zoomvtools 1.1.0

#![allow(clippy::undocumented_unsafe_blocks)]

use std::{arch::x86_64::*, num::NonZeroUsize};

use crate::util::Pixel;

#[target_feature(enable = "avx2")]
pub(super) fn reduce_quadratic<T: Pixel>(
    dest: &mut [T],
    src: &[T],
    dest_pitch: NonZeroUsize,
    src_pitch: NonZeroUsize,
    dest_width: NonZeroUsize,
    dest_height: NonZeroUsize,
) {
    // Check the array bounds once at the start of the loop.
    debug_assert!(src.len() >= src_pitch.get() * dest_height.get() * 2);
    debug_assert!(dest.len() >= dest_pitch.get() * dest_height.get());

    match size_of::<T>() {
        1 => unsafe {
            reduce_quadratic_vertical_u8(
                dest.as_mut_ptr() as *mut u8,
                src.as_ptr() as *const u8,
                dest_pitch,
                src_pitch,
                // SAFETY: non-zero constant
                dest_width.saturating_mul(NonZeroUsize::new_unchecked(2)),
                dest_height,
            );
            reduce_quadratic_horizontal_inplace_u8(
                dest.as_mut_ptr() as *mut u8,
                dest_pitch,
                dest_width,
                dest_height,
            );
        },
        2 => unsafe {
            reduce_quadratic_vertical_u16(
                dest.as_mut_ptr() as *mut u16,
                src.as_ptr() as *const u16,
                dest_pitch,
                src_pitch,
                // SAFETY: non-zero constant
                dest_width.saturating_mul(NonZeroUsize::new_unchecked(2)),
                dest_height,
            );
            reduce_quadratic_horizontal_inplace_u16(
                dest.as_mut_ptr() as *mut u16,
                dest_pitch,
                dest_width,
                dest_height,
            );
        },
        _ => unreachable!(),
    }
}

#[target_feature(enable = "avx2")]
// NOTE: Ported from C implementation
unsafe fn reduce_quadratic_vertical_u8(
    dest: *mut u8,
    src: *const u8,
    dest_pitch: NonZeroUsize,
    src_pitch: NonZeroUsize,
    dest_width: NonZeroUsize,
    dest_height: NonZeroUsize,
) {
    let dest_pitch_val = dest_pitch.get();
    let src_pitch_val = src_pitch.get();
    let dest_width_val = dest_width.get();
    let dest_height_val = dest_height.get();

    let w01 = _mm256_set1_epi16(0x0901);
    let w23 = _mm256_set1_epi16(0x1616);
    let w45 = _mm256_set1_epi16(0x0109);
    let round16 = _mm256_set1_epi16(32);

    // Special case for first line - simple averaging
    {
        let dest_row = dest;
        let src_row0 = src;
        let src_row1 = src.add(src_pitch_val);

        let mut x = 0;
        // Process 32 pixels at a time
        while x + 32 <= dest_width_val {
            let a = _mm256_loadu_si256(src_row0.add(x) as *const __m256i);
            let b = _mm256_loadu_si256(src_row1.add(x) as *const __m256i);

            // Convert to u16 for calculation
            let a_lo = _mm256_unpacklo_epi8(a, _mm256_setzero_si256());
            let a_hi = _mm256_unpackhi_epi8(a, _mm256_setzero_si256());
            let b_lo = _mm256_unpacklo_epi8(b, _mm256_setzero_si256());
            let b_hi = _mm256_unpackhi_epi8(b, _mm256_setzero_si256());

            // Add with rounding: (a + b + 1) / 2
            let sum_lo = _mm256_add_epi16(a_lo, b_lo);
            let sum_hi = _mm256_add_epi16(a_hi, b_hi);
            let one = _mm256_set1_epi16(1);
            let sum_lo_rounded = _mm256_add_epi16(sum_lo, one);
            let sum_hi_rounded = _mm256_add_epi16(sum_hi, one);
            let result_lo = _mm256_srli_epi16(sum_lo_rounded, 1);
            let result_hi = _mm256_srli_epi16(sum_hi_rounded, 1);

