point += i / (length + 1);
                i %= length + 1;
                let c = match char::from_u32(code_point) {
                    Some(c) => c,
                    None => return Err(()),
                };

                // Move earlier insertions farther out in the string
                for (idx, _) in &mut self.insertions {
                    if *idx >= i as usize {
                        *idx += 1;
                    }
                }
                self.insertions.push((i as usize, c));
                length += 1;
                i += 1;
            }

            self.insertions.sort_by_key(|(i, _)| *i);
            Ok(Decode {
                base: base.chars(),
                insertions: &self.insertions,
                inserted: 0,
                position: 0,
                len: base_len + self.insertions.len(),
            })
        }
    }

    pub(crate) struct Decode<'a> {
        base: std::str::Chars<'a>,
        pub(crate) insertions: &'a [(usize, char)],
        inserted: usize,
        position: usize,
        len: usize,
    }

    impl<'a> Iterator for Decode<'a> {
        type Item = char;

        fn next(&mut self) -> Option<Self::Item> {
            loop {
                match self.insertions.get(self.inserted) {
                    Some((pos, c)) if *pos == self.position => {
                        self.inserted += 1;
                        self.position += 1;
                        return Some(*c);
                    }
                    _ => {}
                }
                if let Some(c) = self.base.next() {
                    self.position += 1;
                    return Some(c);
                } else if self.inserted >= self.insertions.len() {
                    return None;
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len - self.position;
            (len, Some(len))
        }
    }

    impl<'a> ExactSizeIterator for Decode<'a> {
        fn len(&self) -> usize {
            self.len - self.position
        }
    }

    /// Convert an Unicode `str` to Punycode.
    ///
    /// This is a convenience wrapper around `encode`.
    #[inline]
    pub fn encode_str(input: &str) -> Option<String> {
        let mut buf = String::with_capacity(input.len());
        encode_into(input.chars(), &mut buf).ok().map(|()| buf)
    }

    /// Convert Unicode to Punycode.
    ///
    /// Return None on overflow, which can only happen on inputs that would take more than
    /// 63 encoded bytes, the DNS limit on domain name labels.
    pub fn encode(input: &[char]) -> Option<String> {
        let mut buf = String::with_capacity(input.len());
        encode_into(input.iter().copied(), &mut buf)
            .ok()
            .map(|()| buf)
    }

    pub(crate) fn encode_into<I>(input: I, output: &mut String) -> Result<(), ()>
    where
        I: Iterator<Item = char> + Clone,
    {
        // Handle "basic" (ASCII) code points. They are encoded as-is.
        let (mut input_length, mut basic_length) = (0, 0);
        for c in input.clone() {
            input_length += 1;
            if c.is_ascii() {
                output.push(c);
                basic_length += 1;
            }
        }

        if basic_length > 0 {
            output.push('-')
        }
        let mut code_point = INITIAL_N;
        let mut delta = 0;
        let mut bias = INITIAL_BIAS;
        let mut processed = basic_length;
        while processed < input_length {
            // All code points < code_point have been handled already.
            // Find the next larger one.
            let min_code_point = input
                .clone()
                .map(|c| c as u32)
                .filter(|&c| c >= code_point)
                .min()
                .unwrap();
            if min_code_point - code_point > (u32::MAX - delta) / (processed + 1) {
                return Err(()); // Overflow
            }
            // Increase delta to advance the decoder’s <code_point,i> state to <min_code_point,0>
            delta += (min_code_point - code_point) * (processed + 1);
            code_point = min_code_point;
            for c in input.clone() {
                let c = c as u32;
                if c < code_point {
                    delta += 1;
                    if delta == 0 {
                        return Err(()); // Overflow
                    }
                }
                if c == code_point {
                    // Represent delta as a generalized variable-length integer:
                    let mut q = delta;
                    let mut k = BASE;
                    loop {
                        let t = if k <= bias {
                            T_MIN
                        } else if k >= bias + T_MAX {
                            T_MAX
                        } else {
                            k - bias
                        };
                        if q < t {
                            break;
                        }
                        let value = t + ((q - t) % (BASE - t));
                        output.push(value_to_digit(value));
                        q = (q - t) / (BASE - t);
                        k += BASE;
                    }
                    output.push(value_to_digit(q));
                    bias = adapt(delta, processed + 1, processed == basic_length);
                    delta = 0;
                    processed += 1;
                }
            }
            delta += 1;
            code_point += 1;
        }
        Ok(())
    }

    #[inline]
    fn value_to_digit(value: u32) -> char {
        match value {
            0..=25 => (value as u8 + b'a') as char,       // a..z
            26..=35 => (value as u8 - 26 + b'0') as char, // 0..9
            _ => panic!(),
        }
    }