//! `str` 方法的迭代器。

use crate::char;
use crate::fmt::{self, Write};
use crate::iter::TrustedRandomAccess;
use crate::iter::{Chain, FlatMap, Flatten};
use crate::iter::{Copied, Filter, FusedIterator, Map, TrustedLen};
use crate::ops::Try;
use crate::option;
use crate::slice::{self, Split as SliceSplit};

use super::from_utf8_unchecked;
use super::pattern::Pattern;
use super::pattern::{DoubleEndedSearcher, ReverseSearcher, Searcher};
use super::validations::{next_code_point, next_code_point_reverse, utf8_is_cont_byte};
use super::LinesAnyMap;
use super::{BytesIsNotEmpty, UnsafeBytesToStr};
use super::{CharEscapeDebugContinue, CharEscapeDefault, CharEscapeUnicode};
use super::{IsAsciiWhitespace, IsNotEmpty, IsWhitespace};

/// 字符串切片的 [`char`] 上的迭代器。
///
///
/// 该结构体是通过 [`str`] 上的 [`chars`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`char`]: prim@char
/// [`chars`]: str::chars
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Chars<'a> {
    pub(super) iter: slice::Iter<'a, u8>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for Chars<'a> {
    type Item = char;

    #[inline]
    fn next(&mut self) -> Option<char> {
        next_code_point(&mut self.iter).map(|ch| {
            // SAFETY: `str` 不变表示 `ch` 是有效的 Unicode 标量值。
            unsafe { char::from_u32_unchecked(ch) }
        })
    }

    #[inline]
    fn count(self) -> usize {
        // `char` 中的长度等于非连续字节数
        self.iter.filter(|&&byte| !utf8_is_cont_byte(byte)).count()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.iter.len();
        // `(len + 3)` 不会溢出，因为我们知道 `slice::Iter` 属于内存中的切片，该切片的最大长度为 `isize::MAX` (远低于 `usize::MAX`)。
        //
        //
        ((len + 3) / 4, Some(len))
    }

    #[inline]
    fn last(mut self) -> Option<char> {
        // 无需遍历整个字符串。
        self.next_back()
    }
}

#[stable(feature = "chars_debug_impl", since = "1.38.0")]
impl fmt::Debug for Chars<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Chars(")?;
        f.debug_list().entries(self.clone()).finish()?;
        write!(f, ")")?;
        Ok(())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for Chars<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<char> {
        next_code_point_reverse(&mut self.iter).map(|ch| {
            // SAFETY: `str` 不变表示 `ch` 是有效的 Unicode 标量值。
            unsafe { char::from_u32_unchecked(ch) }
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Chars<'_> {}

impl<'a> Chars<'a> {
    /// 将基础数据视为原始数据的子切片。
    ///
    /// 它具有与原始切片相同的生命周期，因此迭代器可以在存在时继续使用。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// let mut chars = "abc".chars();
    ///
    /// assert_eq!(chars.as_str(), "abc");
    /// chars.next();
    /// assert_eq!(chars.as_str(), "bc");
    /// chars.next();
    /// chars.next();
    /// assert_eq!(chars.as_str(), "");
    /// ```
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    #[inline]
    pub fn as_str(&self) -> &'a str {
        // SAFETY: `Chars` 仅由 str 制成，这可以确保迭代器是有效的 UTF-8。
        unsafe { from_utf8_unchecked(self.iter.as_slice()) }
    }
}

/// 字符串切片的 ['char`] 及其位置上的迭代器。
///
/// 该结构体是通过 [`str`] 上的 [`char_indices`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`char`]: prim@char
/// [`char_indices`]: str::char_indices
#[derive(Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct CharIndices<'a> {
    pub(super) front_offset: usize,
    pub(super) iter: Chars<'a>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for CharIndices<'a> {
    type Item = (usize, char);

    #[inline]
    fn next(&mut self) -> Option<(usize, char)> {
        let pre_len = self.iter.iter.len();
        match self.iter.next() {
            None => None,
            Some(ch) => {
                let index = self.front_offset;
                let len = self.iter.iter.len();
                self.front_offset += pre_len - len;
                Some((index, ch))
            }
        }
    }

