zenresize 0.3.1

//! Scalar fallback convolution kernels.
#![allow(clippy::too_many_arguments)]

use crate::weights::{F32WeightTable, I16_PRECISION, I16WeightTable};
use archmage::ScalarToken;

/// Horizontal convolution: filter one row of f32 pixels, scalar fallback.
pub(crate) fn filter_h_row_f32_scalar(
    _token: ScalarToken,
    input: &[f32],
    output: &mut [f32],
    weights: &F32WeightTable,
    channels: usize,
) {
    let out_width = weights.len();

    for out_x in 0..out_width {
        let left = weights.left[out_x] as usize;
        let w = weights.weights(out_x);
        let out_offset = out_x * channels;

        // Zero-initialize output pixel
        for c in 0..channels {
            output[out_offset + c] = 0.0;
        }

        // Accumulate weighted contributions
        for (t, &weight) in w.iter().enumerate() {
            let in_offset = (left + t) * channels;
            for c in 0..channels {
                output[out_offset + c] += input[in_offset + c] * weight;
            }
        }
    }
}

/// Vertical convolution: combine rows into one output row, scalar fallback.
pub(crate) fn filter_v_row_f32_scalar(
    _token: ScalarToken,
    rows: &[&[f32]],
    output: &mut [f32],
    weights: &[f32],
) {
    let width = output.len();
    debug_assert_eq!(rows.len(), weights.len());

    // Zero-initialize output
    for v in output.iter_mut() {
        *v = 0.0;
    }

    // Accumulate weighted rows
    for (row, &weight) in rows.iter().zip(weights.iter()) {
        debug_assert!(row.len() >= width);
        for x in 0..width {
            output[x] += row[x] * weight;
        }
    }
}

/// Convert u8 → f32 (divide by 255), scalar fallback.
pub(crate) fn u8_to_f32_row_scalar(_token: ScalarToken, input: &[u8], output: &mut [f32]) {
    debug_assert_eq!(input.len(), output.len());
    for (inp, out) in input.iter().zip(output.iter_mut()) {
        *out = *inp as f32 * (1.0 / 255.0);
    }
}

/// Convert f32 → u8 (multiply by 255, round, clamp), scalar fallback.
pub(crate) fn f32_to_u8_row_scalar(_token: ScalarToken, input: &[f32], output: &mut [u8]) {
    debug_assert_eq!(input.len(), output.len());
    for (inp, out) in input.iter().zip(output.iter_mut()) {
        *out = (*inp * 255.0 + 0.5).clamp(0.0, 255.0) as u8;
    }
}

/// Premultiply alpha in-place, scalar fallback.
pub(crate) fn premultiply_alpha_row_scalar(_token: ScalarToken, row: &mut [f32]) {
    for pixel in row.chunks_exact_mut(4) {
        let a = pixel[3];
        pixel[0] *= a;
        pixel[1] *= a;
        pixel[2] *= a;
    }
}

/// Unpremultiply alpha in-place, scalar fallback.
pub(crate) fn unpremultiply_alpha_row_scalar(_token: ScalarToken, row: &mut [f32]) {
    for pixel in row.chunks_exact_mut(4) {
        let a = pixel[3];
        if a > 1.0 / 1024.0 {
            let inv_a = 1.0 / a;
            pixel[0] *= inv_a;
            pixel[1] *= inv_a;
            pixel[2] *= inv_a;
        }
    }
}

/// Integer horizontal convolution: u8 input → u8 output, scalar fallback.
pub(crate) fn filter_h_u8_i16_scalar(
    _token: ScalarToken,
    input: &[u8],
    output: &mut [u8],
    weights: &I16WeightTable,
    channels: usize,
) {
    let out_width = weights.len();

    for out_x in 0..out_width {
        let left = weights.left[out_x] as usize;
        let w = weights.weights(out_x);
        let out_base = out_x * channels;

        for c in 0..channels {
            let mut acc: i32 = 0;
            for (t, &weight) in w.iter().enumerate() {
                acc += input[(left + t) * channels + c] as i32 * weight as i32;
            }
            let rounded = (acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION;
            output[out_base + c] = rounded.clamp(0, 255) as u8;
        }
    }
}

