Drum machine · qwen3.8-27b

Build a single-file drum sequencer using Web Audio API scheduling (a lookahead scheduler for accurate timing). Let's write the code.

Note: On Stop, the scheduled audio (within 100ms of lookahead) will still be played, but that's not a problem. However, there's a concern about "duplicate sequences." Since there's only one scheduler, no duplication occurs. BPM changes are handled because secondsPerStep is computed every tick. Good.

One issue: scheduleStep contains setTimeout logic, and scheduleVisual is duplicated. scheduleStep itself also sets up a setTimeout for the highlight, and scheduleVisual is also called — visual highlighting is doubled up. Let's clean this up: remove the setTimeout from scheduleStep, and keep only scheduleVisual.

There's a subtle bug: `currentStep` is incremented inside schedulerTick, and scheduleVisual's timeout checks `stepIndex === currentStep`, but by that point currentStep has already advanced. In other words, the highlight check fails! The timeout fires at the time the step is played, and by that point currentStep has advanced to the next step (or even further). Therefore, the highlight won't be displayed.

Proposed fix: instead of a comparison, directly call showStep(stepIndex) and track whether that step is the latest one. A better approach: keep a `lastDisplayed` counter, or simply: if the step hasn't been skipped yet, the timeout fires when that step becomes the playhead. Simplest approach: keep a monotonically increasing step count `totalSteps` in parallel with the currentStep. Inside scheduleVisual: capture totalSteps at schedule time; when the timeout fires, if totalSteps - stepIndex === 1... hmm.

Actually, a simpler approach: use a "visual queue." Each scheduled step has a step number (mod 16) and a sequence index. Maintain a variable `seqCounter` that increments for every scheduled step. Inside the timeout, fire only if playing and the captured seq === seqCounter - 1 (i.e., it's the most recent one scheduled and hasn't been overtaken). At playback speed, each step's timeout fires roughly at the moment the next step gets scheduled (with a 100ms lookahead and lookahead of 25ms, steps are scheduled roughly 100ms ahead; at 120bpm the step time is 125ms — so the next step gets scheduled 25ms after the timeout would fire... no wait.

Lookahead is 100ms, so steps get scheduled up to 100ms ahead. Step time at 120bpm = 125ms. So step N is scheduled around 100ms before its time, and the timeout fires 100ms later — at which point step N+1 is not yet scheduled (that one is scheduled at its time minus 100ms = 25ms later). At 180bpm, step time = 83ms < 100ms lookahead, so multiple steps get scheduled in a single tick. In that case, when the timeout for step N fires, the scheduled seq might already be N+2. Condition: display step N only if seqCounter - seq <= 1? At 180bpm, if you're 1 step ahead, that's fine — the currentStep display would be 1 step ahead of the actual time, but within 83ms. Acceptable as a highlight.

The simplest and most robust approach: use a timestamp-based approach and do the highlighting inside the timeout. Use a `visualStepSeq` counter, and only fire if `seq === visualStepSeq - 1 || seq === visualStepSeq`? Let's implement:

- `let schedSeq = 0;` inside schedulerTick, after scheduling: pass `schedSeq++` to scheduleVisual.
- Inside the timeout: if playing and (schedSeq - seq === 1) → this is the step that just "became current." At a tick where N steps were scheduled at once, the seqs are N, N+1, ..., N+k-1, and the latest is N+k-1 = schedSeq-1. So the last-scheduled step fires, and the others are silently skipped. The highlight advances to the latest — at very high tempos a small jump, but that's fine.

But wait: when the timeout for step N fires at its scheduled time, schedSeq has already advanced (if the next step is scheduled before the timeout fires, at fast tempos it's at least N+1... at 120bpm, the next one is scheduled 25ms later, so at firing time schedSeq = N+1 (after the first schedule)... let's trace: tick 0: schedules step 0 (seq 0), schedSeq=1. timeout(0) fires at t=0+delay. At that moment, schedSeq - 0 = 1 → fires. Good. At high tempo, tick 0: schedules steps 0, 1 (seqs 0, 1), schedSeq=2. timeout(0) fires: schedSeq - 0 = 2 → skip. timeout(1) fires: schedSeq - 1 = 1 → step 1 fires. Good.

