src/shareHalftone.js

export const SHARE_HALFTONE_DOT_RULE = {
  step: 5,
  minRadius: 0.1,
  maxRadiusRatio: 0.46,
  paperThreshold: 0,
  sizeCurve: 0.8,
  alphaCurve: 0.52,
  minAlpha: 0.88,
  maxAlpha: 1,
  luminanceLow: 0.03,
  luminanceHigh: 0.915,
  contrast: 2.09,
  baseInk: 0.01,
  inkGamma: 0.92,
  postCropZoom: 1.05,
  postCropOffsetX: 0,
  postCropOffsetY: 0,
  radiusNoise: 0.08,
  radiusNoiseFloor: 0.2,
  accentWeight: 0.41,
  departureWeight: 0.55,
  toneWeight: 0.64,
  hueDepartureWeight: 0.39,
  saturationDepartureWeight: 0.09,
  darkDepartureWeight: 0.39,
  nearWhiteGate: 0.08,
  paperLightnessGate: 0.885,
  baseLightnessLift: 0.03,
  dotLightnessLift: 0.08,
  baseSaturationScale: 0.84,
  dotSaturationScale: 1.18,
  posterWeight: 0.82,
  posterThreshold: 0.46,
  posterSoftness: 0.15,
  edgeWeight: 0.43,
  edgeRadius: 3,
  weaveBias: 0.08,
  toneBias: 1.3,
  hueBias: 0.34,
  minInterleave: 0.12,
  maxInterleave: 0.78,
};

export function shareHalftoneSamplePoint(x, y, width, height, sampleWidth, sampleHeight, rule = SHARE_HALFTONE_DOT_RULE) {
  return {
    sx: clamp(Math.floor((x / width) * sampleWidth), 0, sampleWidth - 1),
    sy: clamp(Math.floor((y / height) * sampleHeight), 0, sampleHeight - 1),
  };
}

export function shareHalftoneCropRect(width, height, rule = SHARE_HALFTONE_DOT_RULE) {
  const zoom = Math.max(1, rule.postCropZoom || 1);
  const sw = width / zoom;
  const sh = height / zoom;
  const centerX = (width - sw) / 2;
  const centerY = (height - sh) / 2;
  const sx = clamp(centerX + centerX * (rule.postCropOffsetX || 0), 0, width - sw);
  const sy = clamp(centerY + centerY * (rule.postCropOffsetY || 0), 0, height - sh);
  return { sx, sy, sw, sh };
}

export function shareHalftoneDotRadius(radiusTone, rule = SHARE_HALFTONE_DOT_RULE) {
  const safeTone = printableTone(radiusTone, rule);
  if (safeTone === 0) return 0;
  const maxRadius = rule.step * rule.maxRadiusRatio;
  const radiusRange = Math.max(0, maxRadius - rule.minRadius);
  return rule.minRadius + Math.pow(safeTone, rule.sizeCurve) * radiusRange;
}

export function shareHalftoneDotAlpha(radiusTone, rule = SHARE_HALFTONE_DOT_RULE) {
  const safeTone = printableTone(radiusTone, rule);
  if (safeTone === 0) return 0;
  return 1;
}

export function shareHalftoneNoisyTone(radiusTone, x, y, rule = SHARE_HALFTONE_DOT_RULE) {
  const safeTone = clamp(radiusTone, 0, 1);
  const midtoneWeight = 1 - Math.abs(safeTone - 0.5) * 2;
  const noiseWeight = rule.radiusNoiseFloor + (1 - rule.radiusNoiseFloor) * Math.max(0, midtoneWeight);
  const noise = halftoneCellNoise(x, y) * 2 - 1;
  return clamp(safeTone + noise * rule.radiusNoise * noiseWeight, 0, 1);
}

export function shareHalftonePlateColors(primaryInk, secondaryInk, rule = SHARE_HALFTONE_DOT_RULE) {
  return {
    baseInk: normalizePlateInk(primaryInk, "base", rule),
    dotInk: normalizePlateInk(secondaryInk, "dot", rule),
  };
}

export function shareHalftoneFeatureColorPair(colors) {
  if (!colors.length) return [];
  if (colors.length === 1) return [colors[0], colors[0]];

