heros_web/scripts/generate-camp-audio.mjs

import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from 'node:fs';
import { tmpdir } from 'node:os';
import { basename, join, resolve, sep } from 'node:path';
import { spawnSync } from 'node:child_process';

const sampleRate = 44100;
const channels = 2;
const loopTailSeconds = 2;
const bgmOutputDir = join('src', 'assets', 'audio', 'bgm');
const ambienceOutputDir = join('src', 'assets', 'audio', 'ambience');
const ffmpeg = process.env.FFMPEG_PATH?.trim() || 'ffmpeg';

const musicTracks = [
  {
    name: 'camp-rally',
    title: 'Camp Rally',
    tempo: 76,
    bars: 16,
    root: 50,
    progression: [[0, 7, 12], [5, 12, 16], [7, 14, 19], [2, 9, 14]],
    melody: [12, 14, 16, 19, 21, 19, 16, 14, 12, 14, 19, 21, 19, 16, 14, 12],
    mood: 'rally',
    stringGain: 0.02,
    lowGain: 0.024,
    pluckGain: 0.014,
    leadGain: 0.018,
    drumGain: 0.025,
    pluckBeats: [0.12, 0.62, 1.12, 1.62, 2.12, 2.62, 3.12, 3.62],
    drumBeats: [0, 2, 2.5]
  },
  {
    name: 'camp-wayfarer',
    title: 'Camp Wayfarer',
    tempo: 64,
    bars: 16,
    root: 45,
    progression: [[0, 7, 10], [-2, 5, 10], [-5, 2, 7], [0, 5, 10]],
    melody: [12, 15, 17, 19, 17, 15, 12, 10, 7, 10, 12, 15, 12, 10, 7, 5],
    mood: 'wayfarer',
    stringGain: 0.017,
    lowGain: 0.016,
    pluckGain: 0.01,
    leadGain: 0.014,
    drumGain: 0.012,
    pluckBeats: [0.18, 2.18],
    drumBeats: []
  },
  {
    name: 'camp-grand-strategy',
    title: 'Camp Grand Strategy',
    tempo: 70,
    bars: 16,
    root: 43,
    progression: [[0, 7, 12], [5, 12, 17], [2, 9, 14], [-2, 5, 10]],
    melody: [12, 14, 19, 21, 19, 16, 14, 12, 14, 16, 19, 23, 21, 19, 16, 14],
    mood: 'grand',
    stringGain: 0.022,
    lowGain: 0.025,
    pluckGain: 0.015,
    leadGain: 0.019,
    drumGain: 0.028,
    pluckBeats: [0.12, 0.78, 1.45, 2.12, 2.78, 3.45],
    drumBeats: [0, 2, 3.5]
  },
  {
    name: 'camp-frontier',
    title: 'Camp Frontier',
    tempo: 66,
    bars: 16,
    root: 47,
    progression: [[0, 7, 10], [-2, 5, 10], [3, 10, 15], [0, 5, 12]],
    melody: [12, 15, 17, 19, 22, 19, 17, 15, 12, 10, 12, 15, 17, 15, 12, 10],
    mood: 'frontier',
    stringGain: 0.018,
    lowGain: 0.017,
    pluckGain: 0.013,
    leadGain: 0.016,
    drumGain: 0.02,
    pluckBeats: [0.18, 0.92, 1.68, 2.42, 3.18],
    drumBeats: [0, 1.5, 3]
  }
];

const ambienceTracks = [
  { name: 'campfire-ambience', title: 'Campfire Ambience', duration: 24, seed: 0x31a9f27d, kind: 'campfire' },
  { name: 'river-night-ambience', title: 'River Night Ambience', duration: 32, seed: 0x6c4d813b, kind: 'river' },
  { name: 'mountain-wind-ambience', title: 'Mountain Wind Ambience', duration: 36, seed: 0x52f0a7c1, kind: 'wind' },
  { name: 'nanzhong-night-ambience', title: 'Nanzhong Night Ambience', duration: 32, seed: 0x18bd4e63, kind: 'nanzhong' }
];