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

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<(usize, char)> {
        // 无需遍历整个字符串。
        self.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for CharIndices<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<(usize, char)> {
        self.iter.next_back().map(|ch| {
            let index = self.front_offset + self.iter.iter.len();
            (index, ch)
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for CharIndices<'_> {}

impl<'a> CharIndices<'a> {
    /// 将基础数据视为原始数据的子切片。
    ///
    /// 它具有与原始切片相同的生命周期，因此迭代器可以在存在时继续使用。
    ///
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    #[inline]
    pub fn as_str(&self) -> &'a str {
        self.iter.as_str()
    }

    /// 返回下一个字符的字节位置，如果没有更多字符，则返回底层字符串的长度。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(char_indices_offset)]
    /// let mut chars = "a楽".char_indices();
    ///
    /// assert_eq!(chars.offset(), 0);
    /// assert_eq!(chars.next(), Some((0, 'a')));
    ///
    /// assert_eq!(chars.offset(), 1);
    /// assert_eq!(chars.next(), Some((1, '楽')));
    ///
    /// assert_eq!(chars.offset(), 4);
    /// assert_eq!(chars.next(), None);
    /// ```
    #[inline]
    #[unstable(feature = "char_indices_offset", issue = "83871")]
    pub fn offset(&self) -> usize {
        self.front_offset
    }
}

/// 在字符串切片的字节上进行迭代的迭代器。
///
/// 该结构体是通过 [`str`] 上的 [`bytes`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`bytes`]: str::bytes
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone, Debug)]
pub struct Bytes<'a>(pub(super) Copied<slice::Iter<'a, u8>>);

#[stable(feature = "rust1", since = "1.0.0")]
impl Iterator for Bytes<'_> {
    type Item = u8;

    #[inline]
    fn next(&mut self) -> Option<u8> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }

    #[inline]
    fn count(self) -> usize {
        self.0.count()
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        self.0.last()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        self.0.nth(n)
    }

    #[inline]
    fn all<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    {
        self.0.all(f)
    }

    #[inline]
    fn any<F>(&mut self, f: F) -> bool
    where
        F: FnMut(Self::Item) -> bool,
    {
        self.0.any(f)
    }

    #[inline]
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.0.find(predicate)
    }

    #[inline]
    fn position<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
    {
        self.0.position(predicate)
    }

    #[inline]
    fn rposition<P>(&mut self, predicate: P) -> Option<usize>
    where
        P: FnMut(Self::Item) -> bool,
    {
        self.0.rposition(predicate)
    }

    #[inline]
    #[doc(hidden)]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> u8 {
        // SAFETY: 调用者必须坚持 `Iterator::__iterator_get_unchecked` 的安全保证。
        //
        unsafe { self.0.__iterator_get_unchecked(idx) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl DoubleEndedIterator for Bytes<'_> {
    #[inline]
    fn next_back(&mut self) -> Option<u8> {
        self.0.next_back()
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        self.0.nth_back(n)
    }

    #[inline]
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
    {
        self.0.rfind(predicate)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl ExactSizeIterator for Bytes<'_> {
    #[inline]
    fn len(&self) -> usize {
        self.0.len()
    }

    #[inline]
    fn is_empty(&self) -> bool {
        self.0.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Bytes<'_> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl TrustedLen for Bytes<'_> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl TrustedRandomAccess for Bytes<'_> {
    const MAY_HAVE_SIDE_EFFECT: bool = false;
}

/// 此宏为形式为 X <'a，P> 的字符串模式 API 包装类型生成 Clone impl
///
macro_rules! derive_pattern_clone {
    (clone $t:ident with |$s:ident| $e:expr) => {
        impl<'a, P> Clone for $t<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                let $s = self;
                $e
            }
        }
    };
}