            // Pack back to u8
            let result = _mm256_packus_epi16(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, result);

            x += 32;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let a = *src_row0.add(x) as u16;
            let b = *src_row1.add(x) as u16;
            *dest_row.add(x) = ((a + b + 1) / 2) as u8;
            x += 1;
        }
    }

    // Middle lines - full quadratic filter
    for y in 1..(dest_height_val - 1) {
        let dest_row = dest.add(y * dest_pitch_val);
        let src_row_offset = y * 2 * src_pitch_val;
        let src_m2 = src.add(src_row_offset - src_pitch_val * 2);
        let src_m1 = src.add(src_row_offset - src_pitch_val);
        let src_p0 = src.add(src_row_offset);
        let src_p1 = src.add(src_row_offset + src_pitch_val);
        let src_p2 = src.add(src_row_offset + src_pitch_val * 2);
        let src_p3 = src.add(src_row_offset + src_pitch_val * 3);

        let mut x = 0;

        while x + 32 <= dest_width_val {
            let m0 = _mm256_loadu_si256(src_m2.add(x) as *const __m256i);
            let m1 = _mm256_loadu_si256(src_m1.add(x) as *const __m256i);
            let m2 = _mm256_loadu_si256(src_p0.add(x) as *const __m256i);
            let m3 = _mm256_loadu_si256(src_p1.add(x) as *const __m256i);
            let m4 = _mm256_loadu_si256(src_p2.add(x) as *const __m256i);
            let m5 = _mm256_loadu_si256(src_p3.add(x) as *const __m256i);

            let p01_lo = _mm256_unpacklo_epi8(m0, m1);
            let p01_hi = _mm256_unpackhi_epi8(m0, m1);
            let p23_lo = _mm256_unpacklo_epi8(m2, m3);
            let p23_hi = _mm256_unpackhi_epi8(m2, m3);
            let p45_lo = _mm256_unpacklo_epi8(m4, m5);
            let p45_hi = _mm256_unpackhi_epi8(m4, m5);

            let term01_lo = _mm256_maddubs_epi16(p01_lo, w01);
            let term01_hi = _mm256_maddubs_epi16(p01_hi, w01);
            let term23_lo = _mm256_maddubs_epi16(p23_lo, w23);
            let term23_hi = _mm256_maddubs_epi16(p23_hi, w23);
            let term45_lo = _mm256_maddubs_epi16(p45_lo, w45);
            let term45_hi = _mm256_maddubs_epi16(p45_hi, w45);

            let sum_lo = _mm256_add_epi16(_mm256_add_epi16(term01_lo, term23_lo), term45_lo);
            let sum_hi = _mm256_add_epi16(_mm256_add_epi16(term01_hi, term23_hi), term45_hi);

            let result_lo = _mm256_srli_epi16(_mm256_add_epi16(sum_lo, round16), 6);
            let result_hi = _mm256_srli_epi16(_mm256_add_epi16(sum_hi, round16), 6);

            let result = _mm256_packus_epi16(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, result);

            x += 32;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let mut m0 = *src_m2.add(x) as u16;
            let mut m1 = *src_m1.add(x) as u16;
            let mut m2 = *src_p0.add(x) as u16;
            let m3 = *src_p1.add(x) as u16;
            let m4 = *src_p2.add(x) as u16;
            let m5 = *src_p3.add(x) as u16;

            m2 = (m2 + m3) * 22;
            m1 = (m1 + m4) * 9;
            m0 += m5 + m2 + m1 + 32;
            m0 >>= 6;

            *dest_row.add(x) = (m0.min(255)) as u8;
            x += 1;
        }
    }

    // Special case for last line - simple averaging
    if dest_height_val > 1 {
        let dest_row = dest.add((dest_height_val - 1) * dest_pitch_val);
        let src_row_offset = (dest_height_val - 1) * 2 * src_pitch_val;
        let src_row0 = src.add(src_row_offset);
        let src_row1 = src.add(src_row_offset + src_pitch_val);

        let mut x = 0;
        // Process 32 pixels at a time
        while x + 32 <= dest_width_val {
            let a = _mm256_loadu_si256(src_row0.add(x) as *const __m256i);
            let b = _mm256_loadu_si256(src_row1.add(x) as *const __m256i);

