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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // 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.

    //! UTF-8 and UTF-16 decoding iterators

    use fmt;
    use iter::FusedIterator;
    use super::from_u32_unchecked;

    /// An iterator over an iterator of bytes of the characters the bytes represent
    /// as UTF-8
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
        https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
    #[derive(Clone, Debug)]
    #[allow(deprecated)]
    pub struct DecodeUtf8<I: Iterator<Item = u8>>(::iter::Peekable<I>);

    /// Decodes an `Iterator` of bytes as UTF-8.
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
        https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
    #[allow(deprecated)]
    #[inline]
    pub fn decode_utf8<I: IntoIterator<Item = u8>>(i: I) -> DecodeUtf8<I::IntoIter> {
        DecodeUtf8(i.into_iter().peekable())
    }

    /// `<DecodeUtf8 as Iterator>::next` returns this for an invalid input sequence.
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
        https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
    #[derive(PartialEq, Eq, Debug)]
    #[allow(deprecated)]
    pub struct InvalidSequence(());

    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[allow(deprecated)]
    impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
        type Item = Result<char, InvalidSequence>;
        #[inline]

        fn next(&mut self) -> Option<Result<char, InvalidSequence>> {
            self.0.next().map(|first_byte| {
                // Emit InvalidSequence according to
                // Unicode §5.22 Best Practice for U+FFFD Substitution
                // http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf#G40630

                // Roughly: consume at least one byte,
                // then validate one byte at a time and stop before the first unexpected byte
                // (which might be the valid start of the next byte sequence).

                let mut code_point;
                macro_rules! first_byte {
                    ($mask: expr) => {
                        code_point = u32::from(first_byte & $mask)
                    }
                }
                macro_rules! continuation_byte {
                    () => { continuation_byte!(0x80..=0xBF) };
                    ($range: pat) => {
                        match self.0.peek() {
                            Some(&byte @ $range) => {
                                code_point = (code_point << 6) | u32::from(byte & 0b0011_1111);
                                self.0.next();
                            }
                            _ => return Err(InvalidSequence(()))
                        }
                    }
                }

                match first_byte {
                    0x00..=0x7F => {
                        first_byte!(0b1111_1111);
                    }
                    0xC2..=0xDF => {
                        first_byte!(0b0001_1111);
                        continuation_byte!();
                    }
                    0xE0 => {
                        first_byte!(0b0000_1111);
                        continuation_byte!(0xA0..=0xBF);  // 0x80..=0x9F here are overlong
                        continuation_byte!();
                    }
                    0xE1..=0xEC | 0xEE..=0xEF => {
                        first_byte!(0b0000_1111);
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xED => {
                        first_byte!(0b0000_1111);
                        continuation_byte!(0x80..=0x9F);  // 0xA0..0xBF here are surrogates
                        continuation_byte!();
                    }
                    0xF0 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!(0x90..=0xBF);  // 0x80..0x8F here are overlong
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xF1..=0xF3 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!();
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xF4 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!(0x80..=0x8F);  // 0x90..0xBF here are beyond char::MAX
                        continuation_byte!();
                        continuation_byte!();
                    }
                    _ => return Err(InvalidSequence(()))  // Illegal first byte, overlong, or beyond MAX
                }
                unsafe {
                    Ok(from_u32_unchecked(code_point))
                }
            })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lower, upper) = self.0.size_hint();

            // A code point is at most 4 bytes long.
            let min_code_points = lower / 4;

            (min_code_points, upper)
        }
    }

    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[allow(deprecated)]
    impl<I: FusedIterator<Item = u8>> FusedIterator for DecodeUtf8<I> {}

    /// An iterator that decodes UTF-16 encoded code points from an iterator of `u16`s.
    #[stable(feature = "decode_utf16", since = "1.9.0")]
    #[derive(Clone, Debug)]
    pub struct DecodeUtf16<I>
        where I: Iterator<Item = u16>
    {
        iter: I,
        buf: Option<u16>,
    }

