use crate::intrinsics;
use crate::iter::adapters::zip::try_get_unchecked;
use crate::iter::{
    DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen, TrustedRandomAccess,
    TrustedRandomAccessNoCoerce,
};
use crate::ops::Try;

/// 一个迭代器，在底层迭代器产生一次 `None` 之后，永远产生 `None`。
///
///
/// 该 `struct` 由 [`Iterator::fuse`] 创建。有关更多信息，请参见其文档。
///
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Fuse<I> {
    // NOTE: 对于 `I: FusedIterator`，我们从不费心设置 `None`，但由于差异，我们仍然必须为该状态做好准备。
    //
    // 请参见 rust-lang/rust#85863
    iter: Option<I>,
}
impl<I> Fuse<I> {
    pub(in crate::iter) fn new(iter: I) -> Fuse<I> {
        Fuse { iter: Some(iter) }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Fuse<I> where I: Iterator {}

// 此处的任何专门实现都是内部实现的，以避免在此 trait 之外暴露默认 fns。
//
#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Fuse<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        FuseImpl::next(self)
    }

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

    #[inline]
    fn last(self) -> Option<Self::Item> {
        match self.iter {
            Some(iter) => iter.last(),
            None => None,
        }
    }

    #[inline]
    fn count(self) -> usize {
        match self.iter {
            Some(iter) => iter.count(),
            None => 0,
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        match self.iter {
            Some(ref iter) => iter.size_hint(),
            None => (0, Some(0)),
        }
    }

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

    #[inline]
    fn fold<Acc, Fold>(self, mut acc: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if let Some(iter) = self.iter {
            acc = iter.fold(acc, fold);
        }
        acc
    }

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

    #[inline]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        match self.iter {
            // SAFETY: 调用者必须遵守 `Iterator::__iterator_get_unchecked` 的契约。
            //
            Some(ref mut iter) => unsafe { try_get_unchecked(iter, idx) },
            // SAFETY: 调用者断言 `i` 处有一个项目，因此我们不疲倦。
            None => unsafe { intrinsics::unreachable() },
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Fuse<I>
where
    I: DoubleEndedIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        FuseImpl::next_back(self)
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        FuseImpl::nth_back(self, n)
    }

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

    #[inline]
    fn rfold<Acc, Fold>(self, mut acc: Acc, fold: Fold) -> Acc
    where
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if let Some(iter) = self.iter {
            acc = iter.rfold(acc, fold);
        }
        acc
    }

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Fuse<I>
where
    I: ExactSizeIterator,
{
    fn len(&self) -> usize {
        match self.iter {
            Some(ref iter) => iter.len(),
            None => 0,
        }
    }

    fn is_empty(&self) -> bool {
        match self.iter {
            Some(ref iter) => iter.is_empty(),
            None => true,
        }
    }
}

#[stable(feature = "default_iters", since = "1.70.0")]
impl<I: Default> Default for Fuse<I> {
    /// 从默认值 `I` 创建 `Fuse` 迭代器。
    ///
    /// ```
    /// # use core::slice;
    /// # use std::iter::Fuse;
    /// let iter: Fuse<slice::Iter<'_, u8>> = Default::default();
    /// assert_eq!(iter.len(), 0);
    /// ```
    fn default() -> Self {
        Fuse { iter: Default::default() }
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
// SAFETY: `TrustedLen` 要求通过 `size_hint()` 报告准确的长度。
// 由于 `Fuse` 只是将其转发给包装的迭代器 `I`，因此保留了此属性，可以在此处安全地实现 `TrustedLen`。
//
unsafe impl<I> TrustedLen for Fuse<I> where I: TrustedLen {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
// SAFETY: `TrustedRandomAccess` 要求 `size_hint()` 必须精确且便宜，并且 `Iterator::__iterator_get_unchecked()` 必须相应地实现。
//
//
// 这是安全的，因为 `Fuse` 只是将它们转发给包装的迭代器 `I`，后者保留了这些属性。
//
unsafe impl<I> TrustedRandomAccess for Fuse<I> where I: TrustedRandomAccess {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccessNoCoerce for Fuse<I>
where
    I: TrustedRandomAccessNoCoerce,
{
    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
}

