refactor: extract pcm audio helpers

src/audio/pcm.ts

@@ -0,0 +1,28 @@
+export function upsample24kMonoTo48kStereo(mono24k: Buffer): Buffer {
+  const numSamples = mono24k.length / 2;
+  const out = Buffer.alloc(numSamples * 8);
+
+  for (let i = 0; i < numSamples; i++) {
+    const sample = mono24k.readInt16LE(i * 2);
+    const base = i * 8;
+    out.writeInt16LE(sample, base);
+    out.writeInt16LE(sample, base + 2);
+    out.writeInt16LE(sample, base + 4);
+    out.writeInt16LE(sample, base + 6);
+  }
+
+  return out;
+}
+
+export function rmsDb(pcm: Buffer): number {
+  let sum = 0;
+  const samples = pcm.length / 2;
+
+  for (let i = 0; i < samples; i++) {
+    const sample = pcm.readInt16LE(i * 2) / 32768;
+    sum += sample * sample;
+  }
+
+  const rms = Math.sqrt(sum / samples);
+  return 20 * Math.log10(rms);
+}

src/webserver.ts

@@ -12,6 +12,7 @@ import express, {
 import helmet from "helmet";
 import * as prism from "prism-media";
 import { WebSocketServer } from "ws";
+import { rmsDb, upsample24kMonoTo48kStereo } from "./audio/pcm";
 import { config } from "./config";
 import { AppError } from "./errors";
 import { createChildLogger, logger } from "./logger";
@@ -165,32 +166,6 @@ function patchSharedUIState(patch: SharedUIStatePatch) {
   return getSharedUIState();
 }
 
-// Upsample 24kHz mono s16le → 48kHz stereo s16le (pure JS)
-function upsample(mono24k: Buffer): Buffer {
-  const numSamples = mono24k.length / 2;
-  const out = Buffer.alloc(numSamples * 8);
-  for (let i = 0; i < numSamples; i++) {
-    const s = mono24k.readInt16LE(i * 2);
-    const base = i * 8;
-    out.writeInt16LE(s, base);
-    out.writeInt16LE(s, base + 2);
-    out.writeInt16LE(s, base + 4);
-    out.writeInt16LE(s, base + 6);
-  }
-  return out;
-}
-
-// Calculate RMS dB level of a PCM s16le buffer
-function rmsDb(pcm: Buffer): number {
-  let sum = 0;
-  const samples = pcm.length / 2;
-  for (let i = 0; i < samples; i++) {
-    const s = pcm.readInt16LE(i * 2) / 32768;
-    sum += s * s;
-  }
-  const rms = Math.sqrt(sum / samples);
-  return 20 * Math.log10(rms);
-}
 
 export async function startWebserver(
   port: number = 3000,
@@ -517,7 +492,7 @@ export async function startWebserver(
       lastBrowserAudioTime = Date.now();
 
       // Upsample 24kHz mono → 48kHz stereo and add to buffer
-      const upsampled = upsample(data);
+      const upsampled = upsample24kMonoTo48kStereo(data);
 
       // Cap buffer to avoid runaway growth during stall
       if (pcmBuffer.length < MAX_BUF_BYTES) {

tests/audio/pcm.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, it } from "vitest";
+import { rmsDb, upsample24kMonoTo48kStereo } from "../../src/audio/pcm";
+
+describe("PCM helpers", () => {
+  it("upsamples 24kHz mono s16le to 48kHz stereo s16le", () => {
+    const input = Buffer.alloc(4);
+    input.writeInt16LE(1000, 0);
+    input.writeInt16LE(-1000, 2);
+
+    const output = upsample24kMonoTo48kStereo(input);
+
+    expect(output.length).toBe(16);
+    expect(output.readInt16LE(0)).toBe(1000);
+    expect(output.readInt16LE(2)).toBe(1000);
+    expect(output.readInt16LE(4)).toBe(1000);
+    expect(output.readInt16LE(6)).toBe(1000);
+    expect(output.readInt16LE(8)).toBe(-1000);
+    expect(output.readInt16LE(10)).toBe(-1000);
+    expect(output.readInt16LE(12)).toBe(-1000);
+    expect(output.readInt16LE(14)).toBe(-1000);
+  });
+
+  it("calculates RMS dB for non-silent PCM", () => {
+    const input = Buffer.alloc(4);
+    input.writeInt16LE(32767, 0);
+    input.writeInt16LE(32767, 2);
+
+    expect(rmsDb(input)).toBeCloseTo(0, 1);
+  });
+});