  const candidates = colors.filter(isUsableFeatureColor).slice(0, 28);
  if (!candidates.length) return [colors[0], colors[0]];
  const baseCandidates = candidates.slice(0, 6);
  const topWeight = candidates[0]?.weight || 1;
  let bestPair = [candidates[0], candidates[0]];
  let bestScore = -Infinity;

  baseCandidates.forEach((base, baseIndex) => {
    candidates.forEach((accent) => {
      if (accent === base) return;
      const hueGap = hueDistance(base.h, accent.h);
      if (hueGap < 12) return;
      const accentArea = accent.weight / topWeight;
      if (accentArea < 0.03) return;

      const baseArea = base.weight / topWeight;
      const accentPresence = clamp(accentArea / 0.18, 0, 1);
      const hueSeparation = clamp((hueGap - 10) / 42, 0, 1);
      const substrateBonus = baseIndex === 0 ? 0.12 : 0;
      const lightnessContrast = Math.min(Math.abs(base.l - accent.l), 0.42) / 0.42;
      const score = baseArea * 0.34
        + accentPresence * 0.24
        + hueSeparation * 0.16
        + accent.s * 0.1
        + lightnessContrast * 0.08
        + substrateBonus;

      if (score > bestScore) {
        bestScore = score;
        bestPair = [base, accent];
      }
    });
  });

  return bestPair;
}

export function shareHalftoneDotToneForPixel(r, g, b, baseInk, dotInk, luminanceTone, rule = SHARE_HALFTONE_DOT_RULE) {
  const hsl = rgbToHsl(r, g, b);
  const baseHueFit = hueFit(hsl.h, baseInk.h);
  const dotHueFit = hueFit(hsl.h, dotInk.h);
  const chroma = clamp((hsl.s - 0.04) / 0.56, 0, 1);
  const nearWhitePaper = hsl.l > rule.paperLightnessGate ? rule.nearWhiteGate : 1;
  const accentSignal = clamp(0.5 + (dotHueFit - baseHueFit) * 1.55, 0, 1) * (0.55 + chroma * 0.45) * nearWhitePaper;
  const hueDeparture = 1 - baseHueFit;
  const saturationDeparture = Math.abs(hsl.s - baseInk.s);
  const darkDeparture = clamp((baseInk.l - hsl.l) / 0.44, 0, 1);
  const structureDeparture = clamp(
    hueDeparture * rule.hueDepartureWeight
      + saturationDeparture * rule.saturationDepartureWeight
      + darkDeparture * rule.darkDepartureWeight,
    0,
    1,
  ) * nearWhitePaper;
  const structureSignal = dotInk.l >= baseInk.l
    ? clamp(luminanceTone, 0, 1)
    : 1 - clamp(luminanceTone, 0, 1);

  return clamp(
    accentSignal * rule.accentWeight
      + structureDeparture * rule.departureWeight
      + structureSignal * rule.toneWeight
      + rule.baseInk,
    0,
    1,
  );
}

export function shareHalftonePosterDotToneForPixel(
  pixels,
  sx,
  sy,
  sampleWidth,
  sampleHeight,
  lowLum,
  lumRange,
  baseInk,
  dotInk,
  luminanceTone,
  rule = SHARE_HALFTONE_DOT_RULE,
) {
  const { r, g, b } = sampleHalftonePixel(pixels, sx, sy, sampleWidth, sampleHeight);
  const colorTone = shareHalftoneDotToneForPixel(r, g, b, baseInk, dotInk, luminanceTone, rule);
  const structureTone = dotInk.l >= baseInk.l ? luminanceTone : 1 - luminanceTone;
  const posterTone = smoothstep(
    rule.posterThreshold - rule.posterSoftness,
    rule.posterThreshold + rule.posterSoftness,
    structureTone,
  );
  const edgeTone = shareHalftonePosterEdgeTone(
    pixels,
    sx,
    sy,
    sampleWidth,
    sampleHeight,
    lowLum,
    lumRange,
    baseInk,
    dotInk,
    luminanceTone,
    rule,
  );
  return clamp(
    colorTone * (1 - rule.posterWeight)
      + posterTone * rule.posterWeight
      + edgeTone * rule.edgeWeight,
    0,
    1,
  );
}