/// Integer horizontal convolution: u8 input → i16 output (unclamped), scalar fallback.
/// For sRGB path with i16 intermediate: preserves Lanczos ringing without [0,255] clamp.
pub(crate) fn filter_h_u8_to_i16_scalar(
    _token: ScalarToken,
    input: &[u8],
    output: &mut [i16],
    weights: &I16WeightTable,
    channels: usize,
) {
    let out_width = weights.len();

    for out_x in 0..out_width {
        let left = weights.left[out_x] as usize;
        let w = weights.weights(out_x);
        let out_base = out_x * channels;

        for c in 0..channels {
            let mut acc: i32 = 0;
            for (t, &weight) in w.iter().enumerate() {
                acc += input[(left + t) * channels + c] as i32 * weight as i32;
            }
            let rounded = (acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION;
            output[out_base + c] = rounded as i16;
        }
    }
}

/// 4-row batch horizontal convolution: u8 input → i16 output (unclamped), scalar fallback.
#[allow(dead_code)]
pub(crate) fn filter_h_u8_to_i16_4rows_scalar(
    _token: ScalarToken,
    in0: &[u8],
    in1: &[u8],
    in2: &[u8],
    in3: &[u8],
    out0: &mut [i16],
    out1: &mut [i16],
    out2: &mut [i16],
    out3: &mut [i16],
    weights: &I16WeightTable,
) {
    let ch = 4;
    filter_h_u8_to_i16_scalar(_token, in0, out0, weights, ch);
    filter_h_u8_to_i16_scalar(_token, in1, out1, weights, ch);
    filter_h_u8_to_i16_scalar(_token, in2, out2, weights, ch);
    filter_h_u8_to_i16_scalar(_token, in3, out3, weights, ch);
}

/// 4-row batch horizontal convolution, scalar fallback.
/// Just calls filter_h_u8_i16_scalar 4 times.
pub(crate) fn filter_h_u8_i16_4rows_scalar(
    _token: ScalarToken,
    in0: &[u8],
    in1: &[u8],
    in2: &[u8],
    in3: &[u8],
    out0: &mut [u8],
    out1: &mut [u8],
    out2: &mut [u8],
    out3: &mut [u8],
    weights: &I16WeightTable,
) {
    let ch = 4;
    filter_h_u8_i16_scalar(_token, in0, out0, weights, ch);
    filter_h_u8_i16_scalar(_token, in1, out1, weights, ch);
    filter_h_u8_i16_scalar(_token, in2, out2, weights, ch);
    filter_h_u8_i16_scalar(_token, in3, out3, weights, ch);
}

/// Batch vertical filter for all output rows, scalar fallback.
pub(crate) fn filter_v_all_u8_i16_scalar(
    _token: ScalarToken,
    intermediate: &[u8],
    output: &mut [u8],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &crate::weights::I16WeightTable,
) {
    let in_h_i32 = in_h as i32 - 1;

    let mut out_y = 0;
    while out_y < out_h {
        let left = weights.left[out_y];
        let tap_count = weights.tap_count(out_y);
        let w_a = weights.weights(out_y);

        let batch2 = out_y + 1 < out_h
            && weights.left[out_y + 1] == left
            && weights.tap_count(out_y + 1) == tap_count;

        if batch2 {
            let w_b = weights.weights(out_y + 1);
            let out_start_a = out_y * h_row_len;
            let out_start_b = (out_y + 1) * h_row_len;

            for x in 0..h_row_len {
                let mut acc_a: i32 = 0;
                let mut acc_b: i32 = 0;
                for t in 0..tap_count {
                    let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                    let v = intermediate[in_y * h_row_len + x] as i32;
                    acc_a += v * w_a[t] as i32;
                    acc_b += v * w_b[t] as i32;
                }
                output[out_start_a + x] =
                    ((acc_a + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
                output[out_start_b + x] =
                    ((acc_b + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
            }
            out_y += 2;
        } else {
            let out_start = out_y * h_row_len;
            for x in 0..h_row_len {
                let mut acc: i32 = 0;
                for (t, &weight) in w_a[..tap_count].iter().enumerate() {
                    let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                    acc += intermediate[in_y * h_row_len + x] as i32 * weight as i32;
                }
                output[out_start + x] =
                    ((acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
            }
            out_y += 1;
        }
    }
}

