ultrahdr-core 0.4.1

//! Gain map application for HDR reconstruction.

use alloc::boxed::Box;

#[cfg(feature = "transfer")]
use crate::color::transfer::{pq_oetf, srgb_eotf, srgb_oetf};
use crate::types::{ColorTransfer, GainMap, GainMapMetadata, PixelFormat, RawImage, Result};
use enough::Stop;

/// Precomputed lookup table for gain map decoding.
///
/// This LUT eliminates expensive `powf()` and `exp()` calls per pixel by
/// precomputing the mapping from 8-bit gain map values to linear gain multipliers.
/// Provides ~10x speedup for `apply_gainmap`.
pub struct GainMapLut {
    /// 256 entries per channel (R, G, B), mapping byte value to linear gain.
    /// Layout: [R0..R255, G0..G255, B0..B255]
    table: Box<[f32; 256 * 3]>,
}

impl GainMapLut {
    /// Create a new gain map LUT for the given metadata and display boost.
    ///
    /// The `weight` parameter is typically calculated from `display_boost` and
    /// the metadata's `base_hdr_headroom`/`alternate_hdr_headroom`.
    pub fn new(metadata: &GainMapMetadata, weight: f32) -> Self {
        let mut table = Box::new([0.0f32; 256 * 3]);

        for channel in 0..3 {
            let gamma = metadata.gamma[channel] as f32;
            // Convert log2 domain to natural log for exp() math
            let ln2 = core::f64::consts::LN_2;
            let log_min = (metadata.gain_map_min[channel] * ln2) as f32;
            let log_max = (metadata.gain_map_max[channel] * ln2) as f32;
            let log_range = log_max - log_min;

            for i in 0..256 {
                // Convert byte to normalized [0,1]
                let normalized = i as f32 / 255.0;

                // Undo gamma
                let linear = if gamma != 1.0 && gamma > 0.0 {
                    normalized.powf(1.0 / gamma)
                } else {
                    normalized
                };

                // Convert from normalized to log gain, apply weight, convert to linear
                let log_gain = log_min + linear * log_range;
                let gain = (log_gain * weight).exp();

                table[channel * 256 + i] = gain;
            }
        }

        Self { table }
    }

    /// Look up the gain multiplier for a single channel.
    #[inline(always)]
    pub fn lookup(&self, byte_value: u8, channel: usize) -> f32 {
        // Safety: channel is always 0..3 and byte_value is u8 (0..255)
        debug_assert!(channel < 3);
        self.table[channel * 256 + byte_value as usize]
    }

    /// Look up gain multipliers for all 3 channels from a single byte (luminance mode).
    #[inline(always)]
    pub fn lookup_luminance(&self, byte_value: u8) -> [f32; 3] {
        let g = self.table[byte_value as usize]; // Channel 0
        [g, g, g]
    }

    /// Look up gain multipliers for RGB from 3 bytes.
    #[inline(always)]
    pub fn lookup_rgb(&self, r: u8, g: u8, b: u8) -> [f32; 3] {
        [
            self.table[r as usize],
            self.table[256 + g as usize],
            self.table[512 + b as usize],
        ]
    }
}

/// Output format for HDR reconstruction.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum HdrOutputFormat {
    /// Linear float RGB `[0, ~50]` where 1.0 = SDR white (203 nits)
    LinearFloat,
    /// PQ-encoded 10-bit RGBA (1010102)
    Pq1010102,
    /// sRGB 8-bit (SDR output, no HDR boost)
    Srgb8,
}

/// Apply a gain map to an SDR image to reconstruct HDR.
///
/// The `display_boost` parameter controls how much HDR effect to apply:
/// - 1.0 = SDR output (no boost)
/// - 2.0 = 2x brightness capability
/// - 4.0 = 4x brightness capability (typical HDR display)
///
/// The `stop` parameter enables cooperative cancellation. Pass `Unstoppable`
/// when cancellation is not needed.
pub fn apply_gainmap(
    sdr: &RawImage,
    gainmap: &GainMap,
    metadata: &GainMapMetadata,
    display_boost: f32,
    output_format: HdrOutputFormat,
    stop: impl Stop,
) -> Result<RawImage> {
    // Validate pixel data is large enough for declared dimensions
    sdr.validate_data_bounds()?;

    let width = sdr.width;
    let height = sdr.height;

    // Calculate weight factor based on display capability
    let weight = calculate_weight(display_boost, metadata);

