//! This is code from [Tungstenite project](https://github.com/snapview/tungstenite-rs)
#![allow(clippy::cast_ptr_alignment)]
use std::ptr::copy_nonoverlapping;
use std::slice;

// Holds a slice guaranteed to be shorter than 8 bytes
struct ShortSlice<'a>(&'a mut [u8]);

impl<'a> ShortSlice<'a> {
    /// # Safety
    /// Given slice must be shorter than 8 bytes.
    unsafe fn new(slice: &'a mut [u8]) -> Self {
        // Sanity check for debug builds
        debug_assert!(slice.len() < 8);
        ShortSlice(slice)
    }
    fn len(&self) -> usize {
        self.0.len()
    }
}

/// Faster version of `apply_mask()` which operates on 8-byte blocks.
#[inline]
#[allow(clippy::cast_lossless)]
pub(crate) fn apply_mask(buf: &mut [u8], mask_u32: u32) {
    // Extend the mask to 64 bits
    let mut mask_u64 = ((mask_u32 as u64) << 32) | (mask_u32 as u64);
    // Split the buffer into three segments
    let (head, mid, tail) = align_buf(buf);

    // Initial unaligned segment
    let head_len = head.len();
    if head_len > 0 {
        xor_short(head, mask_u64);
        if cfg!(target_endian = "big") {
            mask_u64 = mask_u64.rotate_left(8 * head_len as u32);
        } else {
            mask_u64 = mask_u64.rotate_right(8 * head_len as u32);
        }
    }
    // Aligned segment
    for v in mid {
        *v ^= mask_u64;
    }
    // Final unaligned segment
    if tail.len() > 0 {
        xor_short(tail, mask_u64);
    }
}

// TODO: copy_nonoverlapping here compiles to call memcpy. While it is not so
// inefficient, it could be done better. The compiler does not understand that
// a `ShortSlice` must be smaller than a u64.
#[inline]
#[allow(clippy::needless_pass_by_value)]
fn xor_short(buf: ShortSlice<'_>, mask: u64) {
    // SAFETY: we know that a `ShortSlice` fits in a u64
    unsafe {
        let (ptr, len) = (buf.0.as_mut_ptr(), buf.0.len());
        let mut b: u64 = 0;
        #[allow(trivial_casts)]
        copy_nonoverlapping(ptr, &mut b as *mut _ as *mut u8, len);
        b ^= mask;
        #[allow(trivial_casts)]
        copy_nonoverlapping(&b as *const _ as *const u8, ptr, len);
    }
}

// Splits a slice into three parts: an unaligned short head and tail, plus an aligned
// u64 mid section.
#[inline]
fn align_buf(buf: &mut [u8]) -> (ShortSlice<'_>, &mut [u64], ShortSlice<'_>) {
    // Safety: the only invariant to uphold when transmuting &[u8] to &[u64] is alignment,
    // since all bit patterns are valid for both types and there is no destructor.
    // This unsafe block could be avoided by using the `bytemuck` crate,
    // but it's not clear if eliminating one line of unsafe is worth an extra dependency.
    let (head, mid, tail) = unsafe { buf.align_to_mut::<u64>() };
    (ShortSlice(head), mid, ShortSlice(tail))
}

#[cfg(test)]
mod tests {
    use super::apply_mask;

    /// A safe unoptimized mask application.
    fn apply_mask_fallback(buf: &mut [u8], mask: &[u8; 4]) {
        for (i, byte) in buf.iter_mut().enumerate() {
            *byte ^= mask[i & 3];
        }
    }

    #[test]
    fn test_apply_mask() {
        let mask = [0x6d, 0xb6, 0xb2, 0x80];
        let mask_u32 = u32::from_le_bytes(mask);

        let unmasked = vec![
            0xf3, 0x00, 0x01, 0x02, 0x03, 0x80, 0x81, 0x82, 0xff, 0xfe, 0x00, 0x17,
            0x74, 0xf9, 0x12, 0x03,
        ];

        // Check masking with proper alignment.
        {
            let mut masked = unmasked.clone();
            apply_mask_fallback(&mut masked, &mask);

            let mut masked_fast = unmasked.clone();
            apply_mask(&mut masked_fast, mask_u32);

            assert_eq!(masked, masked_fast);
        }

        // Check masking without alignment.
        {
            let mut masked = unmasked.clone();
            apply_mask_fallback(&mut masked[1..], &mask);

            let mut masked_fast = unmasked;
            apply_mask(&mut masked_fast[1..], mask_u32);

            assert_eq!(masked, masked_fast);
        }
    }
}