If stop() is called, clear the timeouts. And what if a step's timeout fires after a stop and subsequent restart? The timeouts have been cleared. No problem.

Also, if the scheduler runs and steps are skipped due to a timeout delay... acceptable.

Let's rewrite these parts.

Another issue: on Stop, sounds that have already been scheduled (within the 100ms lookahead window) will keep playing. This is a slight leak but acceptable — the spec says "Stop cancels future hits." At 120bpm the step is 125ms so lookahead is less than 1 step; at 180bpm (83ms/step) up to 2 steps might be audible. To really cancel, we need a master gain node that gets muted on stop. Let's add a master gain: connect all voices to a master gain; on stop, ramp the master to 0 to silence the tail, and reset it to 1 on the next start.

Wait a sec — is the re-enabling of masterGain inside `ensureAudio` placed after the `if (!audioCtx)` block? Let me check where I put it. I put it inside ensureAudio after resume — let me check the file around ensureAudio. Also, note that `ensureAudio` is called on every click of any element, so it resets masterGain to 1 on every click during playback — this is mostly harmless, but if you click a step while playing, the cancelScheduledValues after the stop's ramp... if you click a step while playing, the masterGain gets reset to 1 — no problem (it's already 1 anyway). But if you click during the stop's 20ms ramp, it might cause a small pop sound — negligible.

Hmm, actually there's an ordering issue: ensureAudio re-enables masterGain on every click — what if the user clicks Play immediately after Stop? No problem, it gets reset to 1, which is the desired behavior.

Let me verify the structure of the file.

Wait — there's an important bug: `scheduleStep` reads the pattern at the point of scheduling (lookahead 100ms). The spec says "editing steps ... must affect playback" — reading the pattern at scheduling time is fine (100ms latency is negligible). But the requirement "starting again uses the currently displayed pattern" — since we're reading pattern, that's satisfied. OK.

Also, I need to check the placement of the masterGain re-enable inside ensureAudio. Let me take a look at that section.

There's a problem: `ensureAudio` gets called on every click/keydown (a global listener). If it's called while playing (e.g., when clicking on a step during playback), it resets the master gain to 1 — but that's fine. However, if a step is clicked right after a stop (within the 20ms ramp), a tiny click sound might occur — negligible level.

Wait, there's a subtle issue: if the user clicks a step button while playing, ensureAudio re-sets masterGain to 1 at the current time — canceling the pending value of 1. No problem.

But one edge case: does clicking the "Stop" button trigger ensureAudio via the document-level click listener before the button's handler? The document listener fires on bubble — the button's handler runs first (target phase), then the document handler. So stop() runs, and then ensureAudio re-arms masterGain to 1, canceling the silent ramp — if it's within 20ms, a small residual sound might occur. The ramp is 20ms; the re-arm happens immediately within the same task, so basically the residual sound gets fully re-armed! This defeats the purpose of Stop's cancellation.

Fix: In ensureAudio, only re-arm when called from start(). Move the re-arm into start() rather than ensureAudio.

Also, the stop ramp: `linearRampToValueAtTime(0.0001, currentTime+0.02)` — but the value at currentTime might be 1, so the residual sound will be cut within 20ms. Good.

Next, let's run it in the browser and test.

The UI is displayed correctly, and the default pattern also looks correct. The console warnings are from the harness side and are unrelated. Next, click Play and check the playhead progression.

The DOM showed "1 / 16" — currently playing. In the screenshot, the counter at the bottom shows "– / 16", but in the DOM snapshot at the moment of the click it showed "1 / 16". The DOM snapshot is taken at a different moment. Let's take another screenshot to check if the playhead is progressing.

Bug: every step in every row is getting the "current" highlight (a white outline). Step 5's showStep is supposed to clear all "current" and then add it only to the cells of step 5. But in the screenshot, outlines are showing on every cell.

Wait — looking closely: every cell has a white outline. That means the clear in showStep isn't working? Let me look: `ROWS.forEach(row => stepCells[row][i].classList.remove("current"))` then add. That looks correct…

Could it be… the step buttons are being shared? stepCells[row] is a new array per row, each with 16 separate buttons. Hmm.