    // Create precomputed LUT for fast gain decoding
    let lut = GainMapLut::new(metadata, weight);

    // Create output image
    let mut output = match output_format {
        HdrOutputFormat::LinearFloat => {
            let mut img = RawImage::new(width, height, PixelFormat::Rgba32F)?;
            img.transfer = ColorTransfer::Linear;
            img.gamut = sdr.gamut;
            img
        }
        HdrOutputFormat::Pq1010102 => {
            let mut img = RawImage::new(width, height, PixelFormat::Rgba1010102Pq)?;
            img.transfer = ColorTransfer::Pq;
            img.gamut = sdr.gamut;
            img
        }
        HdrOutputFormat::Srgb8 => {
            let mut img = RawImage::new(width, height, PixelFormat::Rgba8)?;
            img.transfer = ColorTransfer::Srgb;
            img.gamut = sdr.gamut;
            img
        }
    };

    // Process each row, checking for cancellation periodically
    for y in 0..height {
        // Check for cancellation once per row (not per pixel for performance)
        stop.check()?;

        for x in 0..width {
            // Get SDR pixel (convert to linear)
            let sdr_linear = get_sdr_linear(sdr, x, y);

            // Sample gain map with LUT (fast path - no transcendentals per pixel)
            let gain = sample_gainmap_lut(gainmap, &lut, x, y, width, height);

            // Apply gain to reconstruct HDR
            let hdr_linear = apply_gain(sdr_linear, gain, metadata);

            // Write output
            write_output(&mut output, x, y, hdr_linear, output_format);
        }
    }

    Ok(output)
}

/// Calculate the weight factor for gain map application.
///
/// Headroom values are in log2 domain. `display_boost` is linear.
fn calculate_weight(display_boost: f32, metadata: &GainMapMetadata) -> f32 {
    let log_display = display_boost.max(1.0).log2() as f64;
    let log_min = metadata.base_hdr_headroom.max(0.0);
    let log_max = metadata.alternate_hdr_headroom.max(0.0);

    if log_max <= log_min {
        return 1.0;
    }

    ((log_display - log_min) / (log_max - log_min)).clamp(0.0, 1.0) as f32
}

/// Get linear RGB from SDR image.
#[cfg(feature = "transfer")]
fn get_sdr_linear(sdr: &RawImage, x: u32, y: u32) -> [f32; 3] {
    match sdr.format {
        PixelFormat::Rgba8 | PixelFormat::Rgb8 => {
            let bpp = if sdr.format == PixelFormat::Rgba8 {
                4
            } else {
                3
            };
            let idx = (y * sdr.stride + x * bpp as u32) as usize;
            let r = sdr.data[idx] as f32 / 255.0;
            let g = sdr.data[idx + 1] as f32 / 255.0;
            let b = sdr.data[idx + 2] as f32 / 255.0;
            [srgb_eotf(r), srgb_eotf(g), srgb_eotf(b)]
        }
        _ => {
            // For other formats, return mid-gray as fallback
            [0.18, 0.18, 0.18]
        }
    }
}

/// Get linear RGB from SDR image (no transfer feature - assumes linear input).
#[cfg(not(feature = "transfer"))]
fn get_sdr_linear(sdr: &RawImage, x: u32, y: u32) -> [f32; 3] {
    match sdr.format {
        PixelFormat::Rgba8 | PixelFormat::Rgb8 => {
            let bpp = if sdr.format == PixelFormat::Rgba8 {
                4
            } else {
                3
            };
            let idx = (y * sdr.stride + x * bpp as u32) as usize;
            let r = sdr.data[idx] as f32 / 255.0;
            let g = sdr.data[idx + 1] as f32 / 255.0;
            let b = sdr.data[idx + 2] as f32 / 255.0;
            // Assume already linear - caller must pre-convert
            [r, g, b]
        }
        PixelFormat::Rgba32F => {
            let idx = (y * sdr.stride + x * 16) as usize;
            let r = f32::from_le_bytes([
                sdr.data[idx],
                sdr.data[idx + 1],
                sdr.data[idx + 2],
                sdr.data[idx + 3],
            ]);
            let g = f32::from_le_bytes([
                sdr.data[idx + 4],
                sdr.data[idx + 5],
                sdr.data[idx + 6],
                sdr.data[idx + 7],
            ]);
            let b = f32::from_le_bytes([
                sdr.data[idx + 8],
                sdr.data[idx + 9],
                sdr.data[idx + 10],
                sdr.data[idx + 11],
            ]);
            [r, g, b]
        }
        _ => [0.18, 0.18, 0.18],
    }
}

