use super::{ArrayLength, Block};

use core::slice;

/// Sealed trait for buffer kinds.

pub trait Sealed {

    /// Invariant guaranteed by a buffer kind, i.e. with correct

    /// buffer code this function always returns true.

    fn invariant(pos: usize, block_size: usize) -> bool;

    /// Split input data into slice of blocks and tail.

    fn split_blocks<N: ArrayLength<u8>>(data: &[u8]) -> (&[Block<N>], &[u8]);

}

impl Sealed for super::Eager {

    #[inline(always)]

    fn invariant(pos: usize, block_size: usize) -> bool {

        pos < block_size

    }

    #[inline(always)]

    fn split_blocks<N: ArrayLength<u8>>(data: &[u8]) -> (&[Block<N>], &[u8]) {

        let nb = data.len() / N::USIZE;

        let blocks_len = nb * N::USIZE;

        let tail_len = data.len() - blocks_len;

        // SAFETY: we guarantee that created slices do not point

        // outside of `data`

        unsafe {

            let blocks_ptr = data.as_ptr() as *const Block<N>;

            let tail_ptr = data.as_ptr().add(blocks_len);

            (

                slice::from_raw_parts(blocks_ptr, nb),

                slice::from_raw_parts(tail_ptr, tail_len),

            )

        }

    }

}

impl Sealed for super::Lazy {

    #[inline(always)]

    fn invariant(pos: usize, block_size: usize) -> bool {

        pos <= block_size

    }

    #[inline(always)]

    fn split_blocks<N: ArrayLength<u8>>(data: &[u8]) -> (&[Block<N>], &[u8]) {

        if data.is_empty() {

            return (&[], &[]);

        }

        let (nb, tail_len) = if data.len() % N::USIZE == 0 {

            (data.len() / N::USIZE - 1, N::USIZE)

        } else {

            let nb = data.len() / N::USIZE;

            (nb, data.len() - nb * N::USIZE)

        };

        let blocks_len = nb * N::USIZE;

        // SAFETY: we guarantee that created slices do not point

        // outside of `data`

        unsafe {

            let blocks_ptr = data.as_ptr() as *const Block<N>;

            let tail_ptr = data.as_ptr().add(blocks_len);

            (

                slice::from_raw_parts(blocks_ptr, nb),

                slice::from_raw_parts(tail_ptr, tail_len),

            )

        }

    }

}