Drum machine · kimi-k3

Let me build a single-file index.html with a 16-step drum sequencer using Web Audio API. Then test with browser tools.

Plan:
- Four rows: kick, snare, hi-hat, clap, 16 toggleable steps each.
- Default pattern: kick on 0,4,8,12; snare on 4,12; hi-hat on every even step (or every 2); clap on 4? Let's do: kick 0,4,8,12; snare 4,12; hi-hat 0,2,4,...14 (eighth notes); clap on 8? Keep simple audible.
- Play/Stop/Clear buttons, BPM input (range slider + number display) 60-180.
- Scheduler: lookahead scheduling with setInterval (25ms), scheduleAheadTime 0.1s. Use AudioContext.currentTime. Each scheduled step triggers voices for enabled rows and schedules playhead highlight via requestAnimationFrame loop based on scheduled step times.
- Highlight: keep array of {step, time} and in rAF, find current step based on audioCtx.currentTime.
- Stop: clear interval, increment a "run id" so scheduled callbacks don't fire, cancel highlight. Since sounds are scheduled via oscillator start/stop with envelopes (short), future hits within lookahead may still play — but small. To "cancel future hits", we can track active source nodes and stop them, or just accept ~100ms tail. Better: keep a generation counter; scheduled events are created only slightly ahead, fine. Also track scheduled nodes to stop them immediately on Stop.
- Distinct sounds:
  - Kick: sine osc pitch drop 150->50Hz, gain envelope.
  - Snare: noise buffer + bandpass/highpass filter, plus a triangle osc body.
  - Hi-hat: noise through highpass filter, short envelope (closed).
  - Clap: noise through bandpass ~1000-1500Hz, several quick bursts envelope.
- BPM change: just update bpm variable; scheduler computes step duration from bpm each step. No duplicate sequences since single interval.
- Editing steps during playback: toggle cell class + state array; scheduler reads state at schedule time.

Write file, then test in browser: check console errors, click Play, screenshot playhead advancing, change BPM, stop/start.

Initial render looks good. No console errors. Now click Play and check playhead advances.

Playback started. I don't see a highlighted step in the screenshot though — maybe timing of screenshot between highlight updates, or highlight isn't showing. Let me take another screenshot to check if the playhead moves. The screenshot may not show "current" box-shadow clearly... Actually looking closely, none of the steps show a white ring. Hmm. Possibly rAF isn't capturing, or the screenshot caught a moment where active = -1 (stepTimes filled -1 then first highlight only when stepTimes[i] <= now... should work).

Wait — one bug: in highlightLoop, the condition `stepTimes[i] > (active >= 0 ? stepTimes[active] : -1)` picks the step with max time <= now, and additionally requires now - stepTimes[i] < stepDuration()*2. But since stepTimes persists from earlier laps, a step from a previous lap may have the same... no, times increase monotonically per lap, so the latest scheduled step <= now is the right one. But after 16 steps, stepTimes[i] all get overwritten each lap, so max time <= now works. However the "now - stepTimes[i] < stepDuration()*2" guard: steps are scheduled up to 0.12s ahead; at 120bpm step duration is 0.125s, so a step time <= now that's up to 2*0.125=0.25s old passes. Fine.

But maybe screenshot timing just missed the visual since the frame captured at a moment... The box-shadow should persist for a full step (125ms). Let me take another screenshot / use motion to check.

Playhead is advancing: frames show highlight at step ~9, ~13, ~2 (wrap), ~6. Good. Now test BPM change — drag the slider. Slider at x≈750, y≈93, range 140px wide starting x≈683. Drag to the right to increase BPM. Then toggle a step while playing. Then Stop, verify pattern retained and playhead resets, then Play again and check highlight starts at step 1.

Let me drag the slider right (to ~180).

BPM updated to 179 live, still playing, playhead still advancing (highlight at step 14). Now toggle a step during playback — e.g., turn on a clap step that's off, say clap step 2 (x≈385, y≈277). Grid: row clap y≈277, steps start x≈351, spacing 39px. Step index 1 (2nd step) center ≈ 351+39=390. Let me toggle clap step at x≈430 (index 3).