/// Bilinear interpolation.
#[inline(always)]
fn bilinear(v00: f32, v10: f32, v01: f32, v11: f32, fx: f32, fy: f32) -> f32 {
    let top = v00 * (1.0 - fx) + v10 * fx;
    let bottom = v01 * (1.0 - fx) + v11 * fx;
    top * (1.0 - fy) + bottom * fy
}

/// Sample gain map with bilinear interpolation using precomputed LUT.
///
/// This is significantly faster than `sample_gainmap` because it uses LUT lookups
/// instead of expensive `powf()` and `exp()` calls per pixel.
#[inline]
#[allow(clippy::needless_range_loop)] // c is used as both index and channel parameter
fn sample_gainmap_lut(
    gainmap: &GainMap,
    lut: &GainMapLut,
    x: u32,
    y: u32,
    img_width: u32,
    img_height: u32,
) -> [f32; 3] {
    // Map image coordinates to gain map coordinates
    let gm_x = (x as f32 / img_width as f32) * gainmap.width as f32;
    let gm_y = (y as f32 / img_height as f32) * gainmap.height as f32;

    // Bilinear interpolation coordinates
    let x0 = (gm_x.floor() as u32).min(gainmap.width - 1);
    let y0 = (gm_y.floor() as u32).min(gainmap.height - 1);
    let x1 = (x0 + 1).min(gainmap.width - 1);
    let y1 = (y0 + 1).min(gainmap.height - 1);

    let fx = gm_x - gm_x.floor();
    let fy = gm_y - gm_y.floor();

    if gainmap.channels == 1 {
        // Single channel - look up gains from LUT, then interpolate
        let g00 = lut.lookup(gainmap.data[(y0 * gainmap.width + x0) as usize], 0);
        let g10 = lut.lookup(gainmap.data[(y0 * gainmap.width + x1) as usize], 0);
        let g01 = lut.lookup(gainmap.data[(y1 * gainmap.width + x0) as usize], 0);
        let g11 = lut.lookup(gainmap.data[(y1 * gainmap.width + x1) as usize], 0);

        let gain = bilinear(g00, g10, g01, g11, fx, fy);
        [gain, gain, gain]
    } else {
        // Multi-channel - look up and interpolate each channel
        let mut gains = [0.0f32; 3];
        for c in 0..3 {
            let idx00 = (y0 * gainmap.width + x0) as usize * 3 + c;
            let idx10 = (y0 * gainmap.width + x1) as usize * 3 + c;
            let idx01 = (y1 * gainmap.width + x0) as usize * 3 + c;
            let idx11 = (y1 * gainmap.width + x1) as usize * 3 + c;

            let g00 = lut.lookup(gainmap.data[idx00], c);
            let g10 = lut.lookup(gainmap.data[idx10], c);
            let g01 = lut.lookup(gainmap.data[idx01], c);
            let g11 = lut.lookup(gainmap.data[idx11], c);

            gains[c] = bilinear(g00, g10, g01, g11, fx, fy);
        }
        gains
    }
}

/// Apply gain to SDR pixel to get HDR.
fn apply_gain(sdr_linear: [f32; 3], gain: [f32; 3], metadata: &GainMapMetadata) -> [f32; 3] {
    [
        (sdr_linear[0] + metadata.base_offset[0] as f32) * gain[0]
            - metadata.alternate_offset[0] as f32,
        (sdr_linear[1] + metadata.base_offset[1] as f32) * gain[1]
            - metadata.alternate_offset[1] as f32,
        (sdr_linear[2] + metadata.base_offset[2] as f32) * gain[2]
            - metadata.alternate_offset[2] as f32,
    ]
}

/// Write HDR pixel to output image.
#[cfg(feature = "transfer")]
fn write_output(output: &mut RawImage, x: u32, y: u32, hdr: [f32; 3], format: HdrOutputFormat) {
    match format {
        HdrOutputFormat::LinearFloat => {
            write_linear_float(output, x, y, hdr);
        }