assertFfmpeg();
mkdirSync(bgmOutputDir, { recursive: true });
mkdirSync(ambienceOutputDir, { recursive: true });

const temporaryRoot = resolve(tmpdir());
const temporaryDir = mkdtempSync(join(temporaryRoot, 'heros-camp-audio-'));

try {
  for (const track of musicTracks) {
    const render = buildMusicTrack(track);
    prepareMaster(render.buffer, 68, 0.62, true);
    const loopBuffer = wrapTailIntoStart(render.buffer, render.loopDuration, loopTailSeconds);
    renderMp3(loopBuffer, track, bgmOutputDir, 192, -16, -1.5, 'music');
  }

  for (const track of ambienceTracks) {
    const render = buildAmbienceTrack(track);
    prepareMaster(render.buffer, 45, 0.54, false);
    const loopBuffer = wrapTailIntoStart(render.buffer, render.loopDuration, loopTailSeconds);
    renderMp3(loopBuffer, track, ambienceOutputDir, 128, -24, -3, 'ambience');
  }
} finally {
  const resolvedTemporaryDir = resolve(temporaryDir);
  const safeTemporaryPrefix = `${temporaryRoot}${sep}`;
  if (resolvedTemporaryDir.startsWith(safeTemporaryPrefix) && basename(resolvedTemporaryDir).startsWith('heros-camp-audio-')) {
    rmSync(resolvedTemporaryDir, { recursive: true, force: true });
  }
}

function buildMusicTrack(track) {
  const beat = 60 / track.tempo;
  const barDuration = beat * 4;
  const loopDuration = barDuration * track.bars;
  const renderBuffer = createBuffer(loopDuration + loopTailSeconds);

  for (let barIndex = 0; barIndex < track.bars; barIndex += 1) {
    const start = barIndex * barDuration;
    const chord = track.progression[barIndex % track.progression.length];
    const dynamics = musicDynamics(track.mood, barIndex, track.bars);

    addStringBed(renderBuffer, start, barDuration + 0.9, track, chord, dynamics);
    addLowPulse(renderBuffer, start, beat, track, chord, dynamics);
    addPluckedFigure(renderBuffer, start, beat, track, chord, barIndex, dynamics);
    addLeadNote(renderBuffer, start, beat, track, barIndex, dynamics);
    addDrumPattern(renderBuffer, start, beat, track, barIndex, dynamics);
    if ((track.mood === 'rally' || track.mood === 'grand') && (barIndex + 1) % 8 === 0) {
      addGong(renderBuffer, start + beat * 3.35, track.mood === 'grand' ? 0.012 : 0.01);
    }
    if (track.mood === 'wayfarer' && (barIndex === 3 || barIndex === 11)) {
      addBell(renderBuffer, start + beat * 3.1, 0.008);
    }
  }

  return { buffer: renderBuffer, loopDuration };
}

function musicDynamics(mood, barIndex, bars) {
  if (mood === 'grand') {
    const rise = Math.min(1, barIndex / Math.max(1, bars - 5));
    const release = barIndex >= bars - 4 ? (bars - barIndex) / 4 : 1;
    return (0.84 + rise * 0.16) * Math.max(0.88, release);
  }
  return 0.88 + Math.sin((barIndex / bars) * Math.PI) * 0.12;
}

function addStringBed(buffer, start, duration, track, chord, dynamics) {
  const notes = [
    { midi: track.root + chord[0] + 12, pan: -0.28, gain: 0.78 },
    { midi: track.root + chord[1] + 12, pan: 0.28, gain: 0.66 },
    { midi: track.root + chord[2] + 12, pan: -0.02, gain: 0.54 },
    { midi: track.root + chord[1] + 24, pan: 0.18, gain: 0.23 }
  ];

  notes.forEach((note, index) => addTone(buffer, {
    start: start + index * 0.04,
    duration,
    frequency: midiToFrequency(note.midi),
    gain: track.stringGain * note.gain * dynamics,
    pan: note.pan,
    attack: track.mood === 'grand' ? 0.8 : 1.15,
    release: 1.2,
    wave: stringWave
  }));
}

function addLowPulse(buffer, start, beat, track, chord, dynamics) {
  const pulses = track.mood === 'wayfarer' ? [0] : [0, 2];
  pulses.forEach((beatOffset) => addTone(buffer, {
    start: start + beat * beatOffset,
    duration: beat * (track.mood === 'wayfarer' ? 2.4 : 1.45),
    frequency: midiToFrequency(Math.max(38, track.root + chord[0])),
    gain: track.lowGain * dynamics,
    pan: -0.08,
    attack: 0.06,
    release: 0.62,
    wave: celloWave
  }));
}

function addPluckedFigure(buffer, start, beat, track, chord, barIndex, dynamics) {
  track.pluckBeats.forEach((beatOffset, index) => {
    const melodicOffset = track.melody[(barIndex + index) % track.melody.length];
    const chordLift = chord[index % chord.length] % 5;
    addTone(buffer, {
      start: start + beat * beatOffset,
      duration: beat * 0.66,
      frequency: midiToFrequency(track.root + melodicOffset + 12 + chordLift),
      gain: track.pluckGain * dynamics,
      pan: index % 2 === 0 ? -0.36 : 0.36,
      attack: 0.008,
      release: 0.2,
      wave: pluckWave,
      exponentialDecay: track.mood === 'wayfarer' ? 5.2 : 6.3
    });
  });
}

function addLeadNote(buffer, start, beat, track, barIndex, dynamics) {
  const noteOffset = track.melody[barIndex % track.melody.length];
  const leadStart = track.mood === 'wayfarer' ? 1.78 : track.mood === 'frontier' ? 1.35 : 1.18;
  const leadDuration = track.mood === 'wayfarer' ? beat * 1.65 : beat * 1.2;
  addTone(buffer, {
    start: start + beat * leadStart,
    duration: leadDuration,
    frequency: midiToFrequency(track.root + noteOffset + 12),
    gain: track.leadGain * dynamics,
    pan: track.mood === 'frontier' ? -0.08 : 0.12,
    attack: 0.12,
    release: 0.48,
    wave: reedWave
  });
}

function addDrumPattern(buffer, start, beat, track, barIndex, dynamics) {
  const beats = track.mood === 'wayfarer' && (barIndex === 7 || barIndex === 15)
    ? [0]
    : track.drumBeats;
  beats.forEach((beatOffset, index) => addTone(buffer, {
    start: start + beat * beatOffset,
    duration: beat * 0.48,
    frequency: index === 0 ? 88 : track.mood === 'frontier' ? 112 : 122,
    gain: track.drumGain * dynamics * (index === 0 ? 1 : 0.68),
    pan: index % 2 === 0 ? -0.04 : 0.04,
    attack: 0.004,
    release: 0.12,
    wave: drumWave,
    exponentialDecay: 8.8
  }));

  if (track.mood === 'frontier') {
    addTone(buffer, {
      start,
      duration: beat * 0.7,
      frequency: 66,
      gain: 0.026 * dynamics,
      pan: 0,
      attack: 0.004,
      release: 0.18,
      wave: drumWave,
      exponentialDecay: 7.4
    });
  }
}

function addGong(buffer, start, gain) {
  [
    { ratio: 1, amount: 0.72, pan: -0.05 },
    { ratio: 1.48, amount: 0.42, pan: 0.08 },
    { ratio: 2.07, amount: 0.24, pan: 0.2 }
  ].forEach(({ ratio, amount, pan }) => addTone(buffer, {
    start,
    duration: 2.2,
    frequency: 164 * ratio,
    gain: gain * amount,
    pan,
    attack: 0.008,
    release: 0.7,
    wave: gongWave,
    exponentialDecay: 1.7
  }));
}

function addBell(buffer, start, gain) {
  [1, 2.02, 3.94].forEach((ratio, index) => addTone(buffer, {
    start,
    duration: 2.1,
    frequency: 520 * ratio,
    gain: gain / (index + 1),
    pan: index % 2 === 0 ? -0.18 : 0.18,
    attack: 0.004,
    release: 0.6,
    wave: sineWave,
    exponentialDecay: 2.4
  }));
}

function buildAmbienceTrack(track) {
  const renderBuffer = createBuffer(track.duration + loopTailSeconds);
  if (track.kind === 'campfire') {
    synthesizeCampfire(renderBuffer, track);
  } else if (track.kind === 'river') {
    synthesizeRiverNight(renderBuffer, track);
  } else if (track.kind === 'wind') {
    synthesizeMountainWind(renderBuffer, track);
  } else {
    synthesizeNanzhongNight(renderBuffer, track);
  }
  return { buffer: renderBuffer, loopDuration: track.duration };
}

function synthesizeCampfire(buffer, track) {
  const random = createRandom(track.seed);
  let leftState = 0;
  let rightState = 0;
  const alpha = onePoleAlpha(2100);
  for (let index = 0; index < buffer.left.length; index += 1) {
    leftState += alpha * ((random() * 2 - 1) - leftState);
    rightState += alpha * ((random() * 2 - 1) - rightState);
    const t = index / sampleRate;
    const breathe = 0.7 + Math.sin(2 * Math.PI * 0.19 * t) * 0.16 + Math.sin(2 * Math.PI * 0.43 * t) * 0.08;
    buffer.left[index] += leftState * 0.035 * breathe + Math.sin(2 * Math.PI * 78 * t) * 0.003;
    buffer.right[index] += rightState * 0.033 * breathe + Math.sin(2 * Math.PI * 81 * t) * 0.0025;
  }

  const crackleCount = Math.floor((track.duration + loopTailSeconds) * 3.1);
  for (let event = 0; event < crackleCount; event += 1) {
    const start = random() * (track.duration + loopTailSeconds - 0.08);
    addNoiseBurst(buffer, random, start, 0.018 + random() * 0.045, 0.07 + random() * 0.1, random() * 1.4 - 0.7, 44 + random() * 34);
  }
}

function synthesizeRiverNight(buffer, track) {
  const random = createRandom(track.seed);
  let lowLeft = 0;
  let lowRight = 0;
  let previousLeft = 0;
  let previousRight = 0;
  const lowAlpha = onePoleAlpha(2200);
  const highAlpha = highPassAlpha(90);
  for (let index = 0; index < buffer.left.length; index += 1) {
    const t = index / sampleRate;
    const inputLeft = random() * 2 - 1;
    const inputRight = random() * 2 - 1;
    lowLeft += lowAlpha * (inputLeft - lowLeft);
    lowRight += lowAlpha * (inputRight - lowRight);
    const highLeft = highAlpha * ((buffer.left[index - 1] ?? 0) + lowLeft - previousLeft);
    const highRight = highAlpha * ((buffer.right[index - 1] ?? 0) + lowRight - previousRight);
    previousLeft = lowLeft;
    previousRight = lowRight;
    const swell = 0.72 + Math.sin(2 * Math.PI * 0.07 * t) * 0.18 + Math.sin(2 * Math.PI * 0.13 * t + 1.1) * 0.09;
    buffer.left[index] = highLeft * 0.055 * swell;
    buffer.right[index] = highRight * 0.055 * (1.46 - swell * 0.56);
  }

  for (let event = 0; event < 12; event += 1) {
    const start = random() * (track.duration + loopTailSeconds - 0.5);
    addTone(buffer, {
      start,
      duration: 0.24 + random() * 0.18,
      frequency: 820 + random() * 620,
      gain: 0.008 + random() * 0.006,
      pan: random() * 1.4 - 0.7,
      attack: 0.006,
      release: 0.16,
      wave: dropletWave,
      exponentialDecay: 7.2
    });
  }
}

function synthesizeMountainWind(buffer, track) {
  const random = createRandom(track.seed);
  let lowLeft = 0;
  let lowRight = 0;
  let highLeft = 0;
  let highRight = 0;
  let previousLowLeft = 0;
  let previousLowRight = 0;
  const lowAlpha = onePoleAlpha(1400);
  const highAlpha = highPassAlpha(180);
  for (let index = 0; index < buffer.left.length; index += 1) {
    const t = index / sampleRate;
    lowLeft += lowAlpha * ((random() * 2 - 1) - lowLeft);
    lowRight += lowAlpha * ((random() * 2 - 1) - lowRight);
    highLeft = highAlpha * (highLeft + lowLeft - previousLowLeft);
    highRight = highAlpha * (highRight + lowRight - previousLowRight);
    previousLowLeft = lowLeft;
    previousLowRight = lowRight;
    const gust = 0.56 + Math.sin(2 * Math.PI * 0.035 * t) * 0.2 + Math.sin(2 * Math.PI * 0.08 * t + 1.7) * 0.16 + Math.sin(2 * Math.PI * 0.17 * t) * 0.06;
    const whistle = Math.sin(2 * Math.PI * (510 + Math.sin(2 * Math.PI * 0.041 * t) * 90) * t) * Math.max(0, gust - 0.65) * 0.004;
    buffer.left[index] = highLeft * 0.05 * gust + whistle;
    buffer.right[index] = highRight * 0.048 * (gust * 0.92 + 0.08) + whistle * 0.72;
  }
}

function synthesizeNanzhongNight(buffer, track) {
  const random = createRandom(track.seed);
  let forestLeft = 0;
  let forestRight = 0;
  const alpha = onePoleAlpha(1250);
  for (let index = 0; index < buffer.left.length; index += 1) {
    const t = index / sampleRate;
    forestLeft += alpha * ((random() * 2 - 1) - forestLeft);
    forestRight += alpha * ((random() * 2 - 1) - forestRight);
    const pulse = 0.68 + Math.sin(2 * Math.PI * 0.09 * t) * 0.16;
    buffer.left[index] = forestLeft * 0.038 * pulse;
    buffer.right[index] = forestRight * 0.036 * (1.24 - pulse * 0.36);
  }

  for (let cluster = 0; cluster < 24; cluster += 1) {
    const clusterStart = random() * (track.duration + loopTailSeconds - 0.8);
    const chirps = 2 + Math.floor(random() * 4);
    for (let chirp = 0; chirp < chirps; chirp += 1) {
      addTone(buffer, {
        start: clusterStart + chirp * (0.055 + random() * 0.045),
        duration: 0.045 + random() * 0.04,
        frequency: 2800 + random() * 2400,
        gain: 0.006 + random() * 0.005,
        pan: random() * 1.6 - 0.8,
        attack: 0.003,
        release: 0.025,
        wave: chirpWave,
        exponentialDecay: 18
      });
    }
  }

  for (let frog = 0; frog < 9; frog += 1) {
    addTone(buffer, {
      start: random() * (track.duration + loopTailSeconds - 0.6),
      duration: 0.28 + random() * 0.24,
      frequency: 140 + random() * 70,
      gain: 0.008,
      pan: random() * 1.2 - 0.6,
      attack: 0.02,
      release: 0.18,
      wave: frogWave,
      exponentialDecay: 4.8
    });
  }
}

function addNoiseBurst(buffer, random, start, duration, gain, pan, decay) {
  const startIndex = Math.floor(start * sampleRate);
  const count = Math.floor(duration * sampleRate);
  const leftGain = Math.cos((pan + 1) * Math.PI * 0.25);
  const rightGain = Math.sin((pan + 1) * Math.PI * 0.25);
  for (let offset = 0; offset < count; offset += 1) {
    const index = startIndex + offset;
    if (index >= buffer.left.length) {
      break;
    }
    const t = offset / sampleRate;
    const sample = (random() * 2 - 1) * gain * Math.exp(-t * decay);
    buffer.left[index] += sample * leftGain;
    buffer.right[index] += sample * rightGain;
  }
}

function createBuffer(duration) {
  const length = Math.ceil(duration * sampleRate);
  return { left: new Float32Array(length), right: new Float32Array(length) };
}

function wrapTailIntoStart(buffer, loopDuration, tailSeconds) {
  const loopLength = Math.floor(loopDuration * sampleRate);
  const tailLength = Math.min(Math.floor(tailSeconds * sampleRate), buffer.left.length - loopLength, loopLength);
  const output = { left: buffer.left.slice(0, loopLength), right: buffer.right.slice(0, loopLength) };
  for (let index = 0; index < tailLength; index += 1) {
    const progress = index / Math.max(1, tailLength - 1);
    const tailGain = Math.cos(progress * Math.PI * 0.5);
    const startGain = Math.sin(progress * Math.PI * 0.5);
    output.left[index] = buffer.left[loopLength + index] * tailGain + output.left[index] * startGain;
    output.right[index] = buffer.right[loopLength + index] * tailGain + output.right[index] * startGain;
  }
  return output;
}

function addTone(buffer, options) {
  const startIndex = Math.max(0, Math.floor(options.start * sampleRate));
  const count = Math.max(1, Math.floor(options.duration * sampleRate));
  const leftGain = Math.cos((options.pan + 1) * Math.PI * 0.25);
  const rightGain = Math.sin((options.pan + 1) * Math.PI * 0.25);
  for (let offset = 0; offset < count; offset += 1) {
    const index = startIndex + offset;
    if (index >= buffer.left.length) {
      break;
    }
    const t = offset / sampleRate;
    const envelopeValue = options.exponentialDecay
      ? Math.min(1, t / Math.max(0.001, options.attack)) * Math.exp(-t * options.exponentialDecay)
      : envelope(t, options.duration, options.attack, options.release);
    const phase = 2 * Math.PI * options.frequency * t;
    const sample = options.wave(phase, t) * options.gain * envelopeValue;
    buffer.left[index] += sample * leftGain;
    buffer.right[index] += sample * rightGain;
  }
}

function envelope(t, duration, attack, release) {
  const attackLevel = Math.sin(Math.min(1, t / Math.max(0.001, attack)) * Math.PI * 0.5);
  const releaseLevel = Math.min(1, Math.max(0, duration - t) / Math.max(0.001, release));
  return Math.min(attackLevel, releaseLevel);
}

function stringWave(phase, t) {
  const bow = 1 + Math.sin(2 * Math.PI * 0.72 * t) * 0.018;
  return Math.sin(phase * bow) * 0.7 + Math.sin(phase * 2) * 0.13 + Math.sin(phase * 3) * 0.045;
}

function celloWave(phase) {
  return Math.sin(phase) * 0.78 + Math.sin(phase * 2) * 0.09 + Math.sin(phase * 0.5) * 0.04;
}

function reedWave(phase, t) {
  const vibrato = Math.sin(2 * Math.PI * 4.5 * t) * 0.006;
  return Math.sin(phase + vibrato) * 0.86 + Math.sin(phase * 2) * 0.045;
}

function pluckWave(phase) {
  return Math.sin(phase) * 0.82 + Math.sin(phase * 2) * 0.12 + Math.sin(phase * 3) * 0.035;
}

function drumWave(phase, t) {
  const pitchDrop = 1 - Math.min(1, t * 5.8) * 0.48;
  return Math.sin(phase * pitchDrop) * 0.92 + Math.sin(phase * 2.02) * 0.06;
}

function gongWave(phase) {
  return Math.sin(phase) * 0.76 + Math.sin(phase * 1.48) * 0.17 + Math.sin(phase * 2.07) * 0.09;
}

function sineWave(phase) {
  return Math.sin(phase);
}

function dropletWave(phase, t) {
  return Math.sin(phase * (1 + t * 0.22)) * 0.88 + Math.sin(phase * 2.2) * 0.08;
}

function chirpWave(phase, t) {
  return Math.sin(phase * (1 + t * 4.8));
}

function frogWave(phase, t) {
  return Math.sin(phase) * 0.72 + Math.sin(phase * 0.5) * 0.18 + Math.sin(phase * (1.5 + t * 0.4)) * 0.08;
}

function midiToFrequency(midi) {
  return 440 * 2 ** ((midi - 69) / 12);
}

function onePoleAlpha(cutoff) {
  return 1 - Math.exp((-2 * Math.PI * cutoff) / sampleRate);
}

function highPassAlpha(cutoff) {
  const rc = 1 / (2 * Math.PI * cutoff);
  const dt = 1 / sampleRate;
  return rc / (rc + dt);
}