/// Tiled batch vertical filter for all output rows, scalar fallback.
///
/// Processes column tiles of width `tile_width` to improve L1 cache reuse
/// across consecutive output rows that share overlapping input row windows.
#[allow(dead_code)]
pub(crate) fn filter_v_all_u8_i16_tiled_scalar(
    _token: ScalarToken,
    intermediate: &[u8],
    output: &mut [u8],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &crate::weights::I16WeightTable,
    tile_width: usize,
) {
    let in_h_i32 = in_h as i32 - 1;

    for tile_start in (0..h_row_len).step_by(tile_width) {
        let tile_end = (tile_start + tile_width).min(h_row_len);

        let mut out_y = 0;
        while out_y < out_h {
            let left = weights.left[out_y];
            let tap_count = weights.tap_count(out_y);
            let w_a = weights.weights(out_y);

            let batch2 = out_y + 1 < out_h
                && weights.left[out_y + 1] == left
                && weights.tap_count(out_y + 1) == tap_count;

            if batch2 {
                let w_b = weights.weights(out_y + 1);
                let out_start_a = out_y * h_row_len;
                let out_start_b = (out_y + 1) * h_row_len;

                for x in tile_start..tile_end {
                    let mut acc_a: i32 = 0;
                    let mut acc_b: i32 = 0;
                    for t in 0..tap_count {
                        let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                        let v = intermediate[in_y * h_row_len + x] as i32;
                        acc_a += v * w_a[t] as i32;
                        acc_b += v * w_b[t] as i32;
                    }
                    output[out_start_a + x] =
                        ((acc_a + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
                    output[out_start_b + x] =
                        ((acc_b + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
                }
                out_y += 2;
            } else {
                let out_start = out_y * h_row_len;
                for x in tile_start..tile_end {
                    let mut acc: i32 = 0;
                    for (t, &weight) in w_a[..tap_count].iter().enumerate() {
                        let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                        acc += intermediate[in_y * h_row_len + x] as i32 * weight as i32;
                    }
                    output[out_start + x] =
                        ((acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION).clamp(0, 255) as u8;
                }
                out_y += 1;
            }
        }
    }
}

// ============================================================================
// Integer i16→i16 path (linear-light i12 values 0-4095)
// ============================================================================

/// Integer horizontal convolution: i16 input → i16 output, scalar fallback.
/// For linear-light i12 path (values 0-4095).
pub(crate) fn filter_h_i16_i16_scalar(
    _token: ScalarToken,
    input: &[i16],
    output: &mut [i16],
    weights: &I16WeightTable,
    channels: usize,
) {
    let out_width = weights.len();

    for out_x in 0..out_width {
        let left = weights.left[out_x] as usize;
        let w = weights.weights(out_x);
        let out_base = out_x * channels;

        for c in 0..channels {
            let mut acc: i32 = 0;
            for (t, &weight) in w.iter().enumerate() {
                acc += input[(left + t) * channels + c] as i32 * weight as i32;
            }
            let rounded = (acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION;
            output[out_base + c] = rounded as i16;
        }
    }
}

/// Batch vertical filter for all output rows (i16 intermediate → i16 output), scalar fallback.
/// For linear-light i12 path (values 0-4095).
pub(crate) fn filter_v_all_i16_i16_scalar(
    _token: ScalarToken,
    intermediate: &[i16],
    output: &mut [i16],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &crate::weights::I16WeightTable,
) {
    let in_h_i32 = in_h as i32 - 1;

    let mut out_y = 0;
    while out_y < out_h {
        let left = weights.left[out_y];
        let tap_count = weights.tap_count(out_y);
        let w_a = weights.weights(out_y);

        let batch2 = out_y + 1 < out_h
            && weights.left[out_y + 1] == left
            && weights.tap_count(out_y + 1) == tap_count;