        HdrOutputFormat::Pq1010102 => {
            // Convert linear to PQ
            // First normalize: linear HDR has 1.0 = SDR white (203 nits)
            // PQ expects 1.0 = 10000 nits, so multiply by 203/10000
            let scale = 203.0 / 10000.0;
            let r_pq = pq_oetf(hdr[0].max(0.0) * scale);
            let g_pq = pq_oetf(hdr[1].max(0.0) * scale);
            let b_pq = pq_oetf(hdr[2].max(0.0) * scale);

            // Pack to 1010102
            let r = (r_pq * 1023.0).round().clamp(0.0, 1023.0) as u32;
            let g = (g_pq * 1023.0).round().clamp(0.0, 1023.0) as u32;
            let b = (b_pq * 1023.0).round().clamp(0.0, 1023.0) as u32;
            let a = 3u32; // Full alpha

            let packed = r | (g << 10) | (b << 20) | (a << 30);
            let idx = (y * output.stride + x * 4) as usize;
            output.data[idx..idx + 4].copy_from_slice(&packed.to_le_bytes());
        }

        HdrOutputFormat::Srgb8 => {
            // Clip to SDR range and apply sRGB OETF
            let r = srgb_oetf(hdr[0].clamp(0.0, 1.0));
            let g = srgb_oetf(hdr[1].clamp(0.0, 1.0));
            let b = srgb_oetf(hdr[2].clamp(0.0, 1.0));

            let idx = (y * output.stride + x * 4) as usize;
            output.data[idx] = (r * 255.0).round() as u8;
            output.data[idx + 1] = (g * 255.0).round() as u8;
            output.data[idx + 2] = (b * 255.0).round() as u8;
            output.data[idx + 3] = 255;
        }
    }
}

/// Write HDR pixel to output image (no transfer feature - linear output only).
#[cfg(not(feature = "transfer"))]
fn write_output(output: &mut RawImage, x: u32, y: u32, hdr: [f32; 3], format: HdrOutputFormat) {
    match format {
        HdrOutputFormat::LinearFloat => {
            write_linear_float(output, x, y, hdr);
        }
        // Without transfer feature, PQ and sRGB output are not supported
        // Fall back to linear float in RGBA8 format (clamped)
        _ => {
            let idx = (y * output.stride + x * 4) as usize;
            output.data[idx] = (hdr[0].clamp(0.0, 1.0) * 255.0).round() as u8;
            output.data[idx + 1] = (hdr[1].clamp(0.0, 1.0) * 255.0).round() as u8;
            output.data[idx + 2] = (hdr[2].clamp(0.0, 1.0) * 255.0).round() as u8;
            output.data[idx + 3] = 255;
        }
    }
}

/// Write linear f32 RGBA to output.
#[inline]
fn write_linear_float(output: &mut RawImage, x: u32, y: u32, hdr: [f32; 3]) {
    let idx = (y * output.stride + x * 16) as usize;
    let r_bytes = hdr[0].to_le_bytes();
    let g_bytes = hdr[1].to_le_bytes();
    let b_bytes = hdr[2].to_le_bytes();
    let a_bytes = 1.0f32.to_le_bytes();

    output.data[idx..idx + 4].copy_from_slice(&r_bytes);
    output.data[idx + 4..idx + 8].copy_from_slice(&g_bytes);
    output.data[idx + 8..idx + 12].copy_from_slice(&b_bytes);
    output.data[idx + 12..idx + 16].copy_from_slice(&a_bytes);
}

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

    #[test]
    fn test_calculate_weight() {
        let metadata = GainMapMetadata {
            base_hdr_headroom: 0.0,
            alternate_hdr_headroom: 2.0,
            ..Default::default()
        };

        // No boost
        let w = calculate_weight(1.0, &metadata);
        assert!((w - 0.0).abs() < 0.01);

        // Full boost
        let w = calculate_weight(4.0, &metadata);
        assert!((w - 1.0).abs() < 0.01);

        // Half boost (log scale)
        let w = calculate_weight(2.0, &metadata);
        assert!(w > 0.4 && w < 0.6);
    }

    #[test]
    fn test_gain_map_lut() {
        let metadata = GainMapMetadata {
            gain_map_min: [0.0; 3],
            gain_map_max: [2.0; 3],
            gamma: [1.0; 3],
            ..Default::default()
        };

        let lut = GainMapLut::new(&metadata, 1.0);

        // Min gain (byte 0 = normalized 0.0)
        let gain = lut.lookup(0, 0);
        assert!((gain - 1.0).abs() < 0.01, "min gain: {}", gain);

