use crate::cmp;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
use crate::num::NonZeroUsize;
use crate::ops::{ControlFlow, Try};

/// 一个仅迭代 `iter` 的前 `n` 迭代的迭代器。
///
/// 该 `struct` 是通过 [`Iterator`] 上的 [`take`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`take`]: Iterator::take
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Take<I> {
    iter: I,
    n: usize,
}

impl<I> Take<I> {
    pub(in crate::iter) fn new(iter: I, n: usize) -> Take<I> {
        Take { iter, n }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Take<I>
where
    I: Iterator,
{
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        if self.n != 0 {
            self.n -= 1;
            self.iter.next()
        } else {
            None
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        if self.n > n {
            self.n -= n + 1;
            self.iter.nth(n)
        } else {
            if self.n > 0 {
                self.iter.nth(self.n - 1);
                self.n = 0;
            }
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.n == 0 {
            return (0, Some(0));
        }

        let (lower, upper) = self.iter.size_hint();

        let lower = cmp::min(lower, self.n);

        let upper = match upper {
            Some(x) if x < self.n => Some(x),
            _ => Some(self.n),
        };

        (lower, upper)
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        fn check<'a, T, Acc, R: Try<Output = Acc>>(
            n: &'a mut usize,
            mut fold: impl FnMut(Acc, T) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
            move |acc, x| {
                *n -= 1;
                let r = fold(acc, x);
                if *n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
            }
        }

        if self.n == 0 {
            try { init }
        } else {
            let n = &mut self.n;
            self.iter.try_fold(init, check(n, fold)).into_try()
        }
    }

    impl_fold_via_try_fold! { fold -> try_fold }

    #[inline]
    fn for_each<F: FnMut(Self::Item)>(mut self, f: F) {
        // 默认实现将使用元累加器，因此我们可以通过折叠我们希望返回的剩余项数来避免有状态闭包。
        //
        //
        fn check<'a, Item>(
            mut action: impl FnMut(Item) + 'a,
        ) -> impl FnMut(usize, Item) -> Option<usize> + 'a {
            move |more, x| {
                action(x);
                more.checked_sub(1)
            }
        }

        let remaining = self.n;
        if remaining > 0 {
            self.iter.try_fold(remaining - 1, check(f));
        }
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        let min = self.n.min(n);
        let rem = match self.iter.advance_by(min) {
            Ok(()) => 0,
            Err(rem) => rem.get(),
        };
        let advanced = min - rem;
        self.n -= advanced;
        NonZeroUsize::new(n - advanced).map_or(Ok(()), Err)
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I> SourceIter for Take<I>
where
    I: SourceIter,
{
    type Source = I::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        // SAFETY: 将不安全的函数转发到具有相同要求的不安全的函数
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I: InPlaceIterable> InPlaceIterable for Take<I> {}

#[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Take<I>
where
    I: DoubleEndedIterator + ExactSizeIterator,
{
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.n == 0 {
            None
        } else {
            let n = self.n;
            self.n -= 1;
            self.iter.nth_back(self.iter.len().saturating_sub(n))
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.iter.len();
        if self.n > n {
            let m = len.saturating_sub(self.n) + n;
            self.n -= n + 1;
            self.iter.nth_back(m)
        } else {
            if len > 0 {
                self.iter.nth_back(len - 1);
            }
            None
        }
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        if self.n == 0 {
            try { init }
        } else {
            let len = self.iter.len();
            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                try { init }
            } else {
                self.iter.try_rfold(init, fold)
            }
        }
    }

    #[inline]
    fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.n == 0 {
            init
        } else {
            let len = self.iter.len();
            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                init
            } else {
                self.iter.rfold(init, fold)
            }
        }
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        // 内部迭代器需要缩短的数量，使其最多与 take() 的长度相同。
        //
        let trim_inner = self.iter.len().saturating_sub(self.n);
        // 我们需要推进内部实现来满足调用者的请求。
        // take()、advance_by()、len() 都最多可以使用，所以我们不用担心这里要提前超过 usize::MAX。
        //
        let advance_by = trim_inner.saturating_add(n);

        let remainder = match self.iter.advance_back_by(advance_by) {
            Ok(()) => 0,
            Err(rem) => rem.get(),
        };
        let advanced_by_inner = advance_by - remainder;
        let advanced_by = advanced_by_inner - trim_inner;
        self.n -= advanced_by;
        NonZeroUsize::new(n - advanced_by).map_or(Ok(()), Err)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}

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

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}