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packages/devtools/tooling/profiler/src/sampler/sampler.md

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+# sampler
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+`Sampler` collects periodic snapshots of runtime metrics (memory usage by default, plus caller-supplied custom metrics). The timer source is injected through a `TimersLike` so tests can advance virtual time deterministically and so non-`setInterval` runtimes (Workers, Deno, embedded JS) can plug in their own scheduler. Sample storage is capped via FIFO eviction so long-running collectors don't leak memory.
+
+## Features
+
+- Zod-validated options (`intervalMs > 0`, optional positive `maxSamples`, optional `captureMemory`).
+- Injectable `Clock`, `TimersLike`, and `MemoryProbe` — the defaults read `Date.now`, `setTimeout`/`clearTimeout`, and `process.memoryUsage`/`performance.memory` respectively.
+- `start()` / `stop()` are idempotent — double-`start` and double-`stop` are no-ops.
+- `takeSample(customMetrics?)` records a single sample on demand; same code path used by the scheduled tick.
+- `getSamples()` / `getSamplesInRange(start, end)` / `clear()` for retrieval and reset.
+- `events: EventBus<SamplerEventMap>` emits `sampler:start`, `sampler:stop`, `sampler:sample`.
+- Telemetry: `profiler_samples_total` counter incremented on every sample.
+
+## Usage: Basic
+
+```ts
+import { Sampler } from "@suaveplan/profiler";
+
+const sampler = new Sampler({ intervalMs: 1000 });
+sampler.start();
+// ...
+sampler.stop();
+console.log(sampler.getSamples());
+```
+
+## Usage: Advanced
+
+```ts
+import { Sampler } from "@suaveplan/profiler";
+import type { TimersLike } from "@suaveplan/types";
+
+const queue: Array<{ cb: () => void; handle: number }> = [];
+let next = 1;
+const fakeTimers: TimersLike = {
+    setTimer(cb) {
+        const handle = next++;
+        queue.push({ cb, handle });
+        return handle;
+    },
+    clearTimer() {},
+};
+
+let clockT = 0;
+const sampler = new Sampler({
+    intervalMs: 10,
+    timers: fakeTimers,
+    clock: () => clockT,
+    memoryProbe: () => 4096,
+});
+sampler.start();
+clockT = 10;
+queue.shift()?.cb();
+```
+
+## API Reference
+
+- `class Sampler` — `new Sampler(options: SamplerOptions)`.
+- `interface SamplerOptions extends SamplerConfig { clock?, timers?, memoryProbe? }`.
+- Methods: `start`, `stop`, `takeSample`, `getSamples`, `getSamplesInRange`, `isRunning`, `clear`.
+- Errors: throws `ProfilerError` with `PROFILER.INVALID_OPTIONS` when options fail Zod validation.
+
+## Implementation Notes
+
+The scheduler uses a self-rescheduling `setTimer` rather than `setInterval` because `TimersLike` only models one-shot timers. Each tick checks `this.running` before taking a sample, which lets a stop() that runs between callback dispatch and execution avoid emitting a stale sample. FIFO eviction uses `splice(0, n)` so the oldest samples drop out when `maxSamples` is exceeded.