Merge PR #93: fix(audio) smooth, monotonic volume curve (#84)

# Conflicts:
#	src/audio/player.test.ts
#	src/audio/player.ts

src/audio/player.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect } from "vitest";
 import { mkdtempSync, writeFileSync, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall } from "./player.js";
+import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, shouldEndOnStall, volumeToFactor } from "./player.js";
 
 function getHeadersArg(args: string[]): string {
   const idx = args.indexOf("-headers");
@@ -89,6 +89,37 @@ describe("buildFfmpegArgs", () => {
   });
 });
 
+describe("volumeToFactor (#84 smooth volume curve)", () => {
+  it("is 0 at vol 0 and exactly 1.0 at vol 100 (full loudness still reserved at 100)", () => {
+    expect(volumeToFactor(0)).toBe(0);
+    expect(volumeToFactor(100)).toBe(1);
+  });
+
+  it("clamps out-of-range input", () => {
+    expect(volumeToFactor(-20)).toBe(0);
+    expect(volumeToFactor(150)).toBe(1);
+  });
+
+  it("is strictly monotonic across the whole range (no dead zone)", () => {
+    for (let v = 0; v < 100; v++) {
+      expect(volumeToFactor(v + 1)).toBeGreaterThan(volumeToFactor(v));
+    }
+  });
+
+  it("removes the old flat 80-99 dead zone", () => {
+    // Old mapping moved only 0.16 -> 0.198 across 80..99; new curve climbs clearly.
+    expect(volumeToFactor(99) - volumeToFactor(80)).toBeGreaterThan(0.3);
+  });
+
+  it("removes the discontinuity at 100 (old jump was ~0.8)", () => {
+    expect(volumeToFactor(100) - volumeToFactor(99)).toBeLessThan(0.1);
+  });
+
+  it("keeps the low range gentle", () => {
+    expect(volumeToFactor(50)).toBeLessThan(0.12);
+  });
+});
+
 describe("shouldUsePowerShellDownload", () => {
   const jdymusicUrl =
     "http://m801.music.126.net/20260507/abc/jdymusic/obj/xyz/song.mp3?vuutv=tok";

src/audio/player.ts

@@ -130,6 +130,21 @@ export function shouldEndOnStall(
   return false;
 }
 
+/**
+ * Maps a 0-100 volume value to a linear PCM gain factor (#84).
+ *
+ * Continuous and strictly monotonic over [0,100]: 0 at vol 0 and exactly 1.0 at
+ * vol 100. The previous mapping was a two-piece step — gain = (vol/100)*0.2 for
+ * vol<100 (so the whole 0-99 range only spanned 0..0.198, making 80->99 feel
+ * flat) then a raw passthrough at vol===100 (a ~5x jump). This single curve keeps
+ * the low end gentle but ramps smoothly toward full loudness near the top, so the
+ * slider feels proportional with no dead zone and no discontinuity at 100.
+ */
+export function volumeToFactor(volume: number): number {
+  const x = Math.max(0, Math.min(100, volume)) / 100;
+  return 0.2 * x + 0.8 * Math.pow(x, 8);
+}
+
 export interface PlayerEvents {
   frame: (opusFrame: Buffer) => void;
   trackEnd: () => void;
@@ -552,8 +567,9 @@ export class AudioPlayer extends EventEmitter {
   }
 
   private applyVolume(pcm: Buffer): Buffer {
-    if (this.volume === 100) return Buffer.from(pcm);
-    const factor = (this.volume / 100) * 0.2;
+    const factor = volumeToFactor(this.volume);
+    // factor === 1 only at volume 100; skip the per-sample loop at full loudness.
+    if (factor >= 1) return Buffer.from(pcm);
     const out = Buffer.alloc(pcm.length);
     for (let i = 0; i < pcm.length; i += 2) {
       let sample = Math.round(pcm.readInt16LE(i) * factor);