/// 此宏生成两个公共迭代器，结构体包装使用 `Pattern` API 的私有内部迭代器。
///
/// 对于所有模式 `P: Pattern<'a>`，将生成以下项 (忽略泛型) :
///
/// struct $forward_iterator($internal_iterator);
/// struct $reverse_iterator($internal_iterator);
///
/// $forward_iterator 的 impl 迭代器
/// { /* internal ends up calling Searcher::next_match() */ }
///
/// impl DoubleEndedIterator for $forward_iterator where P::Searcher: DoubleEndedSearcher
/// { /* internal ends up calling Searcher::next_match_back() */ }
///
/// impl Iterator for $reverse_iterator where P::Searcher: ReverseSearcher
/// { /* internal ends up calling Searcher::next_match_back() */ }
///
/// impl DoubleEndedIterator for $reverse_iterator where P::Searcher: DoubleEndedSearcher
/// { /* internal ends up calling Searcher::next_match() */ }
///
/// 内部宏在宏外部定义，并且通过委派 `pattern::Searcher` 和 `pattern::ReverseSearcher` 进行正向和反向迭代，其语义几乎与 DoubleEndedIterator 相同。
///
/// "Almost", 因为给定 `Pattern` 的 `Searcher` 和 `ReverseSearcher` 可能不会返回相同的元素，所以实际上为其实现 `DoubleEndedIterator` 是不正确的。
/// (有关更多详细信息，请参见 `str::pattern` 中的文档)
///
/// 但是，内部结构体仍然从任一端代表一个单端迭代器，并且取决于模式也是一个有效的双端迭代器，因此两个包装器结构体根据具体的模式类型实现 `Iterator` 和 `DoubleEndedIterator`，从而导致复杂的隐含如上所示。
///
///
///
///
///
///
///
///
///
///
///
///
///
macro_rules! generate_pattern_iterators {
    {
        // 转发迭代器
        forward:
            $(#[$forward_iterator_attribute:meta])*
            struct $forward_iterator:ident;

        // 反向迭代器
        reverse:
            $(#[$reverse_iterator_attribute:meta])*
            struct $reverse_iterator:ident;

        // 所有生成项的稳定性
        stability:
            $(#[$common_stability_attribute:meta])*

        // 内部的几乎迭代器被委派给
        internal:
            $internal_iterator:ident yielding ($iterty:ty);

        // 委托类型 - 单端或双端
        delegate $($t:tt)*
    } => {
        $(#[$forward_iterator_attribute])*
        $(#[$common_stability_attribute])*
        pub struct $forward_iterator<'a, P: Pattern<'a>>(pub(super) $internal_iterator<'a, P>);

        $(#[$common_stability_attribute])*
        impl<'a, P> fmt::Debug for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: fmt::Debug>,
        {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_tuple(stringify!($forward_iterator))
                    .field(&self.0)
                    .finish()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P: Pattern<'a>> Iterator for $forward_iterator<'a, P> {
            type Item = $iterty;

            #[inline]
            fn next(&mut self) -> Option<$iterty> {
                self.0.next()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Clone for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                $forward_iterator(self.0.clone())
            }
        }

        $(#[$reverse_iterator_attribute])*
        $(#[$common_stability_attribute])*
        pub struct $reverse_iterator<'a, P: Pattern<'a>>(pub(super) $internal_iterator<'a, P>);

        $(#[$common_stability_attribute])*
        impl<'a, P> fmt::Debug for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: fmt::Debug>,
        {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_tuple(stringify!($reverse_iterator))
                    .field(&self.0)
                    .finish()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Iterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
        {
            type Item = $iterty;

            #[inline]
            fn next(&mut self) -> Option<$iterty> {
                self.0.next_back()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> Clone for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: Clone>,
        {
            fn clone(&self) -> Self {
                $reverse_iterator(self.0.clone())
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, P: Pattern<'a>> FusedIterator for $forward_iterator<'a, P> {}

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, P> FusedIterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: ReverseSearcher<'a>>,
        {}

        generate_pattern_iterators!($($t)* with $(#[$common_stability_attribute])*,
                                                $forward_iterator,
                                                $reverse_iterator, $iterty);
    };
    {
        double ended; with $(#[$common_stability_attribute:meta])*,
                           $forward_iterator:ident,
                           $reverse_iterator:ident, $iterty:ty
    } => {
        $(#[$common_stability_attribute])*
        impl<'a, P> DoubleEndedIterator for $forward_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>,
        {
            #[inline]
            fn next_back(&mut self) -> Option<$iterty> {
                self.0.next_back()
            }
        }

