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    //! A character type.
    //!
    //! The `char` type represents a single character. More specifically, since
    //! 'character' isn't a well-defined concept in Unicode, `char` is a '[Unicode
    //! scalar value]', which is similar to, but not the same as, a '[Unicode code
    //! point]'.
    //!
    //! [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
    //! [Unicode code point]: http://www.unicode.org/glossary/#code_point
    //!
    //! This module exists for technical reasons, the primary documentation for
    //! `char` is directly on [the `char` primitive type](../../std/primitive.char.html)
    //! itself.
    //!
    //! This module is the home of the iterator implementations for the iterators
    //! implemented on `char`, as well as some useful constants and conversion
    //! functions that convert various types to `char`.

    #![allow(non_snake_case)]
    #![stable(feature = "core_char", since = "1.2.0")]

    mod convert;
    mod decode;
    mod methods;

    // stable re-exports
    #[stable(feature = "char_from_unchecked", since = "1.5.0")]
    pub use self::convert::from_u32_unchecked;
    #[stable(feature = "try_from", since = "1.34.0")]
    pub use self::convert::CharTryFromError;
    #[stable(feature = "char_from_str", since = "1.20.0")]
    pub use self::convert::ParseCharError;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::convert::{from_digit, from_u32};
    #[stable(feature = "decode_utf16", since = "1.9.0")]
    pub use self::decode::{decode_utf16, DecodeUtf16, DecodeUtf16Error};
    #[stable(feature = "unicode_version", since = "1.45.0")]
    pub use crate::unicode::UNICODE_VERSION;

    // perma-unstable re-exports
    #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
    pub use self::methods::encode_utf16_raw;
    #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
    pub use self::methods::encode_utf8_raw;

    use crate::fmt::{self, Write};
    use crate::iter::FusedIterator;

    // UTF-8 ranges and tags for encoding characters
    const TAG_CONT: u8 = 0b1000_0000;
    const TAG_TWO_B: u8 = 0b1100_0000;
    const TAG_THREE_B: u8 = 0b1110_0000;
    const TAG_FOUR_B: u8 = 0b1111_0000;
    const MAX_ONE_B: u32 = 0x80;
    const MAX_TWO_B: u32 = 0x800;
    const MAX_THREE_B: u32 = 0x10000;

    /*
        Lu  Uppercase_Letter        an uppercase letter
        Ll  Lowercase_Letter        a lowercase letter
        Lt  Titlecase_Letter        a digraphic character, with first part uppercase
        Lm  Modifier_Letter         a modifier letter
        Lo  Other_Letter            other letters, including syllables and ideographs
        Mn  Nonspacing_Mark         a nonspacing combining mark (zero advance width)
        Mc  Spacing_Mark            a spacing combining mark (positive advance width)
        Me  Enclosing_Mark          an enclosing combining mark
        Nd  Decimal_Number          a decimal digit
        Nl  Letter_Number           a letterlike numeric character
        No  Other_Number            a numeric character of other type
        Pc  Connector_Punctuation   a connecting punctuation mark, like a tie
        Pd  Dash_Punctuation        a dash or hyphen punctuation mark
        Ps  Open_Punctuation        an opening punctuation mark (of a pair)
        Pe  Close_Punctuation       a closing punctuation mark (of a pair)
        Pi  Initial_Punctuation     an initial quotation mark
        Pf  Final_Punctuation       a final quotation mark
        Po  Other_Punctuation       a punctuation mark of other type
        Sm  Math_Symbol             a symbol of primarily mathematical use
        Sc  Currency_Symbol         a currency sign
        Sk  Modifier_Symbol         a non-letterlike modifier symbol
        So  Other_Symbol            a symbol of other type
        Zs  Space_Separator         a space character (of various non-zero widths)
        Zl  Line_Separator          U+2028 LINE SEPARATOR only
        Zp  Paragraph_Separator     U+2029 PARAGRAPH SEPARATOR only
        Cc  Control                 a C0 or C1 control code
        Cf  Format                  a format control character
        Cs  Surrogate               a surrogate code point
        Co  Private_Use             a private-use character
        Cn  Unassigned              a reserved unassigned code point or a noncharacter
    */

    /// The highest valid code point a `char` can have.
    ///
    /// A [`char`] is a [Unicode Scalar Value], which means that it is a [Code
    /// Point], but only ones within a certain range. `MAX` is the highest valid
    /// code point that's a valid [Unicode Scalar Value].
    ///
    /// [Unicode Scalar Value]: http://www.unicode.org/glossary/#unicode_scalar_value
    /// [Code Point]: http://www.unicode.org/glossary/#code_point
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const MAX: char = char::MAX;

    /// `U+FFFD REPLACEMENT CHARACTER` (�) is used in Unicode to represent a
    /// decoding error.
    ///
    /// It can occur, for example, when giving ill-formed UTF-8 bytes to
    /// [`String::from_utf8_lossy`](../../std/string/struct.String.html#method.from_utf8_lossy).
    #[stable(feature = "decode_utf16", since = "1.9.0")]
    pub const REPLACEMENT_CHARACTER: char = char::REPLACEMENT_CHARACTER;