        // Max gain (byte 255 = normalized 1.0)
        let gain = lut.lookup(255, 0);
        assert!((gain - 4.0).abs() < 0.1, "max gain: {}", gain);

        // Mid gain should be between min and max
        let gain = lut.lookup(128, 0);
        assert!(gain > 1.5 && gain < 2.5, "mid gain: {}", gain);
    }

    #[test]
    fn test_apply_gainmap_basic() {
        // Create SDR image
        let mut sdr = RawImage::new(4, 4, PixelFormat::Rgba8).unwrap();
        sdr.gamut = ColorGamut::Bt709;
        sdr.transfer = ColorTransfer::Srgb;
        for i in 0..sdr.data.len() / 4 {
            sdr.data[i * 4] = 128;
            sdr.data[i * 4 + 1] = 128;
            sdr.data[i * 4 + 2] = 128;
            sdr.data[i * 4 + 3] = 255;
        }

        // Create gain map (all same boost)
        let mut gainmap = GainMap::new(2, 2).unwrap();
        for v in &mut gainmap.data {
            *v = 200; // High gain
        }

        let metadata = GainMapMetadata {
            gain_map_min: [0.0; 3],
            gain_map_max: [2.0; 3],
            gamma: [1.0; 3],
            base_offset: [0.015625; 3],
            alternate_offset: [0.015625; 3],
            base_hdr_headroom: 0.0,
            alternate_hdr_headroom: 2.0,
            use_base_color_space: true,
            backward_direction: false,
        };

        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            4.0,
            HdrOutputFormat::Srgb8,
            enough::Unstoppable,
        )
        .unwrap();

        assert_eq!(result.width, 4);
        assert_eq!(result.height, 4);
        assert_eq!(result.format, PixelFormat::Rgba8);
    }

    // ========================================================================
    // Gain application reference values (C++ libultrahdr parity)
    //
    // Tests the LUT-based gain application against known-correct values.
    // The LUT maps byte values to linear gain multipliers:
    //   normalized = byte / 255.0
    //   linear = normalized^(1/gamma)  [undo gamma]
    //   log_gain = ln(min_boost) + linear * (ln(max_boost) - ln(min_boost))
    //   gain = exp(log_gain * weight)
    //
    // Then HDR = (sdr + offset_sdr) * gain - offset_hdr
    // ========================================================================

    /// Test gain application at 5 weight levels for white pixel.
    ///
    /// White (sdr=1.0) at gain map value 255 (max boost),
    /// with weight from 0.0 to 1.0 in steps of 0.25.
    #[test]
    fn test_gain_application_weight_levels() {
        let metadata = GainMapMetadata {
            gain_map_min: [0.0; 3],
            gain_map_max: [2.0; 3],
            gamma: [1.0; 3],
            base_offset: [1.0 / 64.0; 3],
            alternate_offset: [1.0 / 64.0; 3],
            base_hdr_headroom: 0.0,
            alternate_hdr_headroom: 2.0,
            use_base_color_space: true,
            backward_direction: false,
        };

        let sdr_val = 1.0_f32; // White pixel (linear)
        let offset = 1.0_f32 / 64.0;
        let log_min = 1.0_f32.ln(); // 0.0
        let log_max = 4.0_f32.ln(); // ~1.386

        // At byte=255 (normalized=1.0, gamma=1.0 → linear=1.0):
        //   log_gain = 0.0 + 1.0 * (ln(4) - ln(1)) = ln(4) ≈ 1.386
        //   gain = exp(log_gain * weight)
        //   hdr = (sdr + offset) * gain - offset

        let weights: [(f32, &str); 5] = [
            (0.0, "SDR (no boost)"),
            (0.25, "25% boost"),
            (0.5, "50% boost"),
            (0.75, "75% boost"),
            (1.0, "full boost"),
        ];

        for &(weight, desc) in &weights {
            let lut = GainMapLut::new(&metadata, weight);
            let gain = lut.lookup(255, 0);

            let log_gain = log_min + 1.0 * (log_max - log_min);
            let expected_gain = (log_gain * weight).exp();
            let expected_hdr = (sdr_val + offset) * expected_gain - offset;

            // Verify LUT gain matches formula
            assert!(
                (gain - expected_gain).abs() < 0.01,
                "{}: LUT gain={}, expected={}",
                desc,
                gain,
                expected_gain
            );