/// Fuse 专用 trait
///
/// 我们只需要担心 `&mut self` 方法，它可能会耗尽迭代器而不消耗它。
///
#[doc(hidden)]
trait FuseImpl<I> {
    type Item;

    // 特定于任何普通迭代器的函数
    fn next(&mut self) -> Option<Self::Item>;
    fn nth(&mut self, n: usize) -> Option<Self::Item>;
    fn try_fold<Acc, Fold, R>(&mut self, acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>;
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool;

    // 特定于 DoubleEndedIterators 的函数
    fn next_back(&mut self) -> Option<Self::Item>
    where
        I: DoubleEndedIterator;
    fn nth_back(&mut self, n: usize) -> Option<Self::Item>
    where
        I: DoubleEndedIterator;
    fn try_rfold<Acc, Fold, R>(&mut self, acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
        I: DoubleEndedIterator;
    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
        I: DoubleEndedIterator;
}

/// 通用 `Fuse` impl 在耗尽时设置 `iter = None`。
#[doc(hidden)]
impl<I> FuseImpl<I> for Fuse<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;

    #[inline]
    default fn next(&mut self) -> Option<<I as Iterator>::Item> {
        and_then_or_clear(&mut self.iter, Iterator::next)
    }

    #[inline]
    default fn nth(&mut self, n: usize) -> Option<I::Item> {
        and_then_or_clear(&mut self.iter, |iter| iter.nth(n))
    }

    #[inline]
    default fn try_fold<Acc, Fold, R>(&mut self, mut acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        if let Some(ref mut iter) = self.iter {
            acc = iter.try_fold(acc, fold)?;
            self.iter = None;
        }
        try { acc }
    }

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

    #[inline]
    default fn next_back(&mut self) -> Option<<I as Iterator>::Item>
    where
        I: DoubleEndedIterator,
    {
        and_then_or_clear(&mut self.iter, |iter| iter.next_back())
    }

    #[inline]
    default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item>
    where
        I: DoubleEndedIterator,
    {
        and_then_or_clear(&mut self.iter, |iter| iter.nth_back(n))
    }

    #[inline]
    default fn try_rfold<Acc, Fold, R>(&mut self, mut acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
        I: DoubleEndedIterator,
    {
        if let Some(ref mut iter) = self.iter {
            acc = iter.try_rfold(acc, fold)?;
            self.iter = None;
        }
        try { acc }
    }

    #[inline]
    default fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
    where
        P: FnMut(&Self::Item) -> bool,
        I: DoubleEndedIterator,
    {
        and_then_or_clear(&mut self.iter, |iter| iter.rfind(predicate))
    }
}

/// 专门的 `Fuse` impl，在耗尽时不会费心清除 `iter`。
/// 不过，我们还是要做好准备，它已经被清除了！
#[doc(hidden)]
impl<I> FuseImpl<I> for Fuse<I>
where
    I: FusedIterator,
{
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.as_mut()?.next()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        self.iter.as_mut()?.nth(n)
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, mut acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        if let Some(ref mut iter) = self.iter {
            acc = iter.try_fold(acc, fold)?;
        }
        try { acc }
    }

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

    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item>
    where
        I: DoubleEndedIterator,
    {
        self.iter.as_mut()?.next_back()
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item>
    where
        I: DoubleEndedIterator,
    {
        self.iter.as_mut()?.nth_back(n)
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, mut acc: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
        I: DoubleEndedIterator,
    {
        if let Some(ref mut iter) = self.iter {
            acc = iter.try_rfold(acc, fold)?;
        }
        try { acc }
    }

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

#[inline]
fn and_then_or_clear<T, U>(opt: &mut Option<T>, f: impl FnOnce(&mut T) -> Option<U>) -> Option<U> {
    let x = f(opt.as_mut()?);
    if x.is_none() {
        *opt = None;
    }
    x
}