            // Convert to u16 for calculation
            let a_lo = _mm256_unpacklo_epi8(a, _mm256_setzero_si256());
            let a_hi = _mm256_unpackhi_epi8(a, _mm256_setzero_si256());
            let b_lo = _mm256_unpacklo_epi8(b, _mm256_setzero_si256());
            let b_hi = _mm256_unpackhi_epi8(b, _mm256_setzero_si256());

            // Add with rounding: (a + b + 1) / 2
            let sum_lo = _mm256_add_epi16(a_lo, b_lo);
            let sum_hi = _mm256_add_epi16(a_hi, b_hi);
            let one = _mm256_set1_epi16(1);
            let sum_lo_rounded = _mm256_add_epi16(sum_lo, one);
            let sum_hi_rounded = _mm256_add_epi16(sum_hi, one);
            let result_lo = _mm256_srli_epi16(sum_lo_rounded, 1);
            let result_hi = _mm256_srli_epi16(sum_hi_rounded, 1);

            // Pack back to u8
            let result = _mm256_packus_epi16(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, result);

            x += 32;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let a = *src_row0.add(x) as u16;
            let b = *src_row1.add(x) as u16;
            *dest_row.add(x) = ((a + b + 1) / 2) as u8;
            x += 1;
        }
    }
}

#[target_feature(enable = "avx2")]
// NOTE: Ported from C implementation
unsafe fn reduce_quadratic_horizontal_inplace_u8(
    dest: *mut u8,
    dest_pitch: NonZeroUsize,
    dest_width: NonZeroUsize,
    dest_height: NonZeroUsize,
) {
    let dest_pitch_val = dest_pitch.get();
    let dest_width_val = dest_width.get();
    let dest_height_val = dest_height.get();

    let w01 = _mm256_set1_epi16(0x0901);
    let w23 = _mm256_set1_epi16(0x1616);
    let w45 = _mm256_set1_epi16(0x0109);
    let round16 = _mm256_set1_epi16(32);

    for y in 0..dest_height_val {
        let dest_row = dest.add(y * dest_pitch_val);

        // Special case start of line
        let a = *dest_row as u16;
        let b = *dest_row.add(1) as u16;
        let src0 = ((a + b + 1) / 2) as u8;

        let middle_end = dest_width_val - 1;
        let mut x = 1;

        while x + 16 <= middle_end {
            let p01 = _mm256_loadu_si256(dest_row.add(x * 2 - 2) as *const __m256i);
            let p23 = _mm256_loadu_si256(dest_row.add(x * 2) as *const __m256i);
            let p45 = _mm256_loadu_si256(dest_row.add(x * 2 + 2) as *const __m256i);

            let term01 = _mm256_maddubs_epi16(p01, w01);
            let term23 = _mm256_maddubs_epi16(p23, w23);
            let term45 = _mm256_maddubs_epi16(p45, w45);

            let sum = _mm256_add_epi16(_mm256_add_epi16(term01, term23), term45);
            let reduced = _mm256_srli_epi16(_mm256_add_epi16(sum, round16), 6);

            let packed = _mm256_packus_epi16(reduced, reduced);
            let reordered = _mm256_permute4x64_epi64(packed, 0b0000_1000);
            _mm_storeu_si128(
                dest_row.add(x) as *mut __m128i,
                _mm256_castsi256_si128(reordered),
            );

            x += 16;
        }

        while x < middle_end {
            let mut m0 = *dest_row.add(x * 2 - 2) as u16;
            let mut m1 = *dest_row.add(x * 2 - 1) as u16;
            let mut m2 = *dest_row.add(x * 2) as u16;
            let m3 = *dest_row.add(x * 2 + 1) as u16;
            let m4 = *dest_row.add(x * 2 + 2) as u16;
            let m5 = *dest_row.add(x * 2 + 3) as u16;

            m2 = (m2 + m3) * 22;
            m1 = (m1 + m4) * 9;
            m0 += m5 + m2 + m1 + 32;
            m0 >>= 6;

            *dest_row.add(x) = (m0.min(255)) as u8;
            x += 1;
        }

        *dest_row = src0;

        // Special case end of line
        if dest_width_val > 1 {
            let x = dest_width_val - 1;
            let a = *dest_row.add(x * 2) as u16;
            let b = *dest_row.add(x * 2 + 1) as u16;
            *dest_row.add(x) = ((a + b + 1) / 2) as u8;
        }
    }
}

