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

    use crate::fmt;

    use super::from_u32_unchecked;

    /// 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,
    }

    /// Creates 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;
    ///
    /// // 𝄞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};
    ///
    /// // 𝄞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 {
                // SAFETY: 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;
                // SAFETY: we checked that it's a legal unicode value
                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)
        }
    }