        $(#[$common_stability_attribute])*
        impl<'a, P> DoubleEndedIterator for $reverse_iterator<'a, P>
        where
            P: Pattern<'a, Searcher: DoubleEndedSearcher<'a>>,
        {
            #[inline]
            fn next_back(&mut self) -> Option<$iterty> {
                self.0.next()
            }
        }
    };
    {
        single ended; with $(#[$common_stability_attribute:meta])*,
                           $forward_iterator:ident,
                           $reverse_iterator:ident, $iterty:ty
    } => {}
}

derive_pattern_clone! {
    clone SplitInternal
    with |s| SplitInternal { matcher: s.matcher.clone(), ..*s }
}

pub(super) struct SplitInternal<'a, P: Pattern<'a>> {
    pub(super) start: usize,
    pub(super) end: usize,
    pub(super) matcher: P::Searcher,
    pub(super) allow_trailing_empty: bool,
    pub(super) finished: bool,
}

impl<'a, P> fmt::Debug for SplitInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInternal")
            .field("start", &self.start)
            .field("end", &self.end)
            .field("matcher", &self.matcher)
            .field("allow_trailing_empty", &self.allow_trailing_empty)
            .field("finished", &self.finished)
            .finish()
    }
}

impl<'a, P: Pattern<'a>> SplitInternal<'a, P> {
    #[inline]
    fn get_end(&mut self) -> Option<&'a str> {
        if !self.finished && (self.allow_trailing_empty || self.end - self.start > 0) {
            self.finished = true;
            // SAFETY: `self.start` 和 `self.end` 始终位于 unicode 边界上。
            unsafe {
                let string = self.matcher.haystack().get_unchecked(self.start..self.end);
                Some(string)
            }
        } else {
            None
        }
    }

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        if self.finished {
            return None;
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match() {
            // SAFETY: `Searcher` 保证 `a` 和 `b` 位于 unicode 边界上。
            Some((a, b)) => unsafe {
                let elt = haystack.get_unchecked(self.start..a);
                self.start = b;
                Some(elt)
            },
            None => self.get_end(),
        }
    }

    #[inline]
    fn next_inclusive(&mut self) -> Option<&'a str> {
        if self.finished {
            return None;
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match() {
            // SAFETY: `Searcher` 保证 `b` 位于 unicode 边界上，而 self.start 是原始字符串的开头，或者已为其分配 `b`，因此它也位于 unicode 边界上。
            //
            //
            Some((_, b)) => unsafe {
                let elt = haystack.get_unchecked(self.start..b);
                self.start = b;
                Some(elt)
            },
            None => self.get_end(),
        }
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        if self.finished {
            return None;
        }

        if !self.allow_trailing_empty {
            self.allow_trailing_empty = true;
            match self.next_back() {
                Some(elt) if !elt.is_empty() => return Some(elt),
                _ => {
                    if self.finished {
                        return None;
                    }
                }
            }
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match_back() {
            // SAFETY: `Searcher` 保证 `a` 和 `b` 位于 unicode 边界上。
            Some((a, b)) => unsafe {
                let elt = haystack.get_unchecked(b..self.end);
                self.end = a;
                Some(elt)
            },
            // SAFETY: `self.start` 和 `self.end` 始终位于 unicode 边界上。
            None => unsafe {
                self.finished = true;
                Some(haystack.get_unchecked(self.start..self.end))
            },
        }
    }

    #[inline]
    fn next_back_inclusive(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        if self.finished {
            return None;
        }

        if !self.allow_trailing_empty {
            self.allow_trailing_empty = true;
            match self.next_back_inclusive() {
                Some(elt) if !elt.is_empty() => return Some(elt),
                _ => {
                    if self.finished {
                        return None;
                    }
                }
            }
        }

        let haystack = self.matcher.haystack();
        match self.matcher.next_match_back() {
            // SAFETY: `Searcher` 保证 `b` 位于 unicode 边界上，并且 self.end 是原始字符串的末尾，或者 `b` 被分配给它，因此它也位于 unicode 边界上。
            //
            //
            Some((_, b)) => unsafe {
                let elt = haystack.get_unchecked(b..self.end);
                self.end = b;
                Some(elt)
            },
            // SAFETY: self.start 是原始字符串的开头，或者是表示字符串中尚未迭代的部分的子字符串的开头。
            // 无论哪种方式，都可以保证它位于 unicode 边界上。
            // self.end 是原始字符串的末尾，或者已将 `b` 分配给它，因此它也位于 unicode 边界上。
            //
            //
            None => unsafe {
                self.finished = true;
                Some(haystack.get_unchecked(self.start..self.end))
            },
        }
    }