        if batch2 {
            let w_b = weights.weights(out_y + 1);
            let out_start_a = out_y * h_row_len;
            let out_start_b = (out_y + 1) * h_row_len;

            for x in 0..h_row_len {
                let mut acc_a: i32 = 0;
                let mut acc_b: i32 = 0;
                for t in 0..tap_count {
                    let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                    let v = intermediate[in_y * h_row_len + x] as i32;
                    acc_a += v * w_a[t] as i32;
                    acc_b += v * w_b[t] as i32;
                }
                output[out_start_a + x] =
                    ((acc_a + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
                output[out_start_b + x] =
                    ((acc_b + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
            }
            out_y += 2;
        } else {
            let out_start = out_y * h_row_len;
            for x in 0..h_row_len {
                let mut acc: i32 = 0;
                for (t, &weight) in w_a[..tap_count].iter().enumerate() {
                    let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                    acc += intermediate[in_y * h_row_len + x] as i32 * weight as i32;
                }
                output[out_start + x] =
                    ((acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
            }
            out_y += 1;
        }
    }
}

/// 2D-tiled batch vertical filter: column tiles × row bands.
///
/// Groups consecutive output rows into bands whose combined input row span
/// fits in L1 cache, then processes each band in column tiles.
#[allow(dead_code)]
pub(crate) fn filter_v_all_u8_i16_2d_tiled_scalar(
    _token: ScalarToken,
    intermediate: &[u8],
    output: &mut [u8],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &crate::weights::I16WeightTable,
    tile_width: usize,
    band_size: usize,
) {
    let in_h_i32 = in_h as i32 - 1;

    // Process output rows in bands
    let mut band_start = 0;
    while band_start < out_h {
        let band_end = (band_start + band_size).min(out_h);

        // Process column tiles within each band
        for tile_start in (0..h_row_len).step_by(tile_width) {
            let tile_end = (tile_start + tile_width).min(h_row_len);

            let mut out_y = band_start;
            while out_y < band_end {
                let left = weights.left[out_y];
                let tap_count = weights.tap_count(out_y);
                let w_a = weights.weights(out_y);

                let batch2 = out_y + 1 < band_end
                    && weights.left[out_y + 1] == left
                    && weights.tap_count(out_y + 1) == tap_count;

                if batch2 {
                    let w_b = weights.weights(out_y + 1);
                    let out_start_a = out_y * h_row_len;
                    let out_start_b = (out_y + 1) * h_row_len;

                    for x in tile_start..tile_end {
                        let mut acc_a: i32 = 0;
                        let mut acc_b: i32 = 0;
                        for t in 0..tap_count {
                            let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                            let v = intermediate[in_y * h_row_len + x] as i32;
                            acc_a += v * w_a[t] as i32;
                            acc_b += v * w_b[t] as i32;
                        }
                        output[out_start_a + x] = ((acc_a + (1 << (I16_PRECISION - 1)))
                            >> I16_PRECISION)
                            .clamp(0, 255) as u8;
                        output[out_start_b + x] = ((acc_b + (1 << (I16_PRECISION - 1)))
                            >> I16_PRECISION)
                            .clamp(0, 255) as u8;
                    }
                    out_y += 2;
                } else {
                    let out_start = out_y * h_row_len;
                    for x in tile_start..tile_end {
                        let mut acc: i32 = 0;
                        for (t, &weight) in w_a[..tap_count].iter().enumerate() {
                            let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                            acc += intermediate[in_y * h_row_len + x] as i32 * weight as i32;
                        }
                        output[out_start + x] = ((acc + (1 << (I16_PRECISION - 1)))
                            >> I16_PRECISION)
                            .clamp(0, 255) as u8;
                    }
                    out_y += 1;
                }
            }
        }

        band_start = band_end;
    }
}

/// Tiled batch vertical filter for all output rows (i16→i16), scalar fallback.
#[allow(dead_code)]
pub(crate) fn filter_v_all_i16_i16_tiled_scalar(
    _token: ScalarToken,
    intermediate: &[i16],
    output: &mut [i16],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &crate::weights::I16WeightTable,
    tile_width: usize,
) {
    let in_h_i32 = in_h as i32 - 1;