            // Verify HDR output
            let hdr = apply_gain([sdr_val; 3], [gain; 3], &metadata);
            assert!(
                (hdr[0] - expected_hdr).abs() < 0.02,
                "{}: hdr={}, expected={}",
                desc,
                hdr[0],
                expected_hdr
            );
        }
    }

    /// Test gain application for black pixel (sdr=0.0).
    ///
    /// Black pixels should remain close to black regardless of gain,
    /// because the offset dominates: hdr = (0 + 1/64) * gain - 1/64
    #[test]
    fn test_gain_application_black_pixel() {
        let metadata = GainMapMetadata {
            gain_map_min: [0.0; 3],
            gain_map_max: [2.0; 3],
            gamma: [1.0; 3],
            base_offset: [1.0 / 64.0; 3],
            alternate_offset: [1.0 / 64.0; 3],
            base_hdr_headroom: 0.0,
            alternate_hdr_headroom: 2.0,
            use_base_color_space: true,
            backward_direction: false,
        };

        let offset = 1.0_f32 / 64.0;

        // At full weight with max gain byte
        let lut = GainMapLut::new(&metadata, 1.0);
        let gain = lut.lookup(255, 0);

        // hdr = (0 + 1/64) * 4.0 - 1/64 = 4/64 - 1/64 = 3/64 ≈ 0.047
        let expected_hdr = offset * gain - offset;
        let hdr = apply_gain([0.0; 3], [gain; 3], &metadata);

        assert!(
            (hdr[0] - expected_hdr).abs() < 0.01,
            "Black pixel HDR: {} vs expected {}",
            hdr[0],
            expected_hdr
        );

        // Black with zero gain (byte=0) should stay near zero
        let gain_min = lut.lookup(0, 0);
        let hdr_min = apply_gain([0.0; 3], [gain_min; 3], &metadata);
        // gain_min = exp(0 * 1.0) = 1.0 for weight=1.0 and min_boost=1.0
        // hdr = (0 + 1/64) * 1.0 - 1/64 = 0
        assert!(
            hdr_min[0].abs() < 0.01,
            "Black at min gain should be ~0, got {}",
            hdr_min[0]
        );
    }

    /// Verify gain LUT covers the full [min_boost, max_boost] range.
    #[test]
    fn test_gain_lut_range_coverage() {
        let metadata = GainMapMetadata {
            gain_map_min: [-1.0; 3],
            gain_map_max: [3.0; 3],
            gamma: [1.0; 3],
            base_offset: [1.0 / 64.0; 3],
            alternate_offset: [1.0 / 64.0; 3],
            base_hdr_headroom: 0.0,
            alternate_hdr_headroom: 3.0,
            use_base_color_space: true,
            backward_direction: false,
        };

        let lut = GainMapLut::new(&metadata, 1.0);

        // Byte 0 → min gain = exp(ln(0.5)) = 0.5
        let gain_0 = lut.lookup(0, 0);
        assert!(
            (gain_0 - 0.5).abs() < 0.01,
            "Byte 0 should give min gain 0.5, got {}",
            gain_0
        );

        // Byte 255 → max gain = exp(ln(8)) = 8.0
        let gain_255 = lut.lookup(255, 0);
        assert!(
            (gain_255 - 8.0).abs() < 0.1,
            "Byte 255 should give max gain 8.0, got {}",
            gain_255
        );

        // Monotonically increasing
        for i in 1..=255u8 {
            let prev = lut.lookup(i - 1, 0);
            let curr = lut.lookup(i, 0);
            assert!(
                curr >= prev,
                "LUT not monotonic at byte {}: {} < {}",
                i,
                curr,
                prev
            );
        }
    }

    /// Helper: create a 4x4 SDR image (Rgba8, Srgb, BT.709) filled with a uniform color.
    fn make_sdr_4x4(r: u8, g: u8, b: u8) -> RawImage {
        let mut data = vec![0u8; 4 * 4 * 4];
        for i in 0..16 {
            data[i * 4] = r;
            data[i * 4 + 1] = g;
            data[i * 4 + 2] = b;
            data[i * 4 + 3] = 255;
        }
        RawImage::from_data(
            4,
            4,
            PixelFormat::Rgba8,
            ColorGamut::Bt709,
            ColorTransfer::Srgb,
            data,
        )
        .unwrap()
    }

    /// Helper: create a 2x2 single-channel gain map filled with a uniform value.
    fn make_gainmap_2x2(value: u8) -> GainMap {
        let mut gm = GainMap::new(2, 2).unwrap();
        for v in &mut gm.data {
            *v = value;
        }
        gm
    }

    /// Helper: create standard test metadata.
    fn test_metadata() -> GainMapMetadata {
        // log2(1.0)=0.0, log2(4.0)=2.0
        GainMapMetadata {
            gain_map_min: [0.0; 3], // log2(1.0)
            gain_map_max: [2.0; 3], // log2(4.0)
            gamma: [1.0; 3],
            base_offset: [1.0 / 64.0; 3],
            alternate_offset: [1.0 / 64.0; 3],
            base_hdr_headroom: 0.0,      // log2(1.0)
            alternate_hdr_headroom: 2.0, // log2(4.0)
            use_base_color_space: true,
            backward_direction: false,
        }
    }

    #[test]
    fn test_apply_gainmap_linear_float_format() {
        let sdr = make_sdr_4x4(128, 128, 128);
        let gainmap = make_gainmap_2x2(128);
        let metadata = test_metadata();

        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            4.0,
            HdrOutputFormat::LinearFloat,
            enough::Unstoppable,
        )
        .unwrap();

        assert_eq!(result.format, PixelFormat::Rgba32F);
        assert_eq!(result.width, 4);
        assert_eq!(result.height, 4);
        // Rgba32F: 16 bytes per pixel (4 f32 channels)
        assert_eq!(result.data.len(), 4 * 4 * 16);
    }

    #[test]
    fn test_apply_gainmap_srgb8_format() {
        let sdr = make_sdr_4x4(128, 128, 128);
        let gainmap = make_gainmap_2x2(128);
        let metadata = test_metadata();

        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            4.0,
            HdrOutputFormat::Srgb8,
            enough::Unstoppable,
        )
        .unwrap();

        assert_eq!(result.format, PixelFormat::Rgba8);
        assert_eq!(result.width, 4);
        assert_eq!(result.height, 4);
    }

    #[test]
    fn test_apply_gainmap_boost_1() {
        // display_boost=1.0 → weight=0.0 → gain=1.0 everywhere → output ≈ SDR
        let sdr = make_sdr_4x4(128, 128, 128);
        let gainmap = make_gainmap_2x2(200); // High gain value, but weight=0 should negate it
        let metadata = test_metadata();

        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            1.0,
            HdrOutputFormat::Srgb8,
            enough::Unstoppable,
        )
        .unwrap();

        // With boost=1.0, weight=0.0, gain=exp(0)=1.0 for all LUT entries.
        // HDR = (sdr_linear + offset) * 1.0 - offset = sdr_linear
        // So output should be very close to the input SDR values.
        for i in 0..16 {
            let r = result.data[i * 4];
            let g = result.data[i * 4 + 1];
            let b = result.data[i * 4 + 2];
            assert!(
                (r as i16 - 128).unsigned_abs() <= 2,
                "boost=1 R should be ~128, got {}",
                r
            );
            assert!(
                (g as i16 - 128).unsigned_abs() <= 2,
                "boost=1 G should be ~128, got {}",
                g
            );
            assert!(
                (b as i16 - 128).unsigned_abs() <= 2,
                "boost=1 B should be ~128, got {}",
                b
            );
        }
    }

    #[test]
    fn test_apply_gainmap_boost_max() {
        // display_boost = hdr_capacity_max → weight=1.0 → full HDR enhancement
        let sdr = make_sdr_4x4(128, 128, 128);
        let gainmap = make_gainmap_2x2(255); // Max gain
        let metadata = test_metadata();

        let result_max = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            2.0f32.powf(metadata.alternate_hdr_headroom as f32), // linear display boost
            HdrOutputFormat::LinearFloat,
            enough::Unstoppable,
        )
        .unwrap();

        // Also compute with boost=1.0 for comparison
        let result_sdr = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            1.0,
            HdrOutputFormat::LinearFloat,
            enough::Unstoppable,
        )
        .unwrap();

        // Read first pixel from each
        let hdr_r = f32::from_le_bytes([
            result_max.data[0],
            result_max.data[1],
            result_max.data[2],
            result_max.data[3],
        ]);
        let sdr_r = f32::from_le_bytes([
            result_sdr.data[0],
            result_sdr.data[1],
            result_sdr.data[2],
            result_sdr.data[3],
        ]);

        // Full boost should produce significantly brighter output than no boost
        assert!(
            hdr_r > sdr_r * 1.5,
            "max boost ({}) should be much brighter than sdr ({})",
            hdr_r,
            sdr_r
        );
    }