Wait, look again — the outline in the screenshot: all cells have a white ring. That suggests classList isn't being removed. Or maybe… the `schedSeq - seq <= 1` condition — multiple timeouts fire and each calls showStep with a different stepIndex, and each call first clears. But in the end only one column should be lit up.

Hold on — maybe the issue is: `showStep(i)` removes "current" from `stepCells[row][i]` — only that column. If showStep(5) runs after showStep(4), column 4's highlight stays! Because it only clears its own column. Right! It should clear all columns first. That's the bug: showStep should clear "current" from all 64 cells, then add it to column i.

But wait, if so, the columns that passed through earlier should accumulate — the screenshot shows highlights on all 16 columns, which is consistent with the playhead having passed through about 5–6 steps… but all 16 are highlighted, more than 6. Hmm, the playhead is at 5/16. Steps 0–5 should be highlighted = 6 columns, not 16.

Could it be… the catch-up logic from a previous version? No, that's been deleted.

Hmm, let me reconsider: the `schedSeq - seq <= 1` condition: multiple timeouts fire, but showStep clears its own column each time… actually wait — the remove is executed against the same i that's being added, so after showStep(i), only column i has current. Then showStep(i+1) removes column i+1 (which wasn't there) and adds it to column i+1. So columns 0..i all stay highlighted. At step 5, that should be 6 columns.

But the screenshot has all 16 with outlines. Maybe the counter "5/16" but the display… the playhead has already wrapped around? At 120bpm it's 125ms per step, and about 0.6 seconds have passed since the click… actually the DOM showed 1/16 at the time of the click, and this screenshot might be several seconds later (screenshot calls are spaced out by the round-trip of my messages). Yes! There were multiple tool round-trips. So the playhead wrapped around all the way to the full 16 steps. It's consistent.

Fix: clear all highlights before adding.

Also, the counter was showing 5/16 in the DOM, but the screenshot shows all 16 columns with highlights... well, the counter should match now. Let me reload and test again.

Wait—my fix: in the first loop I used ROWS.length and ROWS to iterate, but the original code was ROWS.forEach. That's fine. But actually there's still a subtle problem: at 120bpm, each step takes 125ms, and the lookahead timeout fires at the time of the step—there might be multiple pending timeouts, but since each showStep clears everything, only the latest one is displayed. Good.

I'll retest playback.

Wait — I edited the file, but the browser reload was using the updated version. I'm currently playing. I'll take a screenshot to check the playhead.

Currently, only one column is highlighted (step 6 is displayed, the counter shows 6/16, and the DOM is at 7/16 — it's advancing).

Next, I'll test BPM changes during playback. Drag the BPM slider to the right, take a screenshot, and confirm it's still running. After that, stop and check whether the counter resets. Then, while the pattern is retained, start from step 1.

BPM is now 180, and it's still playing (counter is 1/16, so it just wrapped around). Single column highlighted. Good. Next, click Stop and confirm that the counter resets to "– / 16" and the highlight is cleared.

Stop worked: status is "Stopped", counter is "– / 16". But there's a residual highlight on column 8! clearHighlight should have removed all of them... wait, the screenshot shows a highlight on column 8 (kick is an outlined white, snare/hihat/clap have scale+glow from .on.current?). Actually clearHighlight removes all "current". But in the screenshot, the column 8 cells are displayed as enlarged with a white outline.

Hmm — maybe it's a pending visual timeout that fired before stop cleared it, after the clear? No: stop clears the timeouts, and clearHighlight removes the classes. The screenshot was taken a moment later; the DOM showed "– / 16" and "Stopped" — the timeout was cleared before the screenshot.

Wait, the highlight appears on column 8. The pending visual timeout with the schedSeq condition: stop clears all pendingVisuals... pendingVisuals holds ids. But `pendingVisuals = pendingVisuals.filter(x => x !== id)` — the ids are numbers, and when a timeout fires, setTimeout's ids are recycled... filter correctly removes the fired ones. stop clears all the ids that were still pending.

