# mask.rs.html -- source
Source to the Rust file `src/ws/mask.rs`.
Keywords: rust, rustlang, rust-lang

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    //! This is code from [Tungstenite project](https://github.com/snapview/tungstenite-rs)
    #![cfg_attr(feature = "cargo-clippy", 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> {
        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]
    #[cfg_attr(feature = "cargo-clippy", 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);
        }
    }

    #[inline]
    // 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.
    #[cfg_attr(
        feature = "cargo-clippy",
        allow(clippy::needless_pass_by_value)
    )]
    fn xor_short(buf: ShortSlice, mask: u64) {
        // Unsafe: 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);
        }
    }

    #[inline]
    // Unsafe: caller must ensure the buffer has the correct size and alignment
    unsafe fn cast_slice(buf: &mut [u8]) -> &mut [u64] {
        // Assert correct size and alignment in debug builds
        debug_assert!(buf.len().trailing_zeros() >= 3);
        debug_assert!((buf.as_ptr() as usize).trailing_zeros() >= 3);

        slice::from_raw_parts_mut(buf.as_mut_ptr() as *mut u64, buf.len() >> 3)
    }

    #[inline]
    // Splits a slice into three parts: an unaligned short head and tail, plus an aligned
    // u64 mid section.
    fn align_buf(buf: &mut [u8]) -> (ShortSlice, &mut [u64], ShortSlice) {
        let start_ptr = buf.as_ptr() as usize;
        let end_ptr = start_ptr + buf.len();

        // Round *up* to next aligned boundary for start
        let start_aligned = (start_ptr + 7) & !0x7;
        // Round *down* to last aligned boundary for end
        let end_aligned = end_ptr & !0x7;

        if end_aligned >= start_aligned {
            // We have our three segments (head, mid, tail)
            let (tmp, tail) = buf.split_at_mut(end_aligned - start_ptr);
            let (head, mid) = tmp.split_at_mut(start_aligned - start_ptr);

            // Unsafe: we know the middle section is correctly aligned, and the outer
            // sections are smaller than 8 bytes
            unsafe { (ShortSlice::new(head), cast_slice(mid), ShortSlice(tail)) }
        } else {
            // We didn't cross even one aligned boundary!

            // Unsafe: The outer sections are smaller than 8 bytes
            unsafe { (ShortSlice::new(buf), &mut [], ShortSlice::new(&mut [])) }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::apply_mask;
        use byteorder::{ByteOrder, LittleEndian};

        /// 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 = LittleEndian::read_u32(&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.clone();
                apply_mask(&mut masked_fast[1..], mask_u32);

                assert_eq!(masked, masked_fast);
            }
        }
    }

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