function createRandom(seed) {
  let state = seed >>> 0;
  return () => {
    state ^= state << 13;
    state ^= state >>> 17;
    state ^= state << 5;
    return (state >>> 0) / 0x100000000;
  };
}

function prepareMaster(buffer, highPassCutoff, targetPeak, room) {
  highPass(buffer.left, highPassCutoff);
  highPass(buffer.right, highPassCutoff);
  if (room) {
    applyRoom(buffer);
  }
  normalize(buffer, targetPeak);
  softLimit(buffer);
}

function highPass(samples, cutoff) {
  const alpha = highPassAlpha(cutoff);
  let previousOutput = 0;
  let previousInput = samples[0] ?? 0;
  for (let index = 0; index < samples.length; index += 1) {
    const input = samples[index];
    const output = alpha * (previousOutput + input - previousInput);
    samples[index] = output;
    previousOutput = output;
    previousInput = input;
  }
}

function applyRoom(buffer) {
  const originalLeft = Float32Array.from(buffer.left);
  const originalRight = Float32Array.from(buffer.right);
  [
    { delay: 0.075, gain: 0.07 },
    { delay: 0.132, gain: 0.045 }
  ].forEach(({ delay, gain }) => {
    const samples = Math.floor(sampleRate * delay);
    for (let index = samples; index < buffer.left.length; index += 1) {
      buffer.left[index] += originalRight[index - samples] * gain;
      buffer.right[index] += originalLeft[index - samples] * gain;
    }
  });
}

function normalize(buffer, targetPeak) {
  let peak = 0;
  for (let index = 0; index < buffer.left.length; index += 1) {
    peak = Math.max(peak, Math.abs(buffer.left[index]), Math.abs(buffer.right[index]));
  }
  if (peak <= 0) {
    return;
  }
  const scale = targetPeak / peak;
  for (let index = 0; index < buffer.left.length; index += 1) {
    buffer.left[index] *= scale;
    buffer.right[index] *= scale;
  }
}

function softLimit(buffer) {
  const divisor = Math.tanh(1.04);
  for (let index = 0; index < buffer.left.length; index += 1) {
    buffer.left[index] = Math.tanh(buffer.left[index] * 1.04) / divisor;
    buffer.right[index] = Math.tanh(buffer.right[index] * 1.04) / divisor;
  }
}

function renderMp3(buffer, track, outputDir, bitrateKbps, loudness, truePeak, kind) {
  const wavPath = join(temporaryDir, `${track.name}.wav`);
  const outputPath = join(outputDir, `${track.name}.mp3`);
  writeFileSync(wavPath, encodeWav(buffer));
  const result = spawnSync(ffmpeg, [
    '-y', '-hide_banner', '-loglevel', 'error',
    '-i', wavPath,
    '-af', `loudnorm=I=${loudness}:LRA=6:TP=${truePeak}`,
    '-c:a', 'libmp3lame',
    '-b:a', `${bitrateKbps}k`,
    '-ar', String(sampleRate),
    '-ac', String(channels),
    '-write_xing', '1',
    '-metadata', `title=${track.title}`,
    '-metadata', 'artist=Heros Web Procedural Ensemble',
    '-metadata', 'comment=Project-original deterministic synthesis; no sampled source audio.',
    outputPath
  ], { encoding: 'utf8' });

  if (result.status !== 0) {
    throw new Error(`ffmpeg failed for ${track.name}: ${result.stderr || result.stdout}`);
  }
  const seam = inspectEncodedLoopSeam(outputPath, track.name, kind);
  console.log(
    `Generated ${outputPath} (seam ${seam.boundaryJump.toFixed(4)} / ${seam.absoluteLimit.toFixed(4)}, ` +
      `peak ${(seam.peakRatio * 100).toFixed(1)}%)`
  );
}

function inspectEncodedLoopSeam(outputPath, trackName, kind) {
  const result = spawnSync(ffmpeg, [
    '-hide_banner', '-loglevel', 'error',
    '-i', outputPath,
    '-f', 'f32le',
    '-c:a', 'pcm_f32le',
    '-ar', String(sampleRate),
    '-ac', String(channels),
    'pipe:1'
  ], { maxBuffer: 96 * 1024 * 1024 });

