feat(streamer): fft + log-bin spectrum analyzer

streamer/tests/fft.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+import { SpectrumAnalyzer } from "../src/fft.ts";
+
+function sineFrame(freqHz: number, sampleRate = 48000, samples = 1024): Buffer {
+  const buf = Buffer.alloc(samples * 2 * 2); // s16le stereo
+  for (let i = 0; i < samples; i++) {
+    const v = Math.round(Math.sin((2 * Math.PI * freqHz * i) / sampleRate) * 16000);
+    buf.writeInt16LE(v, i * 4);
+    buf.writeInt16LE(v, i * 4 + 2);
+  }
+  return buf;
+}
+
+describe("SpectrumAnalyzer", () => {
+  it("produces N bars of normalized [0..1] floats", () => {
+    const a = new SpectrumAnalyzer({ bars: 48, sampleRate: 48000 });
+    a.feed(sineFrame(1000));
+    const bars = a.bars();
+    expect(bars).toHaveLength(48);
+    bars.forEach((v) => {
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThanOrEqual(1);
+    });
+  });
+
+  it("a 1khz sine puts most energy in low bars (logarithmic binning)", () => {
+    const a = new SpectrumAnalyzer({ bars: 48, sampleRate: 48000 });
+    a.feed(sineFrame(1000));
+    const bars = a.bars();
+    const lowSum = bars.slice(0, 16).reduce((s, v) => s + v, 0);
+    const highSum = bars.slice(32).reduce((s, v) => s + v, 0);
+    expect(lowSum).toBeGreaterThan(highSum * 5);
+  });
+
+  it("silence produces all-near-zero bars", () => {
+    const a = new SpectrumAnalyzer({ bars: 72, sampleRate: 48000 });
+    a.feed(Buffer.alloc(1024 * 4));
+    const bars = a.bars();
+    bars.forEach((v) => expect(v).toBeLessThan(0.01));
+  });
+});