    for tile_start in (0..h_row_len).step_by(tile_width) {
        let tile_end = (tile_start + tile_width).min(h_row_len);

        let mut out_y = 0;
        while out_y < out_h {
            let left = weights.left[out_y];
            let tap_count = weights.tap_count(out_y);
            let w_a = weights.weights(out_y);

            let batch2 = out_y + 1 < out_h
                && weights.left[out_y + 1] == left
                && weights.tap_count(out_y + 1) == tap_count;

            if batch2 {
                let w_b = weights.weights(out_y + 1);
                let out_start_a = out_y * h_row_len;
                let out_start_b = (out_y + 1) * h_row_len;

                for x in tile_start..tile_end {
                    let mut acc_a: i32 = 0;
                    let mut acc_b: i32 = 0;
                    for t in 0..tap_count {
                        let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                        let v = intermediate[in_y * h_row_len + x] as i32;
                        acc_a += v * w_a[t] as i32;
                        acc_b += v * w_b[t] as i32;
                    }
                    output[out_start_a + x] =
                        ((acc_a + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
                    output[out_start_b + x] =
                        ((acc_b + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
                }
                out_y += 2;
            } else {
                let out_start = out_y * h_row_len;
                for x in tile_start..tile_end {
                    let mut acc: i32 = 0;
                    for (t, &weight) in w_a[..tap_count].iter().enumerate() {
                        let in_y = (left + t as i32).clamp(0, in_h_i32) as usize;
                        acc += intermediate[in_y * h_row_len + x] as i32 * weight as i32;
                    }
                    output[out_start + x] =
                        ((acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION) as i16;
                }
                out_y += 1;
            }
        }
    }
}

// ============================================================================
// Streaming single-row V-filter kernels (for StreamingResize i16 paths)
// ============================================================================

/// Streaming V-filter: u8 rows → u8 output via i16 weights, scalar fallback.
///
/// For sRGB gamma i16 streaming path (Path 0). Accumulates weighted u8 rows
/// into i32, rounds with 14-bit precision, clamps to [0, 255].
pub(crate) fn filter_v_row_u8_i16_scalar(
    _token: ScalarToken,
    rows: &[&[u8]],
    output: &mut [u8],
    weights: &[i16],
) {
    let width = output.len();
    debug_assert_eq!(rows.len(), weights.len());

    for x in 0..width {
        let mut acc: i32 = 0;
        for (row, &weight) in rows.iter().zip(weights.iter()) {
            acc += row[x] as i32 * weight as i32;
        }
        let rounded = (acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION;
        output[x] = rounded.clamp(0, 255) as u8;
    }
}

/// Streaming V-filter: i16 rows → i16 output via i16 weights, scalar fallback.
///
/// For linear i12 streaming path (Path 1). Accumulates weighted i16 rows
/// into i32, rounds with 14-bit precision, clamps to [0, 4095].
pub(crate) fn filter_v_row_i16_scalar(
    _token: ScalarToken,
    rows: &[&[i16]],
    output: &mut [i16],
    weights: &[i16],
) {
    let width = output.len();
    debug_assert_eq!(rows.len(), weights.len());

    for x in 0..width {
        let mut acc: i32 = 0;
        for (row, &weight) in rows.iter().zip(weights.iter()) {
            acc += row[x] as i32 * weight as i32;
        }
        let rounded = (acc + (1 << (I16_PRECISION - 1))) >> I16_PRECISION;
        output[x] = rounded as i16;
    }
}

/// Premultiply alpha on RGBA u8 row: input → output, scalar fallback.
pub(crate) fn premultiply_u8_row_scalar(_token: ScalarToken, input: &[u8], output: &mut [u8]) {
    debug_assert_eq!(input.len(), output.len());
    for (inp, out) in input.chunks_exact(4).zip(output.chunks_exact_mut(4)) {
        let a = inp[3] as u16;
        // (c * a + 127) / 255 — exact for all u8 inputs
        out[0] = ((inp[0] as u16 * a + 127) / 255) as u8;
        out[1] = ((inp[1] as u16 * a + 127) / 255) as u8;
        out[2] = ((inp[2] as u16 * a + 127) / 255) as u8;
        out[3] = inp[3];
    }
}

