export function buildThumbnailCandidateTimes(durationSeconds) {
  const duration = Number(durationSeconds);
  if (!Number.isFinite(duration) || duration <= 0) {
    return [0.15];
  }

  const upperBound = Math.max(0.05, duration - 0.05);
  const baseCandidates = [0.15, 0.5, 1, 2, 3.5, 5];
  const candidates = [];

  for (const candidate of baseCandidates) {
    const clamped = Math.max(0, Math.min(candidate, upperBound));
    if (candidates.length === 0 || Math.abs(candidates[candidates.length - 1] - clamped) > 0.08) {
      candidates.push(Number(clamped.toFixed(3)));
    }
  }

  if (upperBound > 0.25) {
    const fallback = Number(upperBound.toFixed(3));
    if (Math.abs(candidates[candidates.length - 1] - fallback) > 0.08) {
      candidates.push(fallback);
    }
  }

  return candidates;
}

export function analyzeFrameLuma(data, options = {}) {
  const bytes = data instanceof Uint8ClampedArray ? data : new Uint8ClampedArray(data ?? []);
  const pixelCount = Math.floor(bytes.length / 4);
  if (pixelCount === 0) {
    return {
      sampleCount: 0,
      averageLuma: 0,
      darkRatio: 1,
      isBlackFrame: true
    };
  }

  const {
    darknessThreshold = 18,
    maxDarkRatio = 0.985,
    averageThreshold = 22,
    stride = 16
  } = options;

  let samples = 0;
  let darkSamples = 0;
  let lumaTotal = 0;

  for (let index = 0; index < pixelCount; index += stride) {
    const offset = index * 4;
    const red = bytes[offset];
    const green = bytes[offset + 1];
    const blue = bytes[offset + 2];
    const alpha = bytes[offset + 3];
    const luma = alpha === 0 ? 0 : 0.2126 * red + 0.7152 * green + 0.0722 * blue;
    lumaTotal += luma;
    samples += 1;

    if (luma <= darknessThreshold) {
      darkSamples += 1;
    }
  }

  const averageLuma = samples > 0 ? lumaTotal / samples : 0;
  const darkRatio = samples > 0 ? darkSamples / samples : 1;
  const isBlackFrame = averageLuma <= averageThreshold && darkRatio >= maxDarkRatio;

  return {
    sampleCount: samples,
    averageLuma,
    darkRatio,
    isBlackFrame
  };
}