    #[inline]
    fn as_str(&self) -> &'a str {
        // `Self::get_end` 不会改变 `self.start`
        if self.finished {
            return "";
        }

        // SAFETY: `self.start` 和 `self.end` 始终位于 unicode 边界上。
        unsafe { self.matcher.haystack().get_unchecked(self.start..self.end) }
    }
}

generate_pattern_iterators! {
    forward:
        /// 使用方法 [`split`] 创建。
        ///
        /// [`split`]: str::split
        struct Split;
    reverse:
        /// 使用方法 [`rsplit`] 创建。
        ///
        /// [`rsplit`]: str::rsplit
        struct RSplit;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitInternal yielding (&'a str);
    delegate double ended;
}

impl<'a, P: Pattern<'a>> Split<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "Mary had a little lamb".split(' ');
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    /// split.next();
    /// assert_eq!(split.as_str(), "had a little lamb");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

impl<'a, P: Pattern<'a>> RSplit<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "Mary had a little lamb".rsplit(' ');
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    /// split.next();
    /// assert_eq!(split.as_str(), "Mary had a little");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

generate_pattern_iterators! {
    forward:
        /// 使用方法 [`split_terminator`] 创建。
        ///
        /// [`split_terminator`]: str::split_terminator
        struct SplitTerminator;
    reverse:
        /// 使用方法 [`rsplit_terminator`] 创建。
        ///
        /// [`rsplit_terminator`]: str::rsplit_terminator
        struct RSplitTerminator;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitInternal yielding (&'a str);
    delegate double ended;
}

impl<'a, P: Pattern<'a>> SplitTerminator<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "A..B..".split_terminator('.');
    /// assert_eq!(split.as_str(), "A..B..");
    /// split.next();
    /// assert_eq!(split.as_str(), ".B..");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

impl<'a, P: Pattern<'a>> RSplitTerminator<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "A..B..".rsplit_terminator('.');
    /// assert_eq!(split.as_str(), "A..B..");
    /// split.next();
    /// assert_eq!(split.as_str(), "A..B");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

derive_pattern_clone! {
    clone SplitNInternal
    with |s| SplitNInternal { iter: s.iter.clone(), ..*s }
}

pub(super) struct SplitNInternal<'a, P: Pattern<'a>> {
    pub(super) iter: SplitInternal<'a, P>,
    /// 剩余分割数
    pub(super) count: usize,
}

impl<'a, P> fmt::Debug for SplitNInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitNInternal")
            .field("iter", &self.iter)
            .field("count", &self.count)
            .finish()
    }
}

impl<'a, P: Pattern<'a>> SplitNInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        match self.count {
            0 => None,
            1 => {
                self.count = 0;
                self.iter.get_end()
            }
            _ => {
                self.count -= 1;
                self.iter.next()
            }
        }
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        match self.count {
            0 => None,
            1 => {
                self.count = 0;
                self.iter.get_end()
            }
            _ => {
                self.count -= 1;
                self.iter.next_back()
            }
        }
    }

    #[inline]
    fn as_str(&self) -> &'a str {
        self.iter.as_str()
    }
}

generate_pattern_iterators! {
    forward:
        /// 使用方法 [`splitn`] 创建。
        ///
        /// [`splitn`]: str::splitn
        struct SplitN;
    reverse:
        /// 使用方法 [`rsplitn`] 创建。
        ///
        /// [`rsplitn`]: str::rsplitn
        struct RSplitN;
    stability:
        #[stable(feature = "rust1", since = "1.0.0")]
    internal:
        SplitNInternal yielding (&'a str);
    delegate single ended;
}

impl<'a, P: Pattern<'a>> SplitN<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "Mary had a little lamb".splitn(3, ' ');
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    /// split.next();
    /// assert_eq!(split.as_str(), "had a little lamb");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

