merge.rs

use super::BitChunk;

/// Merges 2 [`BitChunk`]s into a single [`BitChunk`] so that the new items represents
/// the bitmap where bits from `next` are placed in `current` according to `offset`.
/// # Panic
/// The caller must ensure that `0 < offset < size_of::<T>() * 8`
/// # Example
/// ```rust,ignore
/// let current = 0b01011001;
/// let next    = 0b01011011;
/// let result = merge_reversed(current, next, 1);
/// assert_eq!(result, 0b10101100);
/// ```
#[inline]
pub fn merge_reversed<T>(mut current: T, mut next: T, offset: usize) -> T
where
    T: BitChunk,
{
    // 8 _bits_:
    // current = [c0, c1, c2, c3, c4, c5, c6, c7]
    // next =    [n0, n1, n2, n3, n4, n5, n6, n7]
    // offset = 3
    // expected = [n5, n6, n7, c0, c1, c2, c3, c4]

    // 1. unset most significants of `next` up to `offset`
    let inverse_offset = size_of::<T>() * 8 - offset;
    next <<= inverse_offset;
    // next    =  [n5, n6, n7, 0 , 0 , 0 , 0 , 0 ]

    // 2. unset least significants of `current` up to `offset`
    current >>= offset;
    // current =  [0 , 0 , 0 , c0, c1, c2, c3, c4]

    current | next
}

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

    #[test]
    fn test_merge_reversed() {
        let current = 0b00000000;
        let next = 0b00000001;
        let result = merge_reversed::<u8>(current, next, 1);
        assert_eq!(result, 0b10000000);

        let current = 0b01011001;
        let next = 0b01011011;
        let result = merge_reversed::<u8>(current, next, 1);
        assert_eq!(result, 0b10101100);
    }

    #[test]
    fn test_merge_reversed_offset2() {
        let current = 0b00000000;
        let next = 0b00000001;
        let result = merge_reversed::<u8>(current, next, 3);
        assert_eq!(result, 0b00100000);
    }
}