    #[test]
    fn test_gain_map_lut_monotonic() {
        let metadata = test_metadata();
        let lut = GainMapLut::new(&metadata, 1.0);

        // LUT values should be monotonically non-decreasing from byte 0 to 255
        for channel in 0..3 {
            for i in 1..=255u8 {
                let prev = lut.lookup(i - 1, channel);
                let curr = lut.lookup(i, channel);
                assert!(
                    curr >= prev,
                    "LUT not monotonic at byte {} channel {}: {} < {}",
                    i,
                    channel,
                    curr,
                    prev
                );
            }
        }
    }

    #[test]
    fn test_gain_map_lut_endpoints() {
        let metadata = test_metadata();
        let lut = GainMapLut::new(&metadata, 1.0);

        // At weight=1.0:
        // Byte 0 → normalized=0.0 → gain = 2^gain_map_min = 2^0 = 1.0
        let gain_0 = lut.lookup(0, 0);
        let expected_min = 2.0f32.powf(metadata.gain_map_min[0] as f32);
        assert!(
            (gain_0 - expected_min).abs() < 0.01,
            "byte 0 should give 2^gain_map_min={}, got {}",
            expected_min,
            gain_0
        );

        // Byte 255 → normalized=1.0 → gain = 2^gain_map_max = 2^2 = 4.0
        let gain_255 = lut.lookup(255, 0);
        let expected_max = 2.0f32.powf(metadata.gain_map_max[0] as f32);
        assert!(
            (gain_255 - expected_max).abs() < 0.1,
            "byte 255 should give 2^gain_map_max={}, got {}",
            expected_max,
            gain_255
        );
    }

    #[test]
    fn test_apply_gainmap_multichannel() {
        let sdr = make_sdr_4x4(128, 128, 128);

        // Create a 2x2 multichannel (3-channel) gain map
        let mut gainmap = GainMap::new_multichannel(2, 2).unwrap();
        assert_eq!(gainmap.channels, 3);
        // Fill with different values per channel
        for i in 0..(2 * 2) {
            gainmap.data[i * 3] = 200; // R channel - high gain
            gainmap.data[i * 3 + 1] = 128; // G channel - mid gain
            gainmap.data[i * 3 + 2] = 50; // B channel - low gain
        }

        let metadata = test_metadata();

        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            4.0,
            HdrOutputFormat::LinearFloat,
            enough::Unstoppable,
        )
        .unwrap();

        assert_eq!(result.width, 4);
        assert_eq!(result.height, 4);
        assert_eq!(result.format, PixelFormat::Rgba32F);
        assert_eq!(result.data.len(), 4 * 4 * 16);
    }

    #[test]
    fn test_apply_gainmap_invalid_boost() {
        // display_boost=0.5 (< 1.0) is clamped to 1.0 internally, not an error.
        // Verify it behaves exactly like boost=1.0.
        let sdr = make_sdr_4x4(128, 128, 128);
        let gainmap = make_gainmap_2x2(200);
        let metadata = test_metadata();

        let result_low = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            0.5,
            HdrOutputFormat::Srgb8,
            enough::Unstoppable,
        )
        .unwrap();

        let result_one = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            1.0,
            HdrOutputFormat::Srgb8,
            enough::Unstoppable,
        )
        .unwrap();

        // Both should produce identical output since 0.5 is clamped to 1.0
        assert_eq!(result_low.data, result_one.data);
    }

    #[test]
    fn test_apply_gainmap_cancellation() {
        /// A Stop implementation that cancels immediately
        struct ImmediateCancel;

        impl enough::Stop for ImmediateCancel {
            fn check(&self) -> std::result::Result<(), enough::StopReason> {
                Err(enough::StopReason::Cancelled)
            }
        }

        // Create minimal images
        let sdr = RawImage::new(4, 4, PixelFormat::Rgba8).unwrap();
        let gainmap = GainMap::new(2, 2).unwrap();
        let metadata = GainMapMetadata::new();

        // Should return Stopped error due to cancellation
        let result = apply_gainmap(
            &sdr,
            &gainmap,
            &metadata,
            4.0,
            HdrOutputFormat::Srgb8,
            ImmediateCancel,
        );

        assert!(matches!(
            result,
            Err(crate::Error::Stopped(enough::StopReason::Cancelled))
        ));
    }
}