    /// Returns an iterator that yields the hexadecimal Unicode escape of a
    /// character, as `char`s.
    ///
    /// This `struct` is created by the [`escape_unicode`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`escape_unicode`]: char::escape_unicode
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct EscapeUnicode {
        c: char,
        state: EscapeUnicodeState,

        // The index of the next hex digit to be printed (0 if none),
        // i.e., the number of remaining hex digits to be printed;
        // increasing from the least significant digit: 0x543210
        hex_digit_idx: usize,
    }

    // The enum values are ordered so that their representation is the
    // same as the remaining length (besides the hexadecimal digits). This
    // likely makes `len()` a single load from memory) and inline-worth.
    #[derive(Clone, Debug)]
    enum EscapeUnicodeState {
        Done,
        RightBrace,
        Value,
        LeftBrace,
        Type,
        Backslash,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for EscapeUnicode {
        type Item = char;

        fn next(&mut self) -> Option<char> {
            match self.state {
                EscapeUnicodeState::Backslash => {
                    self.state = EscapeUnicodeState::Type;
                    Some('\\')
                }
                EscapeUnicodeState::Type => {
                    self.state = EscapeUnicodeState::LeftBrace;
                    Some('u')
                }
                EscapeUnicodeState::LeftBrace => {
                    self.state = EscapeUnicodeState::Value;
                    Some('{')
                }
                EscapeUnicodeState::Value => {
                    let hex_digit = ((self.c as u32) >> (self.hex_digit_idx * 4)) & 0xf;
                    let c = from_digit(hex_digit, 16).unwrap();
                    if self.hex_digit_idx == 0 {
                        self.state = EscapeUnicodeState::RightBrace;
                    } else {
                        self.hex_digit_idx -= 1;
                    }
                    Some(c)
                }
                EscapeUnicodeState::RightBrace => {
                    self.state = EscapeUnicodeState::Done;
                    Some('}')
                }
                EscapeUnicodeState::Done => None,
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = self.len();
            (n, Some(n))
        }

        #[inline]
        fn count(self) -> usize {
            self.len()
        }

        fn last(self) -> Option<char> {
            match self.state {
                EscapeUnicodeState::Done => None,

                EscapeUnicodeState::RightBrace
                | EscapeUnicodeState::Value
                | EscapeUnicodeState::LeftBrace
                | EscapeUnicodeState::Type
                | EscapeUnicodeState::Backslash => Some('}'),
            }
        }
    }

    #[stable(feature = "exact_size_escape", since = "1.11.0")]
    impl ExactSizeIterator for EscapeUnicode {
        #[inline]
        fn len(&self) -> usize {
            // The match is a single memory access with no branching
            self.hex_digit_idx
                + match self.state {
                    EscapeUnicodeState::Done => 0,
                    EscapeUnicodeState::RightBrace => 1,
                    EscapeUnicodeState::Value => 2,
                    EscapeUnicodeState::LeftBrace => 3,
                    EscapeUnicodeState::Type => 4,
                    EscapeUnicodeState::Backslash => 5,
                }
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for EscapeUnicode {}

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for EscapeUnicode {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            for c in self.clone() {
                f.write_char(c)?;
            }
            Ok(())
        }
    }

    /// An iterator that yields the literal escape code of a `char`.
    ///
    /// This `struct` is created by the [`escape_default`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`escape_default`]: char::escape_default
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct EscapeDefault {
        state: EscapeDefaultState,
    }

    #[derive(Clone, Debug)]
    enum EscapeDefaultState {
        Done,
        Char(char),
        Backslash(char),
        Unicode(EscapeUnicode),
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for EscapeDefault {
        type Item = char;

        fn next(&mut self) -> Option<char> {
            match self.state {
                EscapeDefaultState::Backslash(c) => {
                    self.state = EscapeDefaultState::Char(c);
                    Some('\\')
                }
                EscapeDefaultState::Char(c) => {
                    self.state = EscapeDefaultState::Done;
                    Some(c)
                }
                EscapeDefaultState::Done => None,
                EscapeDefaultState::Unicode(ref mut iter) => iter.next(),
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = self.len();
            (n, Some(n))
        }