impl<'a, P: Pattern<'a>> RSplitN<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_as_str)]
    /// let mut split = "Mary had a little lamb".rsplitn(3, ' ');
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    /// split.next();
    /// assert_eq!(split.as_str(), "Mary had a little");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

derive_pattern_clone! {
    clone MatchIndicesInternal
    with |s| MatchIndicesInternal(s.0.clone())
}

pub(super) struct MatchIndicesInternal<'a, P: Pattern<'a>>(pub(super) P::Searcher);

impl<'a, P> fmt::Debug for MatchIndicesInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("MatchIndicesInternal").field(&self.0).finish()
    }
}

impl<'a, P: Pattern<'a>> MatchIndicesInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<(usize, &'a str)> {
        self.0
            .next_match()
            // SAFETY: `Searcher` 保证 `start` 和 `end` 位于 unicode 边界上。
            .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
    }

    #[inline]
    fn next_back(&mut self) -> Option<(usize, &'a str)>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        self.0
            .next_match_back()
            // SAFETY: `Searcher` 保证 `start` 和 `end` 位于 unicode 边界上。
            .map(|(start, end)| unsafe { (start, self.0.haystack().get_unchecked(start..end)) })
    }
}

generate_pattern_iterators! {
    forward:
        /// 使用方法 [`match_indices`] 创建。
        ///
        /// [`match_indices`]: str::match_indices
        struct MatchIndices;
    reverse:
        /// 使用方法 [`rmatch_indices`] 创建。
        ///
        /// [`rmatch_indices`]: str::rmatch_indices
        struct RMatchIndices;
    stability:
        #[stable(feature = "str_match_indices", since = "1.5.0")]
    internal:
        MatchIndicesInternal yielding ((usize, &'a str));
    delegate double ended;
}

derive_pattern_clone! {
    clone MatchesInternal
    with |s| MatchesInternal(s.0.clone())
}

pub(super) struct MatchesInternal<'a, P: Pattern<'a>>(pub(super) P::Searcher);

impl<'a, P> fmt::Debug for MatchesInternal<'a, P>
where
    P: Pattern<'a, Searcher: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("MatchesInternal").field(&self.0).finish()
    }
}

impl<'a, P: Pattern<'a>> MatchesInternal<'a, P> {
    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        // SAFETY: `Searcher` 保证 `start` 和 `end` 位于 unicode 边界上。
        self.0.next_match().map(|(a, b)| unsafe {
            // 已知索引位于 utf8 边界上
            self.0.haystack().get_unchecked(a..b)
        })
    }

    #[inline]
    fn next_back(&mut self) -> Option<&'a str>
    where
        P::Searcher: ReverseSearcher<'a>,
    {
        // SAFETY: `Searcher` 保证 `start` 和 `end` 位于 unicode 边界上。
        self.0.next_match_back().map(|(a, b)| unsafe {
            // 已知索引位于 utf8 边界上
            self.0.haystack().get_unchecked(a..b)
        })
    }
}

generate_pattern_iterators! {
    forward:
        /// 使用方法 [`matches`] 创建。
        ///
        /// [`matches`]: str::matches
        struct Matches;
    reverse:
        /// 使用方法 [`rmatches`] 创建。
        ///
        /// [`rmatches`]: str::rmatches
        struct RMatches;
    stability:
        #[stable(feature = "str_matches", since = "1.2.0")]
    internal:
        MatchesInternal yielding (&'a str);
    delegate double ended;
}

/// 在字符串的各行上进行迭代的迭代器，作为字符串切片。
///
/// 该结构体是通过 [`str`] 上的 [`lines`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`lines`]: str::lines
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone, Debug)]
pub struct Lines<'a>(pub(super) Map<SplitTerminator<'a, char>, LinesAnyMap>);

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for Lines<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> DoubleEndedIterator for Lines<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Lines<'_> {}

/// 使用方法 [`lines_any`] 创建。
///
/// [`lines_any`]: str::lines_any
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_deprecated(since = "1.4.0", reason = "use lines()/Lines instead now")]
#[derive(Clone, Debug)]
#[allow(deprecated)]
pub struct LinesAny<'a>(pub(super) Lines<'a>);