Could it be: the timeout fired in the same task between my stop() and the screenshot — but the pendingVisuals.forEach(clearTimeout) should cancel it. Unless the highlight was set by a timeout that had already fired and completed (in which case it's no longer in pendingVisuals) — that is, the "current" class was legitimately added by the last showStep before stop, and stop's clearHighlight should have removed it.

Unless... clearHighlight was executed but then... no, nothing re-adds after stop.

Wait, let me look at the screenshot again: kick column 8 has a white outline, and the snare/hihat/clap cells in column 8 look larger (scale 1.08) with a glow. That's the .current styling. Which means clearHighlight didn't clear? Or this is a CSS transition lag? A box-shadow transition over 0.1s — the screenshot was taken right after; but the white outline is fully visible.

Hmm, let me actually double-check: was clearHighlight called? Inside stop(): yes, clearHighlight() is called. Let me check the current file contents around stop() to make sure the edit wasn't broken.

The code looks correct: clearHighlight removes "current" from all cells. But in the screenshot the highlight is displayed on column 8. Explanation: at 180bpm, the visual timeout fires at the moment of the step. The pending timeout fires just before the click, and it's possible the click's clearHighlight was executed within the same frame… No, the click comes later.

Another explanation: a pending visual timeout fires right before my click (the last scheduled step), and then stop clears it — no problem. So why does the highlight remain?

Hmm wait — maybe the timeout fired during the screenshot's rendering after the click? The timeout had already been cleared, so no.

Could it be… the id of the setTimeout: `pendingVisuals = pendingVisuals.filter(x => x !== id)` — no problem.

Oh! I've got it. In `scheduleVisual`'s timeout, the condition is `if (playing && schedSeq - seq <= 1) showStep(stepIndex)`. stop sets playing=false before clearing — no problem.

Let me think about the timing of the screenshot: at the moment of the click, the DOM showed "Stopped" and "– / 16". The screenshot was taken after that. The highlight on column 8 was set by a showStep that ran just before stop… and clearHighlight should have removed it.

However, what if the browser tool's screenshot was taken from a slightly stale frame… or, the visual timeout fired "after" stop, because setTimeout(fn, delayMs) had a small delay that elapsed before my click (the click happened about 100ms later). A timeout that fires "between" the last scheduler tick and the click: it runs, playing is still true, and schedSeq - seq <= 1 → showStep sets the highlight. After that, my click → stop() → clearHighlight should remove it.

But… does the click on the Stop button also trigger the document's "click" listener → ensureAudio — no effect on the class.

Hmm, maybe the browser tool's click was actually dispatched… the DOM snapshot showed "Stopped". Then the screenshot. It should be cleared.

Maybe a race condition: the visual timeout fires in the same millisecond right after clearHighlight? The order can't be guaranteed, but if the timeout was scheduled with delayMs and fires at T, and the click at T+1, then the highlight is set at T, and cleared at T+1. Unless the click's clearHighlight ran "before" the pending timeout fired — but the click should clear the pending timeout… unless the timeout had already started executing (can't be cleared mid-execution). If the timeout fired just before the click handler (same task queue), the class was added, and then cleared. So the final state: no highlight.

So why the highlight in the screenshot? Maybe it's not "current" — look at the image: the kick cell at column 8 has a white outline; snare/hihat/clap at column 8 have a glow (looks like a box-shadow). This is exactly the .current style.

Possibility: two pending timeouts for the same step? No, one per step.

Possibility: schedSeq - seq <= 1 lets an "old" timeout pass through? schedSeq increases with scheduling; the condition is that this step is among the most recent 1. No problem.

Wait — maybe the issue is that `pendingVisuals.filter(x => x !== id)` uses a numeric comparison of the captured `id` — no problem.

Hmm, let me reconsider: the screenshot may have been captured "before" the click handler's effect was rendered, but after the DOM snapshot? The DOM text shows "Stopped" and the text changed, so the handler has run. classList changes and text changes occur within the same task. If the text is showing "Stopped", then clearHighlight has already run.

But… clearHighlight is referencing the wrong variable? Let me check clearHighlight:

ROWS.forEach(row => stepCells[row].forEach(c => c.classList.remove("current")));

This is correct.

Or maybe there's a second timer… the setTimeout in the old scheduleStep — did my edits completely remove it? Let me check the file.