export function halftoneInkForPixel(r, g, b, imageInk, primaryInk, secondaryInk, x, y, rule = SHARE_HALFTONE_DOT_RULE) {
  const hsl = rgbToHsl(r, g, b);
  const ink = clamp(imageInk, 0, 1);
  const secondaryHueFit = hueDistance(hsl.h, secondaryInk.h);
  const primaryHueFit = hueDistance(hsl.h, primaryInk.h);
  const huePreference = hsl.s < 0.055
    ? 0
    : clamp((primaryHueFit - secondaryHueFit) / 180, -1, 1) * rule.hueBias;

  const cellPreference = halftoneCellWeave(x, y, rule.step) ? rule.weaveBias : -rule.weaveBias;
  const tonePreference = (0.5 - ink) * rule.toneBias;
  const secondaryChance = clamp(
    0.5 + cellPreference + tonePreference + huePreference,
    rule.minInterleave,
    rule.maxInterleave,
  );

  return halftoneCellNoise(x, y) < secondaryChance ? secondaryInk : primaryInk;
}

function halftoneCellWeave(x, y, step) {
  const cellX = Math.floor(x / step);
  const cellY = Math.floor(y / step);
  return (cellX + cellY) % 2 === 0;
}

function halftoneCellNoise(x, y) {
  const seed = Math.sin(Math.floor(x) * 12.9898 + Math.floor(y) * 78.233) * 43758.5453;
  return seed - Math.floor(seed);
}

function shareHalftonePosterEdgeTone(
  pixels,
  sx,
  sy,
  sampleWidth,
  sampleHeight,
  lowLum,
  lumRange,
  baseInk,
  dotInk,
  centerLuminanceTone,
  rule,
) {
  const centerShape = generatedDotToneAt(pixels, sx, sy, sampleWidth, sampleHeight, lowLum, lumRange, baseInk, dotInk, rule);
  const radius = Math.max(1, Math.round(rule.edgeRadius || 1));
  const offsets = [
    [-radius, 0],
    [radius, 0],
    [0, -radius],
    [0, radius],
  ];
  const maxEdge = offsets.reduce((peak, [dx, dy]) => {
    const neighborTone = generatedLuminanceToneAt(pixels, sx + dx, sy + dy, sampleWidth, sampleHeight, lowLum, lumRange, rule);
    const neighborShape = generatedDotToneAt(
      pixels,
      sx + dx,
      sy + dy,
      sampleWidth,
      sampleHeight,
      lowLum,
      lumRange,
      baseInk,
      dotInk,
      rule,
    );
    return Math.max(
      peak,
      Math.abs(centerLuminanceTone - neighborTone) * 0.72 + Math.abs(centerShape - neighborShape) * 0.28,
    );
  }, 0);
  return clamp(maxEdge * 1.8, 0, 1);
}

function generatedDotToneAt(pixels, sx, sy, sampleWidth, sampleHeight, lowLum, lumRange, baseInk, dotInk, rule) {
  const { r, g, b } = sampleHalftonePixel(pixels, sx, sy, sampleWidth, sampleHeight);
  const luminanceTone = generatedLuminanceToneAt(pixels, sx, sy, sampleWidth, sampleHeight, lowLum, lumRange, rule);
  return shareHalftoneDotToneForPixel(r, g, b, baseInk, dotInk, luminanceTone, rule);
}

function generatedLuminanceToneAt(pixels, sx, sy, sampleWidth, sampleHeight, lowLum, lumRange, rule) {
  const { r, g, b } = sampleHalftonePixel(pixels, sx, sy, sampleWidth, sampleHeight);
  const luminance = (0.299 * r + 0.587 * g + 0.114 * b) / 255;
  const normalized = clamp((luminance - lowLum) / lumRange, 0, 1);
  return clamp((normalized - 0.5) * rule.contrast + 0.5, 0, 1);
}