/// Unpremultiply alpha in-place on RGBA u8 row, scalar fallback.
pub(crate) fn unpremultiply_u8_row_scalar(_token: ScalarToken, row: &mut [u8]) {
    for pixel in row.chunks_exact_mut(4) {
        let a = pixel[3];
        if a == 0 {
            pixel[0] = 0;
            pixel[1] = 0;
            pixel[2] = 0;
        } else if a < 255 {
            let a16 = a as u16;
            pixel[0] = ((pixel[0] as u16 * 255 + a16 / 2) / a16).min(255) as u8;
            pixel[1] = ((pixel[1] as u16 * 255 + a16 / 2) / a16).min(255) as u8;
            pixel[2] = ((pixel[2] as u16 * 255 + a16 / 2) / a16).min(255) as u8;
        }
        // a == 255: no change needed
    }
}

// ============================================================================
// f16 (IEEE 754 half-precision) support — software conversion
// ============================================================================

/// Convert f32 to f16 (IEEE 754 half-precision) bit pattern stored as u16.
/// Uses round-to-nearest-even to match F16C hardware.
#[inline]
pub(super) fn f32_to_f16_soft(x: f32) -> u16 {
    let b = x.to_bits();
    let sign = (b >> 16) & 0x8000;
    let e = ((b >> 23) & 0xFF) as i32;
    let m = b & 0x007F_FFFF;

    if e == 0xFF {
        // Inf or NaN
        return (sign | 0x7C00 | if m != 0 { (m >> 13).max(1) } else { 0 }) as u16;
    }

    // f16 exponent = f32 exponent - 112 (bias difference: 127 - 15)
    let f16e = e - 112;

    if e == 0 {
        // f32 zero or subnormal → f16 zero
        return sign as u16;
    }

    if f16e >= 31 {
        // Overflow → ±Inf
        return (sign | 0x7C00) as u16;
    }

    if f16e <= 0 {
        if f16e < -10 {
            return sign as u16; // Too small → zero
        }
        // Subnormal f16: prepend implicit 1 bit and right-shift
        let full_m = m | 0x0080_0000;
        let shift = (1 - f16e) as u32 + 13;
        let shifted = full_m >> shift;
        // Round to nearest even
        let half_bit = 1u32 << (shift - 1);
        let remainder = full_m & ((1u32 << shift) - 1);
        let round =
            u32::from(remainder > half_bit || (remainder == half_bit && (shifted & 1) != 0));
        return (sign | (shifted + round)) as u16;
    }

    // Normal case: shift mantissa from 23 to 10 bits
    let shifted_m = m >> 13;
    let remainder = m & 0x1FFF;
    let round = u32::from(remainder > 0x1000 || (remainder == 0x1000 && (shifted_m & 1) != 0));
    let result = sign | ((f16e as u32) << 10) | shifted_m;
    (result + round) as u16 // overflow into exponent is correct (carries to next binade)
}

/// Convert f16 bit pattern (u16) to f32.
#[inline]
pub(super) fn f16_to_f32_soft(h: u16) -> f32 {
    let h32 = h as u32;
    let sign = (h32 & 0x8000) << 16;
    let exp = (h32 >> 10) & 0x1F;
    let mant = h32 & 0x03FF;

    if exp == 0 {
        if mant == 0 {
            return f32::from_bits(sign); // ±0
        }
        // Subnormal: normalize by shifting mantissa left
        let mut e = 1i32;
        let mut m = mant;
        while m & 0x0400 == 0 {
            m <<= 1;
            e -= 1;
        }
        let f32_exp = ((127 - 15 + e) as u32) << 23;
        let f32_mant = (m & 0x03FF) << 13;
        return f32::from_bits(sign | f32_exp | f32_mant);
    }

    if exp == 31 {
        // Inf or NaN
        return f32::from_bits(sign | 0x7F80_0000 | (mant << 13));
    }

    // Normal
    let f32_exp = (exp + 127 - 15) << 23;
    let f32_mant = mant << 13;
    f32::from_bits(sign | f32_exp | f32_mant)
}

// ============================================================================
// f16 conversion row kernels
// ============================================================================

/// Bulk convert f32 → f16 row, scalar fallback.
pub(crate) fn f32_to_f16_row_scalar(_token: ScalarToken, input: &[f32], output: &mut [u16]) {
    debug_assert_eq!(input.len(), output.len());
    for (inp, out) in input.iter().zip(output.iter_mut()) {
        *out = f32_to_f16_soft(*inp);
    }
}

/// Bulk convert f16 → f32 row, scalar fallback.
pub(crate) fn f16_to_f32_row_scalar(_token: ScalarToken, input: &[u16], output: &mut [f32]) {
    debug_assert_eq!(input.len(), output.len());
    for (inp, out) in input.iter().zip(output.iter_mut()) {
        *out = f16_to_f32_soft(*inp);
    }
}

// ============================================================================
// f16 filter kernels
// ============================================================================

/// Horizontal convolution: f32 input → f16 (u16) output, scalar fallback.
/// Accumulates in f32, converts result to f16 for storage.
pub(crate) fn filter_h_row_f32_to_f16_scalar(
    _token: ScalarToken,
    input: &[f32],
    output: &mut [u16],
    weights: &F32WeightTable,
    channels: usize,
) {
    let out_width = weights.len();

    for out_x in 0..out_width {
        let left = weights.left[out_x] as usize;
        let w = weights.weights(out_x);
        let out_offset = out_x * channels;

        for c in 0..channels {
            let mut acc = 0.0f32;
            for (t, &weight) in w.iter().enumerate() {
                acc += input[(left + t) * channels + c] * weight;
            }
            output[out_offset + c] = f32_to_f16_soft(acc);
        }
    }
}

/// Streaming V-filter: f16 rows → f32 output via f32 weights, scalar fallback.
/// Converts each f16 element to f32, accumulates weighted sum.
pub(crate) fn filter_v_row_f16_scalar(
    _token: ScalarToken,
    rows: &[&[u16]],
    output: &mut [f32],
    weights: &[f32],
) {
    let width = output.len();
    debug_assert_eq!(rows.len(), weights.len());

    for v in output.iter_mut() {
        *v = 0.0;
    }

    for (row, &weight) in rows.iter().zip(weights.iter()) {
        debug_assert!(row.len() >= width);
        for x in 0..width {
            output[x] += f16_to_f32_soft(row[x]) * weight;
        }
    }
}

/// Batch V-filter for fullframe: f16 intermediate → f32 output, scalar fallback.
/// Reads f16 (u16) intermediate, accumulates in f32 per output row.
pub(crate) fn filter_v_all_f16_scalar(
    _token: ScalarToken,
    intermediate: &[u16],
    output: &mut [f32],
    h_row_len: usize,
    in_h: usize,
    out_h: usize,
    weights: &F32WeightTable,
) {
    for out_y in 0..out_h {
        let left = weights.left[out_y];
        let tap_count = weights.tap_count(out_y);
        let w = weights.weights(out_y);
        let out_start = out_y * h_row_len;

        for x in 0..h_row_len {
            let mut acc = 0.0f32;
            for (t, &weight) in w[..tap_count].iter().enumerate() {
                let in_y = (left + t as i32).clamp(0, in_h as i32 - 1) as usize;
                acc += f16_to_f32_soft(intermediate[in_y * h_row_len + x]) * weight;
            }
            output[out_start + x] = acc;
        }
    }
}

/// Convert sRGB u8 → linear f32 using LUT.
pub(crate) fn srgb_u8_to_linear_f32_scalar(
    _token: ScalarToken,
    input: &[u8],
    output: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    crate::color::srgb_u8_to_linear_f32_impl(input, output, channels, has_alpha);
}