Clap step 3 toggled on (blue at index 2) while playing; playback continues (highlight at step ~6). Now test Stop: should clear highlight and reset. Then Play again: pattern retained and playhead restarts at step 1.

Stopped. Pattern retained (including the added clap step at index 2). However, I notice the highlight ring is still visible at step 2 in the screenshot! The "current" class should have been removed by clearHighlight(). Look at the screenshot: there's a white box around column index 1 (second step) in all 4 rows. That's the leftover highlight.

Wait — is that the leftover highlight or something else? It looks like a white ring on the second column across all rows — yes, that's the playhead highlight that wasn't cleared. Hmm, why?

In stop(): cancelAnimationFrame(rafId), then clearHighlight(). clearHighlight removes "current" from lastHighlighted column. But there could be a race: a pending rAF callback might have already been queued... cancelAnimationFrame should cancel it. But actually the issue could be different: stop() is called from a click event; the highlight was at some step, clearHighlight removes it. Then... nothing re-adds since isPlaying is false. 

Wait — maybe the screenshot was captured before... no, DOM shows status "Stopped". Hmm.

Actually, look more carefully: could that be focus styling? The clicked element was the Stop button, not a step. The white ring on column 2 across all rows — that matches `.step.current` box-shadow. So clearHighlight didn't work.

Why? Possible bug: in stop(), we do `activeNodes.forEach(node => node.stop(...))` — fine. `clearHighlight()` uses lastHighlighted. But wait: the rAF loop — cancelAnimationFrame(rafId) cancels the most recently scheduled frame. But inside highlightLoop, rafId = requestAnimationFrame(highlightLoop) is scheduled each frame. When stop() cancels it, no more frames. clearHighlight should then remove classes.

Unless... lastHighlighted was -1 at stop time? The screenshot right before stop showed highlight at step ~6. Hmm, but the click on Stop — between the previous screenshot and this click, playback continued. Highlight should be somewhere.

Oh wait! I see the bug. In stop(), I call `clearInterval`, `cancelAnimationFrame`, then stop nodes, then `clearHighlight()`. That's fine. But could a rAF callback run AFTER clearHighlight? cancelAnimationFrame was called before, so no.

Hmm, but actually — is it possible the browser tool's screenshot shows a state where a rAF ran between click handler start and... no, JS is single-threaded; click handler runs to completion.

Wait, maybe the issue: two rAF loops? When I clicked Play only once... isPlaying guard prevents duplicates. Only one loop.

Alternatively — maybe the screenshot frame was captured with a slight delay, and what we see is... no, highlight can't be re-added after stop.

Let me think about clearHighlight: it removes "current" from `stepEls[v][lastHighlighted]` for lastHighlighted >= 0. Should work.

Hmm — unless highlightLoop ran one more time AFTER stop set isPlaying=false... no, it checks `if (!isPlaying) return;` at top.

Wait, actually there might be a subtle race: the click event → stop() runs fully. No more highlight updates. So why is column 2 highlighted?

Could that ring actually be a focus outline? Buttons focused via click... the last clicked step button was the clap step (x=429, y=277) — that's column index 2. And the ring appears on ALL four rows at column... let me look again. The ring appears at column index 1 (second column, x≈390). The clicked clap step was at x=429 (column index 2, third column) — which is blue/on. The ring is at x≈390, column index 1.

Hmm, ring across all 4 rows at the same column = playhead highlight. So it IS the leftover highlight at step index 1.

So clearHighlight failed or something re-added. Let me reconsider: the order in stop()... `cancelAnimationFrame(rafId)`. Is rafId correct? In highlightLoop, `rafId = requestAnimationFrame(highlightLoop)` — yes updated each frame.

OH WAIT. I see it now. The bug: in `play()`, I set `lastHighlighted = -1` AFTER... no.

Hmm, let me check the highlightLoop logic for adding: `if (active !== lastHighlighted)`. When stop happens mid-frame... no race.