#[target_feature(enable = "avx2")]
// NOTE: Custom implementation. No C implementation exists.
unsafe fn reduce_quadratic_vertical_u16(
    dest: *mut u16,
    src: *const u16,
    dest_pitch: NonZeroUsize,
    src_pitch: NonZeroUsize,
    dest_width: NonZeroUsize,
    dest_height: NonZeroUsize,
) {
    let dest_pitch_val = dest_pitch.get();
    let src_pitch_val = src_pitch.get();
    let dest_width_val = dest_width.get();
    let dest_height_val = dest_height.get();

    let zero = _mm256_setzero_si256();
    let weight_9 = _mm256_set1_epi32(9);
    let weight_22 = _mm256_set1_epi32(22);
    let bias_32 = _mm256_set1_epi32(32);

    // Special case for first line - simple averaging
    {
        let dest_row = dest;
        let src_row0 = src;
        let src_row1 = src.add(src_pitch_val);

        let mut x = 0;
        // Process 16 pixels at a time
        while x + 16 <= dest_width_val {
            let a = _mm256_loadu_si256(src_row0.add(x) as *const __m256i);
            let b = _mm256_loadu_si256(src_row1.add(x) as *const __m256i);

            // Convert to u32 for calculation to avoid overflow
            let a_lo = _mm256_unpacklo_epi16(a, _mm256_setzero_si256());
            let a_hi = _mm256_unpackhi_epi16(a, _mm256_setzero_si256());
            let b_lo = _mm256_unpacklo_epi16(b, _mm256_setzero_si256());
            let b_hi = _mm256_unpackhi_epi16(b, _mm256_setzero_si256());

            // Add with rounding: (a + b + 1) / 2
            let sum_lo = _mm256_add_epi32(a_lo, b_lo);
            let sum_hi = _mm256_add_epi32(a_hi, b_hi);
            let one = _mm256_set1_epi32(1);
            let sum_lo_rounded = _mm256_add_epi32(sum_lo, one);
            let sum_hi_rounded = _mm256_add_epi32(sum_hi, one);
            let result_lo = _mm256_srli_epi32(sum_lo_rounded, 1);
            let result_hi = _mm256_srli_epi32(sum_hi_rounded, 1);

            // Pack back to u16
            let result = _mm256_packus_epi32(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, result);

            x += 16;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let a = *src_row0.add(x) as u32;
            let b = *src_row1.add(x) as u32;
            *dest_row.add(x) = ((a + b + 1) / 2) as u16;
            x += 1;
        }
    }

    // Middle lines - full quadratic filter
    for y in 1..(dest_height_val - 1) {
        let dest_row = dest.add(y * dest_pitch_val);
        let src_row_offset = y * 2 * src_pitch_val;
        let src_m2 = src.add(src_row_offset - src_pitch_val * 2);
        let src_m1 = src.add(src_row_offset - src_pitch_val);
        let src_p0 = src.add(src_row_offset);
        let src_p1 = src.add(src_row_offset + src_pitch_val);
        let src_p2 = src.add(src_row_offset + src_pitch_val * 2);
        let src_p3 = src.add(src_row_offset + src_pitch_val * 3);

        let mut x = 0;
        while x + 16 <= dest_width_val {
            let m0 = _mm256_loadu_si256(src_m2.add(x) as *const __m256i);
            let m1 = _mm256_loadu_si256(src_m1.add(x) as *const __m256i);
            let m2 = _mm256_loadu_si256(src_p0.add(x) as *const __m256i);
            let m3 = _mm256_loadu_si256(src_p1.add(x) as *const __m256i);
            let m4 = _mm256_loadu_si256(src_p2.add(x) as *const __m256i);
            let m5 = _mm256_loadu_si256(src_p3.add(x) as *const __m256i);

            let m0_lo = _mm256_unpacklo_epi16(m0, zero);
            let m1_lo = _mm256_unpacklo_epi16(m1, zero);
            let m2_lo = _mm256_unpacklo_epi16(m2, zero);
            let m3_lo = _mm256_unpacklo_epi16(m3, zero);
            let m4_lo = _mm256_unpacklo_epi16(m4, zero);
            let m5_lo = _mm256_unpacklo_epi16(m5, zero);