The code looks correct. So why is the highlight still remaining? Let me check the screenshot again: in column 8, the kick cell has a white border, and the other cells have a soft glow. That's the `.step.current` box-shadow: 0 0 0 2px #fff, 0 0 12px. And the snare/hihat/clap cells are enlarged — that's the scale 1.08 of `.step.on.current`. So the "current" class was present on the column 8 cell at the time of the screenshot.

Maybe it's a timing issue: the screenshot tool might capture a frame from before the click. But the DOM text shows Stopped. DOM text and CSS classes are in the same task…

Or: the visual timeout for step 8 (0-indexed, i.e., the 9th column? The screenshot shows the 8th column (0-indexed 7)? Let me count: the highlighted column is the 8th cell (1-indexed index 8). Hmm.

Actually — could the setTimeout have fired in between: the click occurred, stop() was executed, and then a microtask/macrotask… no, that's not it.

Another hypothesis: `pendingVisuals.forEach(clearTimeout)` — setTimeout ids are integers. No problem there.

Wait, is there a possibility that a new scheduler tick scheduled a visual after stop's clearing? No, the interval is cleared first, and clearInterval prevents any further ticks… Actually clearInterval(lookaheadTimer) — but if the interval callback is already executing (in the middle of schedulerTick), it can't be interrupted; it might schedule 1-2 more steps and push new visual timeouts after stop's pendingVisuals.clear? No wait, the order: the click handler runs on the main thread; the interval callback can't be running simultaneously. But what if the interval callback runs just before the click? Then the new timeout gets pushed to pendingVisuals, and then the click clears all of them. No problem.

Hmm, or maybe this: the timeout fired just before the click (showStep(8)), and then the click cleared it… in which case there shouldn't be a highlight.

Anyway, let me inspect the current DOM state of the class list and test again.

Currently, the state after stopping is clean — the leftover highlight from before was due to a pending timeout that fired in the gap between the screenshot frame and the DOM snapshot. The current state shows no highlighting, the pattern is preserved, and it's 180 BPM.

Next, I'll click Play again to verify that the playhead restarts from step 1 and that the pattern is preserved.

Playback was resumed (pattern is maintained, 180 BPM is retained). After confirming that the playhead reaches step 1, I'll test editing the step during playback and the Clear button.

The playhead is at 15/16 — it was reset and restarted from step 1. The pattern is retained, and the BPM is also retained at 180.

Next, test step toggling and Clear. Click an unlit hihat step (e.g., the 2nd cell of the hihat row), verify it lights up, then press Clear and verify all go out.

Hmm — the hi-hat step at (384, 459) is... looking at the hi-hat row: cell 1 is on, cell 2 looks like a dim blue (partially?), cell 3 is on. The cell I clicked was at x=384, which is the 2nd cell. It's showing a dim/intermediate color — wait, it looks like a dark blue (a color close to the background?). Actually, compared to the original off-state #1a1f2f, it looks kind of like a muted cyan. Maybe I toggled on... no, it wasn't on before. The screenshot might be showing a CSS transition mid-way. transition background .08s — a very short duration. It looks like a darker cyan — maybe it's the "on" state, but the image is showing... the on-state color for hi-hat is #4cc9f0 (bright cyan). Cell 2 looks like a dark teal — that's neither. Hmm, maybe I hit the boundary. Let me click the Clear button and verify everything turns off, then turn it back on afterwards.

Clear disabled all the steps (they've all become dark) — status shows "Pattern cleared (still playing)". Playback is continuing, playhead is at 2/16.

Interesting point: the cells look slightly colored (dimmed)? Looks like the transition is still in progress. OK.

Next, stop, and re-verify the playhead reset and the retained (now cleared) pattern, and additionally verify the BPM lower-bound value of 60. Finally: stop → playhead resets to the first step on restart. Already verified once. Do a final stop check and set the BPM to 60.

Stop succeeded: all steps turned off (cleared), counter "– / 16". The highlight in column 8 is showing again in this screenshot — this is a pending visual timeout firing between the screenshot capture and the clear? Actually, stop() clears the pending timeouts and also clears the highlights. The screenshot was captured after the DOM snapshot showing "Stopped". This suggests that the pending visual timeout fired after clearHighlight ran… but we're calling clearTimeout on all the pending ones.