Actually, maybe the actual sequence: browser tool clicked Stop; the click handler ran; but the screenshot rendering shows CSS class still applied because... Let me just inspect the DOM to check whether "current" classes are present.

Actually, another thought: maybe stop() threw an exception before clearHighlight! In stop(): `activeNodes.forEach(node => { try { node.stop(...) } catch(e){} })` — safe. `clearInterval(schedulerTimer)` fine. `cancelAnimationFrame(rafId)` fine. clearHighlight fine. `playBtn.textContent`... all fine. The status DID update to "Stopped. Pattern retained..." which is the LAST line of stop(). So stop() completed fully, including clearHighlight. So clearHighlight ran but lastHighlighted must have been -1?? But highlight was visible...

Unless the highlight visible at stop time was a DIFFERENT mechanism. Hmm... wait — is it possible there are TWO rAF loops running? Play clicked once... but I clicked Play at the start, then... only once. isPlaying guard.

Hold on — what about the SPACE bar handler? No.

Let me just inspect the DOM/classes now to see the current state.

The highlight is now gone — no leftover "current" ring. The earlier screenshot must have been captured one frame before the class removal rendered (or it was transient). Actually in the previous screenshot the ring was present; in this one it's gone. It was likely a screenshot timing artifact — or possibly the removal happened one frame later. Either way, current state is clean: pattern retained (including the added clap step 3), highlight cleared.

Hmm, but was it a real one-frame lag? clearHighlight runs synchronously in stop(), and the screenshot was taken after the click completed... The previous screenshot showed status text already "Stopped" AND ring present. That means at render time the class was still applied. But now it's gone. Since no JS runs after stop to remove it... unless a pending rAF callback ran AFTER stop. Wait — highlightLoop checks `if (!isPlaying) return;` at the top, so a stray callback wouldn't re-add. But could a stray rAF callback have run BETWEEN the click's class removal and the screenshot, re-adding? No — it returns early.

Alternatively: two rAF loops? Only one. Hmm, actually there is a subtle possibility: rAF callbacks scheduled before cancelAnimationFrame... cancelAnimationFrame cancels the pending one. OK.

Another explanation: the browser tool captured the screenshot from a compositor frame that was produced slightly before the DOM update but with DOM text already updated? The text update and class removal happen in the same synchronous handler, so they'd render together. Odd, but the current verified state is clean, and re-inspection shows no highlight. It's fine — possibly the tool's screenshot pipeline raced. 

To be safe against any stale-frame weirdness, I could also clear highlight in a rAF after stop. Not necessary. Let's move on: click Play again and confirm playhead restarts at step 1 with retained pattern.

Restarted, pattern retained. The screenshot was taken right at start, before highlight appears (step 1 plays at currentTime+0.05). Let me grab one more screenshot quickly to see highlight at an early step, and then test Clear. Actually, let me do a quick screenshot then stop, then clear.

Restarted at an early step (step 6 here — taken ~0.5s after click at 179 BPM ≈ 84ms/step, so step ~6 makes sense for restart from step 1). Pattern retained. Now test Stop then Clear.

The Clear click registered ("Pattern cleared." status), but the screenshot still shows the steps in their colored state — they look dimmed/desaturated though. Actually looking closely, the colored cells appear faded (opacity reduced?), suggesting mid-transition (CSS transition 0.08s on background). The screenshot may have caught the transition. Let me take another screenshot to confirm steps are off.

Clear works — all steps disabled. Everything verified:

- Initial render: 4 labeled rows × 16 steps, default pattern, no console errors.
- Play: playhead advances and wraps (motion frames showed steps 9→13→2→6).
- BPM drag live 120→179 while playing; single scheduler interval (no duplicates possible).
- Step toggled on during playback; playback continued.
- Stop: highlight cleared, status shows retained pattern; playhead resets to step 1 on next Play (restarted and highlight near start).
- Clear: all steps off.

Unverified: actual audio output (can't hear in headless browser, but AudioContext code paths executed without console errors — no exceptions thrown, meaning oscillators/noise were created and scheduled). Space bar shortcut untested but non-essential.

Done. Final report.