            let sum_05_lo = _mm256_add_epi32(m0_lo, m5_lo);
            let sum_14_lo = _mm256_mullo_epi32(_mm256_add_epi32(m1_lo, m4_lo), weight_9);
            let sum_23_lo = _mm256_mullo_epi32(_mm256_add_epi32(m2_lo, m3_lo), weight_22);
            let result_lo = _mm256_srli_epi32(
                _mm256_add_epi32(
                    _mm256_add_epi32(sum_05_lo, sum_14_lo),
                    _mm256_add_epi32(sum_23_lo, bias_32),
                ),
                6,
            );

            let m0_hi = _mm256_unpackhi_epi16(m0, zero);
            let m1_hi = _mm256_unpackhi_epi16(m1, zero);
            let m2_hi = _mm256_unpackhi_epi16(m2, zero);
            let m3_hi = _mm256_unpackhi_epi16(m3, zero);
            let m4_hi = _mm256_unpackhi_epi16(m4, zero);
            let m5_hi = _mm256_unpackhi_epi16(m5, zero);

            let sum_05_hi = _mm256_add_epi32(m0_hi, m5_hi);
            let sum_14_hi = _mm256_mullo_epi32(_mm256_add_epi32(m1_hi, m4_hi), weight_9);
            let sum_23_hi = _mm256_mullo_epi32(_mm256_add_epi32(m2_hi, m3_hi), weight_22);
            let result_hi = _mm256_srli_epi32(
                _mm256_add_epi32(
                    _mm256_add_epi32(sum_05_hi, sum_14_hi),
                    _mm256_add_epi32(sum_23_hi, bias_32),
                ),
                6,
            );

            let packed_result = _mm256_packus_epi32(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, packed_result);

            x += 16;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let mut m0 = *src_m2.add(x) as u32;
            let mut m1 = *src_m1.add(x) as u32;
            let mut m2 = *src_p0.add(x) as u32;
            let m3 = *src_p1.add(x) as u32;
            let m4 = *src_p2.add(x) as u32;
            let m5 = *src_p3.add(x) as u32;

            m2 = (m2 + m3) * 22;
            m1 = (m1 + m4) * 9;
            m0 += m5 + m2 + m1 + 32;
            m0 >>= 6;

            *dest_row.add(x) = (m0.min(65535)) as u16;
            x += 1;
        }
    }

    // Special case for last line - simple averaging
    if dest_height_val > 1 {
        let dest_row = dest.add((dest_height_val - 1) * dest_pitch_val);
        let src_row_offset = (dest_height_val - 1) * 2 * src_pitch_val;
        let src_row0 = src.add(src_row_offset);
        let src_row1 = src.add(src_row_offset + src_pitch_val);

        let mut x = 0;
        // Process 16 pixels at a time
        while x + 16 <= dest_width_val {
            let a = _mm256_loadu_si256(src_row0.add(x) as *const __m256i);
            let b = _mm256_loadu_si256(src_row1.add(x) as *const __m256i);

            // Convert to u32 for calculation to avoid overflow
            let a_lo = _mm256_unpacklo_epi16(a, _mm256_setzero_si256());
            let a_hi = _mm256_unpackhi_epi16(a, _mm256_setzero_si256());
            let b_lo = _mm256_unpacklo_epi16(b, _mm256_setzero_si256());
            let b_hi = _mm256_unpackhi_epi16(b, _mm256_setzero_si256());

            // Add with rounding: (a + b + 1) / 2
            let sum_lo = _mm256_add_epi32(a_lo, b_lo);
            let sum_hi = _mm256_add_epi32(a_hi, b_hi);
            let one = _mm256_set1_epi32(1);
            let sum_lo_rounded = _mm256_add_epi32(sum_lo, one);
            let sum_hi_rounded = _mm256_add_epi32(sum_hi, one);
            let result_lo = _mm256_srli_epi32(sum_lo_rounded, 1);
            let result_hi = _mm256_srli_epi32(sum_hi_rounded, 1);