        #[inline]
        fn count(self) -> usize {
            self.len()
        }

        fn nth(&mut self, n: usize) -> Option<char> {
            match self.state {
                EscapeDefaultState::Backslash(c) if n == 0 => {
                    self.state = EscapeDefaultState::Char(c);
                    Some('\\')
                }
                EscapeDefaultState::Backslash(c) if n == 1 => {
                    self.state = EscapeDefaultState::Done;
                    Some(c)
                }
                EscapeDefaultState::Backslash(_) => {
                    self.state = EscapeDefaultState::Done;
                    None
                }
                EscapeDefaultState::Char(c) => {
                    self.state = EscapeDefaultState::Done;

                    if n == 0 { Some(c) } else { None }
                }
                EscapeDefaultState::Done => None,
                EscapeDefaultState::Unicode(ref mut i) => i.nth(n),
            }
        }

        fn last(self) -> Option<char> {
            match self.state {
                EscapeDefaultState::Unicode(iter) => iter.last(),
                EscapeDefaultState::Done => None,
                EscapeDefaultState::Backslash(c) | EscapeDefaultState::Char(c) => Some(c),
            }
        }
    }

    #[stable(feature = "exact_size_escape", since = "1.11.0")]
    impl ExactSizeIterator for EscapeDefault {
        fn len(&self) -> usize {
            match self.state {
                EscapeDefaultState::Done => 0,
                EscapeDefaultState::Char(_) => 1,
                EscapeDefaultState::Backslash(_) => 2,
                EscapeDefaultState::Unicode(ref iter) => iter.len(),
            }
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for EscapeDefault {}

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for EscapeDefault {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            for c in self.clone() {
                f.write_char(c)?;
            }
            Ok(())
        }
    }

    /// An iterator that yields the literal escape code of a `char`.
    ///
    /// This `struct` is created by the [`escape_debug`] method on [`char`]. See its
    /// documentation for more.
    ///
    /// [`escape_debug`]: char::escape_debug
    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    #[derive(Clone, Debug)]
    pub struct EscapeDebug(EscapeDefault);

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl Iterator for EscapeDebug {
        type Item = char;
        fn next(&mut self) -> Option<char> {
            self.0.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.0.size_hint()
        }
    }

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl ExactSizeIterator for EscapeDebug {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for EscapeDebug {}

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl fmt::Display for EscapeDebug {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    /// Returns an iterator that yields the lowercase equivalent of a `char`.
    ///
    /// This `struct` is created by the [`to_lowercase`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`to_lowercase`]: char::to_lowercase
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug, Clone)]
    pub struct ToLowercase(CaseMappingIter);

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for ToLowercase {
        type Item = char;
        fn next(&mut self) -> Option<char> {
            self.0.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.0.size_hint()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for ToLowercase {}

    #[stable(feature = "exact_size_case_mapping_iter", since = "1.35.0")]
    impl ExactSizeIterator for ToLowercase {}

    /// Returns an iterator that yields the uppercase equivalent of a `char`.
    ///
    /// This `struct` is created by the [`to_uppercase`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`to_uppercase`]: char::to_uppercase
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug, Clone)]
    pub struct ToUppercase(CaseMappingIter);

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for ToUppercase {
        type Item = char;
        fn next(&mut self) -> Option<char> {
            self.0.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.0.size_hint()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for ToUppercase {}

    #[stable(feature = "exact_size_case_mapping_iter", since = "1.35.0")]
    impl ExactSizeIterator for ToUppercase {}

    #[derive(Debug, Clone)]
    enum CaseMappingIter {
        Three(char, char, char),
        Two(char, char),
        One(char),
        Zero,
    }

    impl CaseMappingIter {
        fn new(chars: [char; 3]) -> CaseMappingIter {
            if chars[2] == '\0' {
                if chars[1] == '\0' {
                    CaseMappingIter::One(chars[0]) // Including if chars[0] == '\0'
                } else {
                    CaseMappingIter::Two(chars[0], chars[1])
                }
            } else {
                CaseMappingIter::Three(chars[0], chars[1], chars[2])
            }
        }
    }

    impl Iterator for CaseMappingIter {
        type Item = char;
        fn next(&mut self) -> Option<char> {
            match *self {
                CaseMappingIter::Three(a, b, c) => {
                    *self = CaseMappingIter::Two(b, c);
                    Some(a)
                }
                CaseMappingIter::Two(b, c) => {
                    *self = CaseMappingIter::One(c);
                    Some(b)
                }
                CaseMappingIter::One(c) => {
                    *self = CaseMappingIter::Zero;
                    Some(c)
                }
                CaseMappingIter::Zero => None,
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let size = match self {
                CaseMappingIter::Three(..) => 3,
                CaseMappingIter::Two(..) => 2,
                CaseMappingIter::One(_) => 1,
                CaseMappingIter::Zero => 0,
            };
            (size, Some(size))
        }
    }

    impl fmt::Display for CaseMappingIter {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                CaseMappingIter::Three(a, b, c) => {
                    f.write_char(a)?;
                    f.write_char(b)?;
                    f.write_char(c)
                }
                CaseMappingIter::Two(b, c) => {
                    f.write_char(b)?;
                    f.write_char(c)
                }
                CaseMappingIter::One(c) => f.write_char(c),
                CaseMappingIter::Zero => Ok(()),
            }
        }
    }

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for ToLowercase {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for ToUppercase {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }