src/audio/AudioPlayer.ts

const DEFAULT_SAMPLE_RATE = 16000;

export class AudioPlayer {
  private context: AudioContext | null = null;
  private nextPlayTime = 0;
  private started = false;
  private resumed = false;
  private pendingBytes = 0;
  private bufferThresholdBytes = 6400; // 200ms at 16kHz default
  private pendingBuffers: AudioBuffer[] = [];
  // Carries the leftover byte when a chunk has odd byte length, so PCM sample
  // boundaries stay aligned across chunk splits from the HTTP stream.
  private leftoverByte: number | null = null;

  init(sampleRate = DEFAULT_SAMPLE_RATE): void {
    if (this.context) {
      if (this.context.sampleRate === sampleRate) return;
      this.context.close();
      this.context = null;
    }
    this.context = new AudioContext({ sampleRate });
    this.bufferThresholdBytes = Math.floor(sampleRate * 0.5) * 2; // 500ms
    this.nextPlayTime = 0;
    this.started = false;
    this.resumed = false;
    this.pendingBytes = 0;
    this.pendingBuffers = [];
    this.leftoverByte = null;
  }

  enqueue(rawPcm: ArrayBuffer, onStarted?: () => void): void {
    if (!this.context) return;

    // Prepend any leftover byte from the previous chunk so that Int16 sample
    // boundaries are always aligned, regardless of how the HTTP stream splits.
    let pcmBytes: Uint8Array;
    if (this.leftoverByte !== null) {
      const combined = new Uint8Array(1 + rawPcm.byteLength);
      combined[0] = this.leftoverByte;
      combined.set(new Uint8Array(rawPcm), 1);
      pcmBytes = combined;
      this.leftoverByte = null;
    } else {
      pcmBytes = new Uint8Array(rawPcm);
    }

    // If still odd, save the trailing byte for the next chunk.
    if (pcmBytes.byteLength % 2 !== 0) {
      this.leftoverByte = pcmBytes[pcmBytes.byteLength - 1];
      pcmBytes = pcmBytes.slice(0, pcmBytes.byteLength - 1);
    }

    if (pcmBytes.byteLength === 0) return;

    const int16 = new Int16Array(pcmBytes.buffer, pcmBytes.byteOffset, pcmBytes.byteLength / 2);
    const float32 = new Float32Array(int16.length);
    for (let i = 0; i < int16.length; i++) {
      float32[i] = int16[i] / 32768;
    }

    const audioBuffer = this.context.createBuffer(1, float32.length, this.context.sampleRate);
    audioBuffer.copyToChannel(float32, 0);

    this.pendingBytes += rawPcm.byteLength;

    if (this.resumed) {
      // AudioContext is running — schedule immediately
      const source = this.context.createBufferSource();
      source.buffer = audioBuffer;
      source.connect(this.context.destination);
      source.start(this.nextPlayTime);
      this.nextPlayTime += audioBuffer.duration;
    } else {
      // Still buffering: waiting for threshold or waiting for ctx.resume() to complete
      this.pendingBuffers.push(audioBuffer);

      if (!this.started && this.pendingBytes >= this.bufferThresholdBytes) {
        this.started = true;
        void this.context.resume().then(() => {
          if (!this.context) return;
          this.resumed = true;
          this.nextPlayTime = this.context.currentTime;
          onStarted?.();
          const toSchedule = this.pendingBuffers.splice(0);
          this.pendingBuffers = [];
          for (const buf of toSchedule) {
            const source = this.context.createBufferSource();
            source.buffer = buf;
            source.connect(this.context.destination);
            source.start(this.nextPlayTime);
            this.nextPlayTime += buf.duration;
          }
        });
      }
    }
  }

  // Call after the stream ends to play any buffered audio that hasn't started yet
  // (handles responses shorter than the buffer threshold)
  flush(onStarted?: () => void): void {
    if (!this.context || this.started || this.pendingBuffers.length === 0) return;
    this.started = true;
    void this.context.resume().then(() => {
      if (!this.context) return;
      this.resumed = true;
      this.nextPlayTime = this.context.currentTime;
      onStarted?.();
      const toSchedule = this.pendingBuffers.splice(0);
      this.pendingBuffers = [];
      for (const buf of toSchedule) {
        const source = this.context.createBufferSource();
        source.buffer = buf;
        source.connect(this.context.destination);
        source.start(this.nextPlayTime);
        this.nextPlayTime += buf.duration;
      }
    });
  }

  drain(): void {
    // Let queued buffers play out — nothing to do, the AudioContext schedule handles it
  }

  stopImmediately(): void {
    if (!this.context) return;
    this.context.close();
    this.context = null;
    this.nextPlayTime = 0;
    this.started = false;
    this.resumed = false;
    this.pendingBytes = 0;
    this.pendingBuffers = [];
    this.leftoverByte = null;
  }
}

export function replayAudio(
  chunks: ArrayBuffer[],
  sampleRate: number
): () => void {
  const ctx = new AudioContext({ sampleRate });
  let nextTime = ctx.currentTime + 0.05;

  for (const chunk of chunks) {
    const int16 = new Int16Array(chunk, 0, Math.floor(chunk.byteLength / 2));
    const float32 = new Float32Array(int16.length);
    for (let i = 0; i < int16.length; i++) float32[i] = int16[i] / 32768;
    const buf = ctx.createBuffer(1, float32.length, sampleRate);
    buf.copyToChannel(float32, 0);
    const src = ctx.createBufferSource();
    src.buffer = buf;
    src.connect(ctx.destination);
    src.start(nextTime);
    nextTime += buf.duration;
  }

  return () => ctx.close();
}