use crate::iter::adapters::{
    zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
};
use crate::iter::{FusedIterator, TrustedLen};
use crate::mem::MaybeUninit;
use crate::mem::SizedTypeProperties;
use crate::num::NonZeroUsize;
use crate::ops::Try;
use crate::{array, ptr};

/// 一个可以复制底层迭代器元素的迭代器。
///
/// 该 `struct` 是通过 [`Iterator`] 上的 [`copied`] 方法创建的。
/// 有关更多信息，请参见其文档。
///
/// [`copied`]: Iterator::copied
/// [`Iterator`]: trait.Iterator.html
#[stable(feature = "iter_copied", since = "1.36.0")]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone, Debug)]
pub struct Copied<I> {
    it: I,
}

impl<I> Copied<I> {
    pub(in crate::iter) fn new(it: I) -> Copied<I> {
        Copied { it }
    }
}

fn copy_fold<T: Copy, Acc>(mut f: impl FnMut(Acc, T) -> Acc) -> impl FnMut(Acc, &T) -> Acc {
    move |acc, &elt| f(acc, elt)
}

fn copy_try_fold<T: Copy, Acc, R>(mut f: impl FnMut(Acc, T) -> R) -> impl FnMut(Acc, &T) -> R {
    move |acc, &elt| f(acc, elt)
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> Iterator for Copied<I>
where
    I: Iterator<Item = &'a T>,
    T: Copy,
{
    type Item = T;

    fn next(&mut self) -> Option<T> {
        self.it.next().copied()
    }

    fn next_chunk<const N: usize>(
        &mut self,
    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
    where
        Self: Sized,
    {
        <I as SpecNextChunk<'_, N, T>>::spec_next_chunk(&mut self.it)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.it.size_hint()
    }

    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        self.it.try_fold(init, copy_try_fold(f))
    }

    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.fold(init, copy_fold(f))
    }

    fn nth(&mut self, n: usize) -> Option<T> {
        self.it.nth(n).copied()
    }

    fn last(self) -> Option<T> {
        self.it.last().copied()
    }

    fn count(self) -> usize {
        self.it.count()
    }

    #[inline]
    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        self.it.advance_by(n)
    }

    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> T
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        // SAFETY: 调用者必须遵守 `Iterator::__iterator_get_unchecked` 的契约。
        //
        *unsafe { try_get_unchecked(&mut self.it, idx) }
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> DoubleEndedIterator for Copied<I>
where
    I: DoubleEndedIterator<Item = &'a T>,
    T: Copy,
{
    fn next_back(&mut self) -> Option<T> {
        self.it.next_back().copied()
    }

    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> R,
        R: Try<Output = B>,
    {
        self.it.try_rfold(init, copy_try_fold(f))
    }

    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
    where
        F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.rfold(init, copy_fold(f))
    }

    #[inline]
    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
        self.it.advance_back_by(n)
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> ExactSizeIterator for Copied<I>
where
    I: ExactSizeIterator<Item = &'a T>,
    T: Copy,
{
    fn len(&self) -> usize {
        self.it.len()
    }

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

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> FusedIterator for Copied<I>
where
    I: FusedIterator<Item = &'a T>,
    T: Copy,
{
}

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

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

#[stable(feature = "iter_copied", since = "1.36.0")]
unsafe impl<'a, I, T: 'a> TrustedLen for Copied<I>
where
    I: TrustedLen<Item = &'a T>,
    T: Copy,
{
}

trait SpecNextChunk<'a, const N: usize, T: 'a>: Iterator<Item = &'a T>
where
    T: Copy,
{
    fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>>;
}

impl<'a, const N: usize, I, T: 'a> SpecNextChunk<'a, N, T> for I
where
    I: Iterator<Item = &'a T>,
    T: Copy,
{
    default fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>> {
        array::iter_next_chunk(&mut self.map(|e| *e))
    }
}

impl<'a, const N: usize, T: 'a> SpecNextChunk<'a, N, T> for crate::slice::Iter<'a, T>
where
    T: Copy,
{
    fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>> {
        let mut raw_array = MaybeUninit::uninit_array();

        let len = self.len();

        if T::IS_ZST {
            if len < N {
                let _ = self.advance_by(len);
                // SAFETY: ZST 可以凭空变出来; 只有数量是正确的
                return Err(unsafe { array::IntoIter::new_unchecked(raw_array, 0..len) });
            }

            let _ = self.advance_by(N);
            // SAFETY: ditto
            return Ok(unsafe { MaybeUninit::array_assume_init(raw_array) });
        }

        if len < N {
            // SAFETY: `len` 表示有这么多元素可用，我们刚刚检查了它是否适合数组。
            //
            unsafe {
                ptr::copy_nonoverlapping(
                    self.as_ref().as_ptr(),
                    raw_array.as_mut_ptr() as *mut T,
                    len,
                );
                let _ = self.advance_by(len);
                return Err(array::IntoIter::new_unchecked(raw_array, 0..len));
            }
        }

        // SAFETY: `len` 大于数组大小。
        // 在此处复制固定数量以完全初始化数组。
        unsafe {
            ptr::copy_nonoverlapping(self.as_ref().as_ptr(), raw_array.as_mut_ptr() as *mut T, N);
            let _ = self.advance_by(N);
            Ok(MaybeUninit::array_assume_init(raw_array))
        }
    }
}

#[stable(feature = "default_iters", since = "1.70.0")]
impl<I: Default> Default for Copied<I> {
    /// 从 `I` 的默认值创建一个 `Copied` 迭代器
    /// ```
    /// # use core::slice;
    /// # use core::iter::Copied;
    /// let iter: Copied<slice::Iter<'_, u8>> = Default::default();
    /// assert_eq!(iter.len(), 0);
    /// ```
    fn default() -> Self {
        Self::new(Default::default())
    }
}