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use std::convert::TryInto;

#[derive(Debug, Clone)]
pub struct ChunksExacter<'a, T: 'a, const N: usize> {
    v: &'a [T],
    rem: &'a [T],
}

impl<'a, T, const N: usize> ChunksExacter<'a, T, N> {
    pub fn new(slice: &'a [T]) -> Self {
        let rem = slice.len() % N;
        let fst_len = slice.len() - rem;

        let (fst, snd) = slice.split_at(fst_len);
        Self { v: fst, rem: snd }
    }

    pub fn remainder(&self) -> &[T] {
        self.rem
    }
}

impl<'a, T, const N: usize> Iterator for ChunksExacter<'a, T, N> {
    type Item = &'a [T; N];

    fn next(&mut self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let (fst, snd) = self.v.split_at(N);
            self.v = snd;
            Some(fst.try_into().unwrap())
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / N;
        (n, Some(n))
    }

    fn count(self) -> usize {
        self.size_hint().0
    }
}

impl<'a, T, const N: usize> ExactSizeIterator for ChunksExacter<'a, T, N> {}

struct ChunksExactest<'a, T: 'a, const N: usize> {
    v: &'a [T],
}

impl<'a, T, const N: usize> ChunksExactest<'a, T, N> {
    fn new(slice: &'a [T]) -> Self {
        let rem = slice.len() % N;
        assert_eq!(rem, 0);

        Self { v: slice }
    }
}

impl<'a, T, const N: usize> Iterator for ChunksExactest<'a, T, N> {
    type Item = &'a [T; N];

    fn next(&mut self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let (fst, snd) = self.v.split_at(N);
            self.v = snd;
            Some(fst.try_into().unwrap())
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / N;
        (n, Some(n))
    }

    fn count(self) -> usize {
        self.size_hint().0
    }
}

impl<'a, T, const N: usize> ExactSizeIterator for ChunksExactest<'a, T, N> {}

trait ChunksExactExt<'a, T, const N: usize> {
    fn chunks_exacter(&'a self) -> ChunksExacter<'a, T, N>;
    fn chunks_exactest(&'a self) -> ChunksExactest<'a, T, N>;
}

impl<'a, T, const N: usize> ChunksExactExt<'a, T, N> for [T] {
    fn chunks_exacter(&'a self) -> ChunksExacter<'a, T, N> {
        ChunksExacter::new(self)
    }

    fn chunks_exactest(&'a self) -> ChunksExactest<'a, T, N> {
        ChunksExactest::new(self)
    }
}

#[test]
fn test_construct() {
    let bytes = &[
        1,0,0,1,0,0,0,0,
        2,0,0,2,0,0,0,0,
    ];

    let new_things: Vec<_> = bytes
        .chunks_exacter()
        .map(|x: &[_; 8]| x[3])
        .collect();

    assert_eq!(&new_things, &[1, 2]);

    let new_things: Vec<_> = bytes
        .chunks_exactest()
        .map(|x| u32::from_le_bytes(*x))
        .collect();

    assert_eq!(&new_things, &[0x01000001, 0, 0x02000002, 0]);
}