    /// An error that can be returned when decoding UTF-16 code points.
    #[stable(feature = "decode_utf16", since = "1.9.0")]
    #[derive(Debug, Clone, Eq, PartialEq)]
    pub struct DecodeUtf16Error {
        code: u16,
    }

    /// Create an iterator over the UTF-16 encoded code points in `iter`,
    /// returning unpaired surrogates as `Err`s.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::char::decode_utf16;
    ///
    /// fn main() {
    ///     // 𝄞mus<invalid>ic<invalid>
    ///     let v = [0xD834, 0xDD1E, 0x006d, 0x0075,
    ///              0x0073, 0xDD1E, 0x0069, 0x0063,
    ///              0xD834];
    ///
    ///     assert_eq!(decode_utf16(v.iter().cloned())
    ///                            .map(|r| r.map_err(|e| e.unpaired_surrogate()))
    ///                            .collect::<Vec<_>>(),
    ///                vec![Ok('𝄞'),
    ///                     Ok('m'), Ok('u'), Ok('s'),
    ///                     Err(0xDD1E),
    ///                     Ok('i'), Ok('c'),
    ///                     Err(0xD834)]);
    /// }
    /// ```
    ///
    /// A lossy decoder can be obtained by replacing `Err` results with the replacement character:
    ///
    /// ```
    /// use std::char::{decode_utf16, REPLACEMENT_CHARACTER};
    ///
    /// fn main() {
    ///     // 𝄞mus<invalid>ic<invalid>
    ///     let v = [0xD834, 0xDD1E, 0x006d, 0x0075,
    ///              0x0073, 0xDD1E, 0x0069, 0x0063,
    ///              0xD834];
    ///
    ///     assert_eq!(decode_utf16(v.iter().cloned())
    ///                    .map(|r| r.unwrap_or(REPLACEMENT_CHARACTER))
    ///                    .collect::<String>(),
    ///                "𝄞mus�ic�");
    /// }
    /// ```
    #[stable(feature = "decode_utf16", since = "1.9.0")]
    #[inline]
    pub fn decode_utf16<I: IntoIterator<Item = u16>>(iter: I) -> DecodeUtf16<I::IntoIter> {
        DecodeUtf16 {
            iter: iter.into_iter(),
            buf: None,
        }
    }

    #[stable(feature = "decode_utf16", since = "1.9.0")]
    impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
        type Item = Result<char, DecodeUtf16Error>;

        fn next(&mut self) -> Option<Result<char, DecodeUtf16Error>> {
            let u = match self.buf.take() {
                Some(buf) => buf,
                None => self.iter.next()?
            };

            if u < 0xD800 || 0xDFFF < u {
                // not a surrogate
                Some(Ok(unsafe { from_u32_unchecked(u as u32) }))
            } else if u >= 0xDC00 {
                // a trailing surrogate
                Some(Err(DecodeUtf16Error { code: u }))
            } else {
                let u2 = match self.iter.next() {
                    Some(u2) => u2,
                    // eof
                    None => return Some(Err(DecodeUtf16Error { code: u })),
                };
                if u2 < 0xDC00 || u2 > 0xDFFF {
                    // not a trailing surrogate so we're not a valid
                    // surrogate pair, so rewind to redecode u2 next time.
                    self.buf = Some(u2);
                    return Some(Err(DecodeUtf16Error { code: u }));
                }

                // all ok, so lets decode it.
                let c = (((u - 0xD800) as u32) << 10 | (u2 - 0xDC00) as u32) + 0x1_0000;
                Some(Ok(unsafe { from_u32_unchecked(c) }))
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (low, high) = self.iter.size_hint();
            // we could be entirely valid surrogates (2 elements per
            // char), or entirely non-surrogates (1 element per char)
            (low / 2, high)
        }
    }

    impl DecodeUtf16Error {
        /// Returns the unpaired surrogate which caused this error.
        #[stable(feature = "decode_utf16", since = "1.9.0")]
        pub fn unpaired_surrogate(&self) -> u16 {
            self.code
        }
    }

    #[stable(feature = "decode_utf16", since = "1.9.0")]
    impl fmt::Display for DecodeUtf16Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "unpaired surrogate found: {:x}", self.code)
        }
    }

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