const-arrayvec 0.2.1

A vec-like type backed by an array on the stack.
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use crate::ArrayVec;
use core::{
    iter::{DoubleEndedIterator, FusedIterator},
    mem,
    ops::Range,
    ptr,
};

#[derive(Debug, PartialEq)]
pub struct Drain<'a, T, const N: usize> {
    inner: &'a mut ArrayVec<T, { N }>,
    /// The index of the first item being removed.
    drain_range_start: usize,
    /// The index of the first item after the drained range.
    tail_start: usize,
    tail_length: usize,
    /// The front of the remaining drained range.
    head: *mut T,
    /// One after the last item in the range being drained.
    tail: *mut T,
}

impl<'a, T, const N: usize> Drain<'a, T, { N }> {
    pub(crate) fn with_range(
        vector: &'a mut ArrayVec<T, { N }>,
        range: Range<usize>,
    ) -> Self {
        debug_assert!(
            range.start <= range.end,
            "The range start must be before end"
        );
        debug_assert!(range.end <= vector.len(), "The range is out of bounds");
        debug_assert!(
            core::mem::size_of::<T>() != 0,
            "We can't deal with zero-sized types"
        );

        unsafe {
            let head = vector.as_mut_ptr().add(range.start);
            let tail = vector.as_mut_ptr().add(range.end);
            let tail_length = vector.len() - (range.end - range.start);

            // prevent a leaked Drain from letting users read from uninitialized
            // memory
            vector.set_len(range.start);

            Drain {
                inner: vector,
                drain_range_start: range.start,
                tail_start: range.end,
                tail_length,
                head,
                tail,
            }
        }
    }

    pub fn as_slice(&self) -> &[T] { unimplemented!() }

    pub fn as_mut_slice(&mut self) -> &mut [T] { unimplemented!() }
}

impl<'a, T, const N: usize> Iterator for Drain<'a, T, { N }> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        if self.head == self.tail {
            // No more items
            return None;
        }

        unsafe {
            // The tail points at tne end of our
            // copy the item onto the stack. The tail
            let item = self.head.read();
            // increment the head pointer
            self.head = self.head.add(1);
            Some(item)
        }
    }

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

impl<'a, T, const N: usize> DoubleEndedIterator for Drain<'a, T, { N }> {
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.head == self.tail {
            // No more items
            return None;
        }

        unsafe {
            // the tail pointer is one PAST the end of our selection.
            // Pre-decrement so we're pointing at a valid item before reading
            self.tail = self.tail.sub(1);
            let item = self.tail.read();
            Some(item)
        }
    }
}

impl<'a, T, const N: usize> FusedIterator for Drain<'a, T, { N }> {}

impl<'a, T, const N: usize> ExactSizeIterator for Drain<'a, T, { N }> {
    fn len(&self) -> usize {
        let size = mem::size_of::<T>();
        assert!(0 < size && size <= isize::max_value() as usize);

        let difference = (self.tail as isize) - (self.head as isize);
        debug_assert!(difference >= 0, "Tail should always be after head");

        difference as usize / size
    }
}

impl<'a, T, const N: usize> Drop for Drain<'a, T, { N }> {
    fn drop(&mut self) {
        // remove any remaining items so their destructors can run
        while let Some(item) = self.next() {
            mem::drop(item);
        }

        if self.tail_length == 0 {
            // there are no items after the drained range
            return;
        }

        unsafe {
            let tail_start = self.inner.as_ptr().add(self.tail_start);
            let drain_range_start =
                self.inner.as_mut_ptr().add(self.drain_range_start);

            // moves the tail (items after drained range) forwards now that the
            // drained items are destroyed
            ptr::copy(tail_start, drain_range_start, self.tail_length);

            // we can now update the length
            self.inner
                .set_len(self.drain_range_start + self.tail_length);
        }
    }
}