#[derive(Debug, Clone, Copy)]
pub struct Image<'a> {
    pub width: u32,
    pub height: u32,
    pub data: &'a [u8],
}

#[derive(Debug)]
pub struct ImageMut<'a> {
    pub width: u32,
    pub height: u32,
    pub data: &'a mut [u8],
}

impl ImageMut<'_> {
    pub fn composite_alpha(&mut self, src: &Image<'_>) {
        assert_eq!(self.width, src.width);
        assert_eq!(self.height, src.height);

        fn fixp_mul(a: u8, b: u8) -> u8 {
            ((a as u16 * b as u16 + 255) >> 8) as u8
        }

        fn alpha_blend(dst: u8, src: u8, alpha: u8) -> u8 {
            fixp_mul(src, alpha) + fixp_mul(dst, 255 - alpha)
        }

        for (dst_rgba, src_rgba) in self.data.chunks_exact_mut(4).zip(src.data.chunks_exact(4)) {
            let src_alpha = src_rgba[3];
            dst_rgba[0] = alpha_blend(dst_rgba[0], src_rgba[0], src_alpha);
            dst_rgba[1] = alpha_blend(dst_rgba[1], src_rgba[1], src_alpha);
            dst_rgba[2] = alpha_blend(dst_rgba[2], src_rgba[2], src_alpha);
            dst_rgba[3] = alpha_blend(dst_rgba[3], src_rgba[3], src_alpha);
        }
    }
}