#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated)]
impl<'a> Iterator for LinesAny<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.0.size_hint()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated)]
impl<'a> DoubleEndedIterator for LinesAny<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
#[allow(deprecated)]
impl FusedIterator for LinesAny<'_> {}

/// 字符串的非空白子字符串上的迭代器，以任意数量的空格分隔。
///
///
/// 该结构体是通过 [`str`] 上的 [`split_whitespace`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`split_whitespace`]: str::split_whitespace
#[stable(feature = "split_whitespace", since = "1.1.0")]
#[derive(Clone, Debug)]
pub struct SplitWhitespace<'a> {
    pub(super) inner: Filter<Split<'a, IsWhitespace>, IsNotEmpty>,
}

/// 字符串的非 ASCII 空格子字符串上的迭代器，以任意数量的 ASCII 空格分隔。
///
///
/// 该结构体是通过 [`str`] 上的 [`split_ascii_whitespace`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`split_ascii_whitespace`]: str::split_ascii_whitespace
#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct SplitAsciiWhitespace<'a> {
    pub(super) inner:
        Map<Filter<SliceSplit<'a, u8, IsAsciiWhitespace>, BytesIsNotEmpty>, UnsafeBytesToStr>,
}

/// 字符串子字符串上的迭代器，由与谓词函数匹配的子字符串终止，与 `Split` 不同，它包含匹配的部分作为子片段的终止符。
///
///
/// 该结构体是通过 [`str`] 上的 [`split_inclusive`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`split_inclusive`]: str::split_inclusive
///
///
#[stable(feature = "split_inclusive", since = "1.51.0")]
pub struct SplitInclusive<'a, P: Pattern<'a>>(pub(super) SplitInternal<'a, P>);

#[stable(feature = "split_whitespace", since = "1.1.0")]
impl<'a> Iterator for SplitWhitespace<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.inner.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "split_whitespace", since = "1.1.0")]
impl<'a> DoubleEndedIterator for SplitWhitespace<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.inner.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for SplitWhitespace<'_> {}

impl<'a> SplitWhitespace<'a> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_whitespace_as_str)]
    ///
    /// let mut split = "Mary had a little lamb".split_whitespace();
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    ///
    /// split.next();
    /// assert_eq!(split.as_str(), "had a little lamb");
    ///
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.inner.iter.as_str()
    }
}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl<'a> Iterator for SplitAsciiWhitespace<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.inner.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<&'a str> {
        self.next_back()
    }
}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl<'a> DoubleEndedIterator for SplitAsciiWhitespace<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.inner.next_back()
    }
}

#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
impl FusedIterator for SplitAsciiWhitespace<'_> {}

impl<'a> SplitAsciiWhitespace<'a> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_whitespace_as_str)]
    ///
    /// let mut split = "Mary had a little lamb".split_ascii_whitespace();
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    ///
    /// split.next();
    /// assert_eq!(split.as_str(), "had a little lamb");
    ///
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        if self.inner.iter.iter.finished {
            return "";
        }

        // SAFETY: 切片是从 str 创建的。
        unsafe { crate::str::from_utf8_unchecked(&self.inner.iter.iter.v) }
    }
}

#[stable(feature = "split_inclusive", since = "1.51.0")]
impl<'a, P: Pattern<'a>> Iterator for SplitInclusive<'a, P> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<&'a str> {
        self.0.next_inclusive()
    }
}

#[stable(feature = "split_inclusive", since = "1.51.0")]
impl<'a, P: Pattern<'a, Searcher: fmt::Debug>> fmt::Debug for SplitInclusive<'a, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInclusive").field("0", &self.0).finish()
    }
}

// FIXME(#26925) 删除以支持 `#[derive(Clone)]`
#[stable(feature = "split_inclusive", since = "1.51.0")]
impl<'a, P: Pattern<'a, Searcher: Clone>> Clone for SplitInclusive<'a, P> {
    fn clone(&self) -> Self {
        SplitInclusive(self.0.clone())
    }
}