Ah, I see a possible race condition: `scheduleVisual`'s timeout fires and calls showStep. stop() calls clearTimeout on all pendingVisuals. But what if the timeout fires between clearTimeout and clearHighlight? Not possible — it's single-threaded, and these run synchronously within the same handler.

But wait: pendingVisuals.filter removes the entry when the timeout fires. If a timeout fires just before the stop click, it runs showStep (sets the highlight). Then stop runs: clears the remaining pending ones, and removes the highlight via clearHighlight. The highlight should be gone.

The only explanation: the screenshot was taken just at a moment when a timeout scheduled by the final scheduler tick fired… but that would have been cleared. Hmm.

Another possibility: the click handler ran, but the screenshot tool's frame is a composite of a slightly earlier state. The DOM text in the snapshot ("Stopped", "– / 16") and the classes are updated in the same task… The screenshot might be capturing a paint that occurred between showStep (from the pending timeout) and the click. The DOM snapshot and screenshot are captured almost simultaneously, but the snapshot might reflect the post-click state, while the paint reflects an earlier rAF? Actually, paint is also discrete.

Wait — actually there's another real bug: `showStep` is guarded with `if (playing && schedSeq - seq <= 1)`. When stop sets schedSeq = 0… if a pending timeout fires after that (which shouldn't be possible since it was cleared).

Hmm, but note: pendingVisuals only contains timeouts pushed by scheduleVisual. All good.

Actually, let's see: is it possible the click didn't hit the Stop button and hit the page background? No, the DOM shows "Stopped" and "– / 16" — stop was executed.

So in this frame the highlight is visible, but the text is "Stopped". In the previous inspect (post-stop) screenshot the highlight was not visible. Both are post-stop. The difference: this time, the playhead was at 2/16 right before stop; the pending visual timeout for the next step (seq) had about 33ms left (180bpm = 83ms/step, ~100ms lookahead → up to 2 steps scheduled). stop clears those…

Maybe the problem is: setTimeout ids! `pendingVisuals = pendingVisuals.filter(x => x !== id)` — when a timeout fires and gets removed from the array, the id is freed. Later a new setTimeout could reuse that id number. In that case, the old cleared entry… no, the old entry was removed from the array upon firing.

But there's a more subtle problem: `clearTimeout` is called on ids in the array. If two entries share the same id (one fired and removed, the other new) — the fired one was already removed so no problem.

OK, another thought: maybe it's a transition. `.step.current` has box-shadow transition .1s. When clearHighlight removes the class, the shadow fades over 100ms. If the screenshot was captured within that 100ms, it would show the fading highlight. That's it! The box-shadow transition produces a ghost image for 100ms. The previous post-stop screenshot was taken longer after the click (after inspect roundtrip), so the ghost had faded.

This is a cosmetic-only issue lasting 100ms. Acceptable, but we could disable the transition on removal… this is fine as-is; the highlight fades out quickly. Not a functional bug.

Now let's set the BPM to 60, hit Play, and check step 1.

The drag didn't move the slider (it's still at 180). Dragging on range inputs is tricky to handle with mouse events. Instead, let's set the value via keyboard: click on the slider to focus it, then press the Home key. The browser tool's "type" action can probably input into focused elements, but range inputs don't accept text. Let me try the "key" action with a left-arrow key... First, click on the slider to focus it.

The slider responded to the click (currently 135). Since the slider is focused, pressing the Home key should bring it down to 60.

BPM is now 60 (the lower bound works). All patterns have been cleared. Let me toggle a few steps (kick at step 1, snare at step 5) to make sure something sounds, press Play, and check step 1.

Kick step 1 is on. Enable snare step 5 (x≈478, y≈403) and play.

Toggle behavior (kick 1 on, snare 5 on). At 60 BPM, press Play and confirm that step 1 is highlighted. After that, wrap up.

Playing at 60 BPM, status is "1/16", new pattern (kick 1, snare 5) is in use. All verified. No console errors (only unrelated sandbox warnings). index.html has been saved. Done — going to report now.