  if (result.status !== 0 || !Buffer.isBuffer(result.stdout)) {
    throw new Error(`ffmpeg could not decode ${trackName} for loop seam verification: ${result.stderr?.toString() || ''}`);
  }

  const bytesPerFrame = channels * 4;
  const frameCount = Math.floor(result.stdout.length / bytesPerFrame);
  if (frameCount < 2) {
    throw new Error(`${trackName} did not decode enough samples for loop seam verification.`);
  }

  const sample = (frame, channel) => result.stdout.readFloatLE(frame * bytesPerFrame + channel * 4);
  let signalPeak = 0;
  let deltaSquaredSum = 0;
  let deltaCount = 0;
  for (let channel = 0; channel < channels; channel += 1) {
    signalPeak = Math.max(signalPeak, Math.abs(sample(0, channel)));
  }
  for (let frame = 1; frame < frameCount; frame += 1) {
    for (let channel = 0; channel < channels; channel += 1) {
      const current = sample(frame, channel);
      const delta = current - sample(frame - 1, channel);
      signalPeak = Math.max(signalPeak, Math.abs(current));
      deltaSquaredSum += delta * delta;
      deltaCount += 1;
    }
  }

  let boundaryJump = 0;
  for (let channel = 0; channel < channels; channel += 1) {
    boundaryJump = Math.max(boundaryJump, Math.abs(sample(0, channel) - sample(frameCount - 1, channel)));
  }
  const deltaRms = Math.sqrt(deltaSquaredSum / Math.max(1, deltaCount));
  const absoluteLimit = kind === 'music'
    ? Math.min(0.04, Math.max(0.02, deltaRms * 4))
    : Math.min(0.12, Math.max(0.04, deltaRms * 4));
  const peakRatioLimit = kind === 'music' ? 0.1 : 0.75;
  const peakRatio = boundaryJump / Math.max(signalPeak, Number.EPSILON);
  if (
    !Number.isFinite(boundaryJump) ||
    boundaryJump > absoluteLimit ||
    peakRatio > peakRatioLimit
  ) {
    throw new Error(
      `${trackName} loop seam is too abrupt after MP3 encoding: jump ${boundaryJump.toFixed(6)}, ` +
        `limit ${absoluteLimit.toFixed(6)}, peak ratio ${(peakRatio * 100).toFixed(2)}%.`
    );
  }

  return { boundaryJump, absoluteLimit, peakRatio };
}

function encodeWav(buffer) {
  const bytesPerSample = 2;
  const frameCount = buffer.left.length;
  const dataSize = frameCount * channels * bytesPerSample;
  const wav = Buffer.alloc(44 + dataSize);
  wav.write('RIFF', 0);
  wav.writeUInt32LE(36 + dataSize, 4);
  wav.write('WAVE', 8);
  wav.write('fmt ', 12);
  wav.writeUInt32LE(16, 16);
  wav.writeUInt16LE(1, 20);
  wav.writeUInt16LE(channels, 22);
  wav.writeUInt32LE(sampleRate, 24);
  wav.writeUInt32LE(sampleRate * channels * bytesPerSample, 28);
  wav.writeUInt16LE(channels * bytesPerSample, 32);
  wav.writeUInt16LE(16, 34);
  wav.write('data', 36);
  wav.writeUInt32LE(dataSize, 40);
  for (let index = 0; index < frameCount; index += 1) {
    const offset = 44 + index * channels * bytesPerSample;
    wav.writeInt16LE(Math.round(Math.max(-1, Math.min(1, buffer.left[index])) * 32767), offset);
    wav.writeInt16LE(Math.round(Math.max(-1, Math.min(1, buffer.right[index])) * 32767), offset + bytesPerSample);
  }
  return wav;
}

function assertFfmpeg() {
  const result = spawnSync(ffmpeg, ['-version'], { encoding: 'utf8' });
  if (result.status !== 0) {
    throw new Error(`ffmpeg is required to generate camp audio. Set FFMPEG_PATH when it is not on PATH. ${result.stderr || ''}`);
  }
}