use crate::fmt;
use crate::iter::adapters::{
    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
};
use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen, UncheckedIterator};
use crate::ops::Try;

/// 将 `iter` 的值与 `f` 映射的迭代器。
///
/// 该 `struct` 是通过 [`Iterator`] 上的 [`map`] 方法创建的。有关更多信息，请参见其文档。
///
/// [`map`]: Iterator::map
/// [`Iterator`]: trait.Iterator.html
///
/// # 有关副作用的注意事项
///
/// [`map`] 迭代器实现 [`DoubleEndedIterator`]，这意味着您还可以向后 [`map`]：
///
/// ```rust
/// let v: Vec<i32> = [1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
///
/// assert_eq!(v, [4, 3, 2]);
/// ```
///
/// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
///
/// 但是，如果您的闭包具有状态，则向后迭代可能会以您所不期望的方式起作用。让我们来看一个例子。首先，在前进方向上：
///
/// ```rust
/// let mut c = 0;
///
/// for pair in ['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) }) {
///     println!("{pair:?}");
/// }
/// ```
///
/// 这将打印 `('a', 1), ('b', 2), ('c', 3)`。
///
/// 现在考虑这种转折，我们在 `rev` 上添加了一个调用。此版本将打印 `('c', 1), ('b', 2), ('a', 3)`。
/// 请注意，字母是相反的，但计数器的值仍然按顺序排列。
/// 这是因为 `map()` 仍在每个项上被延迟调用，但是现在我们将项从 vector 的后面弹出，而不是从正面移出它们。
///
///
/// ```rust
/// let mut c = 0;
///
/// for pair in ['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) })
///                                .rev() {
///     println!("{pair:?}");
/// }
/// ```
///
///
///
///
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Map<I, F> {
    // 用于 `SplitWhitespace` 和 `SplitAsciiWhitespace` `as_str` 方法
    pub(crate) iter: I,
    f: F,
}

impl<I, F> Map<I, F> {
    pub(in crate::iter) fn new(iter: I, f: F) -> Map<I, F> {
        Map { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Map").field("iter", &self.iter).finish()
    }
}

fn map_fold<T, B, Acc>(
    mut f: impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, elt| g(acc, f(elt))
}

fn map_try_fold<'a, T, B, Acc, R>(
    f: &'a mut impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, elt| g(acc, f(elt))
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().map(&mut self.f)
    }

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

    fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_fold(init, map_try_fold(&mut self.f, g))
    }

    fn fold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, map_fold(self.f, g))
    }

    #[inline]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        // SAFETY: 调用者必须遵守 `Iterator::__iterator_get_unchecked` 的契约。
        //
        unsafe { (self.f)(try_get_unchecked(&mut self.iter, idx)) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        self.iter.next_back().map(&mut self.f)
    }

    fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_rfold(init, map_try_fold(&mut self.f, g))
    }

    fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, map_fold(self.f, g))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for Map<I, F> where F: FnMut(I::Item) -> B {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<B, I, F> TrustedLen for Map<I, F>
where
    I: TrustedLen,
    F: FnMut(I::Item) -> B,
{
}

impl<B, I, F> UncheckedIterator for Map<I, F>
where
    I: UncheckedIterator,
    F: FnMut(I::Item) -> B,
{
    unsafe fn next_unchecked(&mut self) -> B {
        // SAFETY: `Map` 与内部迭代器是 1:1，所以如果调用者承诺还有一个元素，则内部迭代器也有一个。
        //
        let item = unsafe { self.iter.next_unchecked() };
        (self.f)(item)
    }
}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I, F> TrustedRandomAccess for Map<I, F> where I: TrustedRandomAccess {}

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

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I, F> SourceIter for Map<I, F>
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<B, I: InPlaceIterable, F> InPlaceIterable for Map<I, F> where F: FnMut(I::Item) -> B {}