v3-api/src/audio/audio-normalization.service.ts

import { spawn } from 'node:child_process';
import { stat, unlink } from 'node:fs/promises';
import { tmpdir } from 'node:os';
import { join } from 'node:path';
import { randomUUID } from 'node:crypto';
import { Injectable, Logger } from '@nestjs/common';
import { ConfigService } from '@nestjs/config';
import { AudioNormalizationError } from './audio-normalization.errors';

/**
 * Normalizes arbitrary browser voice captures to **AAC-LC in MP4** (`.m4a`, `audio/mp4`).
 *
 * Rationale: AAC-LC in an MP4 container is natively supported by iOS Safari, Android Chrome,
 * and desktop browsers; `-movflags +faststart` moves metadata to the head of the file for
 * faster start/seek over HTTP. Mono + 96k is a strong voice default (size vs quality).
 */
@Injectable()
export class AudioNormalizationService {
  private readonly logger = new Logger(AudioNormalizationService.name);
  private readonly ffmpegPath: string;
  private readonly ffprobePath: string;
  private readonly transcodeTimeoutMs: number;

  constructor(private readonly config: ConfigService) {
    this.ffmpegPath = this.config.get<string>('AUDIO_FFMPEG_PATH')?.trim() || 'ffmpeg';
    this.ffprobePath = this.config.get<string>('AUDIO_FFPROBE_PATH')?.trim() || 'ffprobe';
    const raw = this.config.get<string>('AUDIO_TRANSCODE_TIMEOUT_MS');
    const n = raw != null ? Number(raw) : NaN;
    this.transcodeTimeoutMs = Number.isFinite(n) && n > 0 ? n : 120_000;
  }

  /**
   * Transcode any input FFmpeg can demux to single-channel AAC in MP4 (`.m4a`).
   * Caller must delete `outputAbsolutePath` and the input temp file after upload.
   */
  async transcodeIncomingVoiceToM4a(
    inputAbsolutePath: string,
  ): Promise<{ outputAbsolutePath: string; durationSec?: number }> {
    const outputAbsolutePath = join(tmpdir(), `voice-norm-${randomUUID()}.m4a`);

    const args = [
      '-nostdin',
      '-hide_banner',
      '-loglevel',
      'error',
      '-y',
      '-i',
      inputAbsolutePath,
      '-vn',
      '-sn',
      '-dn',
      '-ac',
      '1',
      '-ar',
      '48000',
      '-c:a',
      'aac',
      '-b:a',
      '96k',
      '-profile:a',
      'aac_low',
      '-movflags',
      '+faststart',
      outputAbsolutePath,
    ];

    try {
      await this.runProcess(this.ffmpegPath, args, 'ffmpeg');
    } catch (e: unknown) {
      await unlink(outputAbsolutePath).catch(() => {});
      throw e;
    }

    try {
      const st = await stat(outputAbsolutePath);
      if (!st.size) {
        throw new AudioNormalizationError('normalized_audio_empty');
      }
    } catch (e: unknown) {
      await unlink(outputAbsolutePath).catch(() => {});
      throw e;
    }

    let durationSec: number | undefined;
    try {
      durationSec = await this.probeDurationSec(outputAbsolutePath);
    } catch (err: unknown) {
      this.logger.debug(`ffprobe duration skipped: ${(err as Error)?.message}`);
    }

    return { outputAbsolutePath, durationSec };
  }

  private async probeDurationSec(filePath: string): Promise<number | undefined> {
    const args = [
      '-v',
      'error',
      '-show_entries',
      'format=duration',
      '-of',
      'default=noprint_wrappers=1:nokey=1',
      filePath,
    ];
    const stdout = await this.runProcessCaptureStdout(this.ffprobePath, args, 'ffprobe');
    const n = parseFloat(String(stdout).trim());
    return Number.isFinite(n) ? n : undefined;
  }

  private runProcess(
    command: string,
    args: string[],
    label: string,
  ): Promise<void> {
    return new Promise((resolve, reject) => {
      const child = spawn(command, args, {
        stdio: ['ignore', 'pipe', 'pipe'],
        windowsHide: true,
      });

      let stderr = '';
      const onData = (c: Buffer) => {
        stderr += c.toString();
        if (stderr.length > 64_000) stderr = stderr.slice(-32_000);
      };
      child.stderr?.on('data', onData);

      const timer = setTimeout(() => {
        child.kill('SIGKILL');
        reject(
          new AudioNormalizationError(`${label}_timeout_${this.transcodeTimeoutMs}ms`),
        );
      }, this.transcodeTimeoutMs);

      child.on('error', (err: NodeJS.ErrnoException) => {
        clearTimeout(timer);
        if (err?.code === 'ENOENT') {
          reject(
            new AudioNormalizationError(
              `${label}_binary_not_found_install_ffmpeg_package`,
            ),
          );
        } else {
          reject(err);
        }
      });

      child.on('close', (code) => {
        clearTimeout(timer);
        if (code === 0) {
          resolve();
          return;
        }
        const tail = stderr.trim().slice(-800);
        reject(
          new AudioNormalizationError(
            `${label}_failed_exit_${code}${tail ? `:${tail}` : ''}`,
          ),
        );
      });
    });
  }

  private runProcessCaptureStdout(
    command: string,
    args: string[],
    label: string,
  ): Promise<string> {
    return new Promise((resolve, reject) => {
      const child = spawn(command, args, {
        stdio: ['ignore', 'pipe', 'pipe'],
        windowsHide: true,
      });
      let stdout = '';
      let stderr = '';
      child.stdout?.on('data', (c: Buffer) => {
        stdout += c.toString();
      });
      child.stderr?.on('data', (c: Buffer) => {
        stderr += c.toString();
      });

      const timer = setTimeout(() => {
        child.kill('SIGKILL');
        reject(
          new AudioNormalizationError(`${label}_timeout_${this.transcodeTimeoutMs}ms`),
        );
      }, this.transcodeTimeoutMs);

      child.on('error', (err: NodeJS.ErrnoException) => {
        clearTimeout(timer);
        if (err?.code === 'ENOENT') {
          reject(
            new AudioNormalizationError(
              `${label}_binary_not_found_install_ffmpeg_package`,
            ),
          );
        } else {
          reject(err);
        }
      });

      child.on('close', (code) => {
        clearTimeout(timer);
        if (code === 0) {
          resolve(stdout);
          return;
        }
        reject(
          new AudioNormalizationError(
            `${label}_failed_exit_${code}:${stderr.trim().slice(-400)}`,
          ),
        );
      });
    });
  }
}