            // Pack back to u16
            let result = _mm256_packus_epi32(result_lo, result_hi);
            _mm256_storeu_si256(dest_row.add(x) as *mut __m256i, result);

            x += 16;
        }

        // Handle remaining pixels
        while x < dest_width_val {
            let a = *src_row0.add(x) as u32;
            let b = *src_row1.add(x) as u32;
            *dest_row.add(x) = ((a + b + 1) / 2) as u16;
            x += 1;
        }
    }
}

#[target_feature(enable = "avx2")]
// NOTE: Custom implementation. No C implementation exists.
unsafe fn reduce_quadratic_horizontal_inplace_u16(
    dest: *mut u16,
    dest_pitch: NonZeroUsize,
    dest_width: NonZeroUsize,
    dest_height: NonZeroUsize,
) {
    let dest_pitch_val = dest_pitch.get();
    let dest_width_val = dest_width.get();
    let dest_height_val = dest_height.get();

    let zero = _mm256_setzero_si256();
    let mask_lo16 = _mm256_set1_epi32(0x0000_FFFF);
    let weight_9 = _mm256_set1_epi32(9);
    let weight_22 = _mm256_set1_epi32(22);
    let bias_32 = _mm256_set1_epi32(32);

    for y in 0..dest_height_val {
        let dest_row = dest.add(y * dest_pitch_val);

        // Special case start of line
        let a = *dest_row as u32;
        let b = *dest_row.add(1) as u32;
        let src0 = ((a + b + 1) / 2) as u16;

        let middle_end = dest_width_val - 1;
        let mut x = 1;

        while x + 8 <= middle_end {
            let v01 = _mm256_loadu_si256(dest_row.add(x * 2 - 2) as *const __m256i);
            let v23 = _mm256_loadu_si256(dest_row.add(x * 2) as *const __m256i);
            let v45 = _mm256_loadu_si256(dest_row.add(x * 2 + 2) as *const __m256i);

            let m0 = _mm256_and_si256(v01, mask_lo16);
            let m1 = _mm256_srli_epi32(v01, 16);
            let m2 = _mm256_and_si256(v23, mask_lo16);
            let m3 = _mm256_srli_epi32(v23, 16);
            let m4 = _mm256_and_si256(v45, mask_lo16);
            let m5 = _mm256_srli_epi32(v45, 16);

            let sum_05 = _mm256_add_epi32(m0, m5);
            let sum_14 = _mm256_mullo_epi32(_mm256_add_epi32(m1, m4), weight_9);
            let sum_23 = _mm256_mullo_epi32(_mm256_add_epi32(m2, m3), weight_22);
            let result = _mm256_srli_epi32(
                _mm256_add_epi32(
                    _mm256_add_epi32(sum_05, sum_14),
                    _mm256_add_epi32(sum_23, bias_32),
                ),
                6,
            );

            let packed = _mm256_packus_epi32(result, zero);
            let lower_half = _mm256_castsi256_si128(packed);
            let upper_half = _mm256_extracti128_si256(packed, 1);
            let combined = _mm_unpacklo_epi64(lower_half, upper_half);
            _mm_storeu_si128(dest_row.add(x) as *mut __m128i, combined);

            x += 8;
        }

        while x < middle_end {
            let mut m0 = *dest_row.add(x * 2 - 2) as u32;
            let mut m1 = *dest_row.add(x * 2 - 1) as u32;
            let mut m2 = *dest_row.add(x * 2) as u32;
            let m3 = *dest_row.add(x * 2 + 1) as u32;
            let m4 = *dest_row.add(x * 2 + 2) as u32;
            let m5 = *dest_row.add(x * 2 + 3) as u32;

            m2 = (m2 + m3) * 22;
            m1 = (m1 + m4) * 9;
            m0 += m5 + m2 + m1 + 32;
            m0 >>= 6;

            *dest_row.add(x) = (m0.min(65535)) as u16;
            x += 1;
        }

        *dest_row = src0;

        // Special case end of line
        if dest_width_val > 1 {
            let x = dest_width_val - 1;
            let a = *dest_row.add(x * 2) as u32;
            let b = *dest_row.add(x * 2 + 1) as u32;
            *dest_row.add(x) = ((a + b + 1) / 2) as u16;
        }
    }
}