/// Convert linear f32 → sRGB u8 using LUT.
pub(crate) fn linear_f32_to_srgb_u8_scalar(
    _token: ScalarToken,
    input: &[f32],
    output: &mut [u8],
    channels: usize,
    has_alpha: bool,
) {
    crate::color::linear_f32_to_srgb_u8_impl(input, output, channels, has_alpha);
}

// ============================================================================
// Transfer function batch processors (scalar fallback)
// ============================================================================

fn tf_row_inplace_scalar(row: &mut [f32], channels: usize, has_alpha: bool, tf: fn(f32) -> f32) {
    if has_alpha && channels >= 2 {
        for pixel in row.chunks_exact_mut(channels) {
            for v in &mut pixel[..channels - 1] {
                *v = tf(*v);
            }
        }
    } else {
        for v in row.iter_mut() {
            *v = tf(*v);
        }
    }
}

pub(crate) fn srgb_to_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::srgb_to_linear);
}

pub(crate) fn srgb_from_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::srgb_from_linear);
}

pub(crate) fn bt709_to_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::bt709_to_linear);
}

pub(crate) fn bt709_from_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::bt709_from_linear);
}

pub(crate) fn pq_to_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::pq_to_linear);
}

pub(crate) fn pq_from_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::pq_from_linear);
}

pub(crate) fn hlg_to_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::hlg_to_linear);
}

pub(crate) fn hlg_from_linear_row_scalar(
    _token: ScalarToken,
    row: &mut [f32],
    channels: usize,
    has_alpha: bool,
) {
    tf_row_inplace_scalar(row, channels, has_alpha, crate::fastmath::hlg_from_linear);
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::filter::{Filter, InterpolationDetails};
    #[cfg(not(feature = "std"))]
    use alloc::{vec, vec::Vec};

    #[test]
    fn test_horizontal_convolution_identity() {
        let filter = InterpolationDetails::create(Filter::Box);
        let width = 10;
        let channels = 4;
        let weights = F32WeightTable::new(width, width, &filter);

        let input: Vec<f32> = (0..width as usize * channels)
            .map(|i| (i as f32) / (width as f32 * channels as f32))
            .collect();
        let mut output = vec![0.0f32; width as usize * channels];

        filter_h_row_f32_scalar(ScalarToken, &input, &mut output, &weights, channels);

        for i in 0..input.len() {
            assert!(
                (input[i] - output[i]).abs() < 0.01,
                "Mismatch at {}: {} vs {}",
                i,
                input[i],
                output[i]
            );
        }
    }

    #[test]
    fn test_vertical_convolution_single_row() {
        let width = 40;
        let row: Vec<f32> = (0..width).map(|i| i as f32).collect();
        let mut output = vec![0.0f32; width];

        filter_v_row_f32_scalar(ScalarToken, &[&row], &mut output, &[1.0]);

        for i in 0..width {
            assert!(
                (row[i] - output[i]).abs() < 1e-6,
                "Mismatch at {}: {} vs {}",
                i,
                row[i],
                output[i]
            );
        }
    }

    #[test]
    fn test_vertical_convolution_average() {
        let width = 8;
        let row_a: Vec<f32> = vec![1.0; width];
        let row_b: Vec<f32> = vec![3.0; width];
        let mut output = vec![0.0f32; width];

        filter_v_row_f32_scalar(ScalarToken, &[&row_a, &row_b], &mut output, &[0.5, 0.5]);

        for v in &output {
            assert!((*v - 2.0).abs() < 1e-6);
        }
    }

    #[test]
    fn test_horizontal_downscale() {
        let filter = InterpolationDetails::create(Filter::Lanczos);
        let in_width = 100u32;
        let out_width = 50u32;
        let channels = 4;
        let weights = F32WeightTable::new(in_width, out_width, &filter);

        let input = vec![0.5f32; in_width as usize * channels];
        let mut output = vec![0.0f32; out_width as usize * channels];

        filter_h_row_f32_scalar(ScalarToken, &input, &mut output, &weights, channels);

        for (i, &v) in output.iter().enumerate() {
            assert!(
                (v - 0.5).abs() < 0.01,
                "Constant input should produce constant output at {}: {}",
                i,
                v
            );
        }
    }
}