function sampleHalftonePixel(pixels, sx, sy, sampleWidth, sampleHeight) {
  const x = clamp(Math.round(sx), 0, sampleWidth - 1);
  const y = clamp(Math.round(sy), 0, sampleHeight - 1);
  const offset = (y * sampleWidth + x) * 4;
  return {
    r: pixels[offset],
    g: pixels[offset + 1],
    b: pixels[offset + 2],
  };
}

function normalizePlateInk(color, role, rule) {
  const hsl = color.h === undefined || color.s === undefined || color.l === undefined
    ? rgbToHsl(color.r, color.g, color.b)
    : color;
  const saturation = role === "base"
    ? hsl.s < 0.16
      ? clamp(hsl.s * 1.02, 0.04, 0.16)
      : clamp(hsl.s * rule.baseSaturationScale, 0.18, 0.5)
    : hsl.s < 0.18
      ? clamp(hsl.s * rule.dotSaturationScale, 0.04, 0.18)
      : clamp(hsl.s * rule.dotSaturationScale, 0.22, 0.74);
  const lightness = role === "base"
    ? clamp(hsl.l + rule.baseLightnessLift, 0.38, 0.82)
    : hsl.l < 0.2
      ? clamp(hsl.l + rule.dotLightnessLift * 0.22, 0.08, 0.28)
      : clamp(hsl.l + rule.dotLightnessLift, 0.48, 0.92);
  const [r, g, b] = hslToRgb(hsl.h, saturation, lightness);
  return { r, g, b, h: hsl.h, s: saturation, l: lightness };
}

function smoothstep(edge0, edge1, value) {
  if (edge0 === edge1) return value >= edge1 ? 1 : 0;
  const t = clamp((value - edge0) / (edge1 - edge0), 0, 1);
  return t * t * (3 - 2 * t);
}

function printableTone(radiusTone, rule) {
  const safeTone = clamp(radiusTone, 0, 1);
  if (safeTone < rule.paperThreshold) return 0;
  return clamp((safeTone - rule.paperThreshold) / (1 - rule.paperThreshold), 0, 1);
}

function rgbToHsl(r, g, b) {
  const red = r / 255;
  const green = g / 255;
  const blue = b / 255;
  const max = Math.max(red, green, blue);
  const min = Math.min(red, green, blue);
  let hue = 0;
  let saturation = 0;
  const lightness = (max + min) / 2;

  if (max !== min) {
    const delta = max - min;
    saturation = lightness > 0.5 ? delta / (2 - max - min) : delta / (max + min);
    if (max === red) {
      hue = (green - blue) / delta + (green < blue ? 6 : 0);
    } else if (max === green) {
      hue = (blue - red) / delta + 2;
    } else {
      hue = (red - green) / delta + 4;
    }
    hue *= 60;
  }

  return { h: hue, s: saturation, l: lightness };
}

function hueDistance(firstHue, secondHue) {
  const difference = Math.abs(firstHue - secondHue) % 360;
  return Math.min(difference, 360 - difference);
}

function hueFit(firstHue, secondHue) {
  return 1 - clamp(hueDistance(firstHue, secondHue) / 90, 0, 1);
}

function isUsableFeatureColor(color) {
  return color.l < 0.94;
}

function hslToRgb(hue, saturation, lightness) {
  const chroma = (1 - Math.abs(2 * lightness - 1)) * saturation;
  const huePrime = hue / 60;
  const x = chroma * (1 - Math.abs((huePrime % 2) - 1));
  let red = 0;
  let green = 0;
  let blue = 0;

  if (huePrime < 1) {
    red = chroma;
    green = x;
  } else if (huePrime < 2) {
    red = x;
    green = chroma;
  } else if (huePrime < 3) {
    green = chroma;
    blue = x;
  } else if (huePrime < 4) {
    green = x;
    blue = chroma;
  } else if (huePrime < 5) {
    red = x;
    blue = chroma;
  } else {
    red = chroma;
    blue = x;
  }

  const match = lightness - chroma / 2;
  return [
    Math.round((red + match) * 255),
    Math.round((green + match) * 255),
    Math.round((blue + match) * 255),
  ];
}

function clamp(value, min, max) {
  return Math.min(max, Math.max(min, value));
}