#[stable(feature = "split_inclusive", since = "1.51.0")]
impl<'a, P: Pattern<'a, Searcher: ReverseSearcher<'a>>> DoubleEndedIterator
    for SplitInclusive<'a, P>
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a str> {
        self.0.next_back_inclusive()
    }
}

#[stable(feature = "split_inclusive", since = "1.51.0")]
impl<'a, P: Pattern<'a>> FusedIterator for SplitInclusive<'a, P> {}

impl<'a, P: Pattern<'a>> SplitInclusive<'a, P> {
    /// 返回分割后的字符串的其余部分
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(str_split_inclusive_as_str)]
    /// let mut split = "Mary had a little lamb".split_inclusive(' ');
    /// assert_eq!(split.as_str(), "Mary had a little lamb");
    /// split.next();
    /// assert_eq!(split.as_str(), "had a little lamb");
    /// split.by_ref().for_each(drop);
    /// assert_eq!(split.as_str(), "");
    /// ```
    #[inline]
    #[unstable(feature = "str_split_inclusive_as_str", issue = "77998")]
    pub fn as_str(&self) -> &'a str {
        self.0.as_str()
    }
}

/// [`u16`] 的迭代器，编码为 UTF-16 的字符串。
///
/// 该结构体是通过 [`str`] 上的 [`encode_utf16`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`encode_utf16`]: str::encode_utf16
#[derive(Clone)]
#[stable(feature = "encode_utf16", since = "1.8.0")]
pub struct EncodeUtf16<'a> {
    pub(super) chars: Chars<'a>,
    pub(super) extra: u16,
}

#[stable(feature = "collection_debug", since = "1.17.0")]
impl fmt::Debug for EncodeUtf16<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("EncodeUtf16").finish_non_exhaustive()
    }
}

#[stable(feature = "encode_utf16", since = "1.8.0")]
impl<'a> Iterator for EncodeUtf16<'a> {
    type Item = u16;

    #[inline]
    fn next(&mut self) -> Option<u16> {
        if self.extra != 0 {
            let tmp = self.extra;
            self.extra = 0;
            return Some(tmp);
        }

        let mut buf = [0; 2];
        self.chars.next().map(|ch| {
            let n = ch.encode_utf16(&mut buf).len();
            if n == 2 {
                self.extra = buf[1];
            }
            buf[0]
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (low, high) = self.chars.size_hint();
        // 每个字符获得一个 u16 或两个 u16，因此此迭代器的长度是基础迭代器的 1 到 2 倍。
        //
        //
        (low, high.and_then(|n| n.checked_mul(2)))
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for EncodeUtf16<'_> {}

/// [`str::escape_debug`] 的返回类型。
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeDebug<'a> {
    pub(super) inner: Chain<
        Flatten<option::IntoIter<char::EscapeDebug>>,
        FlatMap<Chars<'a>, char::EscapeDebug, CharEscapeDebugContinue>,
    >,
}

/// [`str::escape_default`] 的返回类型。
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeDefault<'a> {
    pub(super) inner: FlatMap<Chars<'a>, char::EscapeDefault, CharEscapeDefault>,
}

/// [`str::escape_unicode`] 的返回类型。
#[stable(feature = "str_escape", since = "1.34.0")]
#[derive(Clone, Debug)]
pub struct EscapeUnicode<'a> {
    pub(super) inner: FlatMap<Chars<'a>, char::EscapeUnicode, CharEscapeUnicode>,
}

macro_rules! escape_types_impls {
    ($( $Name: ident ),+) => {$(
        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> fmt::Display for $Name<'a> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                self.clone().try_for_each(|c| f.write_char(c))
            }
        }

        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> Iterator for $Name<'a> {
            type Item = char;

            #[inline]
            fn next(&mut self) -> Option<char> { self.inner.next() }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }

            #[inline]
            fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
                Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Output = Acc>
            {
                self.inner.try_fold(init, fold)
            }

            #[inline]
            fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
                where Fold: FnMut(Acc, Self::Item) -> Acc,
            {
                self.inner.fold(init, fold)
            }
        }

        #[stable(feature = "str_escape", since = "1.34.0")]
        impl<'a> FusedIterator for $Name<'a> {}
    )+}
}

escape_types_impls!(EscapeDebug, EscapeDefault, EscapeUnicode);