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use std::{convert::TryInto, slice::ChunksExact};

use crate::trusted_len::TrustedLen;

use super::{BitChunk, BitChunkIterExact};

/// An iterator over a slice of bytes in [`BitChunk`]s.
#[derive(Debug)]
pub struct BitChunksExact<'a, T: BitChunk> {
    iter: ChunksExact<'a, u8>,
    remainder: &'a [u8],
    remainder_len: usize,
    phantom: std::marker::PhantomData<T>,
}

impl<'a, T: BitChunk> BitChunksExact<'a, T> {
    /// Creates a new [`BitChunksExact`].
    #[inline]
    pub fn new(bitmap: &'a [u8], length: usize) -> Self {
        assert!(length <= bitmap.len() * 8);
        let size_of = std::mem::size_of::<T>();

        let bitmap = &bitmap[..length.saturating_add(7) / 8];

        let split = (length / 8 / size_of) * size_of;
        let (chunks, remainder) = bitmap.split_at(split);
        let remainder_len = length - chunks.len() * 8;
        let iter = chunks.chunks_exact(size_of);

        Self {
            iter,
            remainder,
            remainder_len,
            phantom: std::marker::PhantomData,
        }
    }

    /// Returns the number of chunks of this iterator
    #[inline]
    pub fn len(&self) -> usize {
        self.iter.len()
    }

    /// Returns whether there are still elements in this iterator
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns the remaining [`BitChunk`]. It is zero iff `len / 8 == 0`.
    #[inline]
    pub fn remainder(&self) -> T {
        let remainder_bytes = self.remainder;
        if remainder_bytes.is_empty() {
            return T::zero();
        }
        let remainder = match remainder_bytes.try_into() {
            Ok(a) => a,
            Err(_) => {
                let mut remainder = T::zero().to_ne_bytes();
                remainder_bytes
                    .iter()
                    .enumerate()
                    .for_each(|(index, b)| remainder[index] = *b);
                remainder
            }
        };
        T::from_ne_bytes(remainder)
    }
}

impl<T: BitChunk> Iterator for BitChunksExact<'_, T> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next().map(|x| match x.try_into() {
            Ok(a) => T::from_ne_bytes(a),
            Err(_) => unreachable!(),
        })
    }

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

unsafe impl<T: BitChunk> TrustedLen for BitChunksExact<'_, T> {}

impl<T: BitChunk> BitChunkIterExact<T> for BitChunksExact<'_, T> {
    #[inline]
    fn remainder(&self) -> T {
        self.remainder()
    }

    #[inline]
    fn remainder_len(&self) -> usize {
        self.remainder_len
    }
}