Add regression test scaffolding

AGENTS.md

@@ -0,0 +1,4 @@
+# Contributor Guidelines
+
+- Every code change must be accompanied by regression tests.
+- Any user-reported bug signals a failure in both implementation and testing. When a bug is discovered, document the testing gap and add guidelines here to prevent similar issues in the future. This file is our living record of lessons learned and improved testing strategies; keep it updated.

package.json

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+{
+  "name": "audio-interpolator",
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "test": "node --test"
+  }
+}

src/index.html

@@ -86,7 +86,8 @@
     </div>
   </div>
 
-<script>
+<script type="module">
+import { TARGET_SAMPLE_RATE, humanTime, clamp, bytesHuman } from './utils.js';
 (() => {
   // ---- Utilities -----------------------------------------------------------
   const $ = (sel) => document.querySelector(sel);
@@ -106,16 +107,6 @@
   let files = { A: null, B: null };
   let lastBlobUrl = null;
 
-  const TARGET_SAMPLE_RATE = 44100; // Common denominator; browsers will resample if needed
-
-  function humanTime(seconds) {
-    if (!isFinite(seconds)) return '—';
-    const m = Math.floor(seconds / 60);
-    const s = Math.round(seconds % 60).toString().padStart(2,'0');
-    return `${m}:${s}`;
-  }
-  function clamp(v, lo, hi){ return Math.min(hi, Math.max(lo, v)); }
-
   function setStatus(text, kind='info') {
     statusEl.textContent = text;
     statusEl.style.color = kind === 'error' ? '#ff9c9c' : kind === 'ok' ? '#8ef18e' : '#aab6c4';
@@ -209,12 +200,6 @@
     }
   });
 
-  function bytesHuman(n){
-    if (n < 1024) return n + ' B';
-    const u = ['KB','MB','GB']; let i = -1; do { n/=1024; i++; } while (n>=1024 && i<u.length-1);
-    return n.toFixed(n<10?2:1)+' '+u[i];
-  }
-
   function suggestOutName(a, b){
     function base(n){ return (n||'track').replace(/\.[^.]+$/, ''); }
     return `${base(a)}__AVERAGE__${base(b)}.wav`;

src/utils.js

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+export const TARGET_SAMPLE_RATE = 44100; // Common denominator; browsers will resample if needed
+
+export function humanTime(seconds) {
+  if (!isFinite(seconds)) return '—';
+  const m = Math.floor(seconds / 60);
+  const s = Math.round(seconds % 60).toString().padStart(2,'0');
+  return `${m}:${s}`;
+}
+
+export function clamp(v, lo, hi) {
+  return Math.min(hi, Math.max(lo, v));
+}
+
+export function bytesHuman(n){
+  if (n < 1024) return n + ' B';
+  const u = ['KB','MB','GB']; let i = -1; do { n/=1024; i++; } while (n>=1024 && i<u.length-1);
+  return n.toFixed(n<10?2:1)+' '+u[i];
+}

test/utils.test.js

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+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { humanTime, clamp, bytesHuman } from '../src/utils.js';
+
+test('humanTime formats seconds', () => {
+  assert.equal(humanTime(0), '0:00');
+  assert.equal(humanTime(65), '1:05');
+});
+
+test('clamp limits values', () => {
+  assert.equal(clamp(5, 0, 3), 3);
+  assert.equal(clamp(-1, 0, 3), 0);
+});
+
+test('bytesHuman converts bytes to human-readable form', () => {
+  assert.equal(bytesHuman(500), '500 B');
+  assert.equal(bytesHuman(1024), '1.00 KB');
+});