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docs/media/audio-fx/audio-fx-plugin-sampler/src/sampler/sampler.md

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+# `sampler`
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+`SamplerPlugin` is the universal polyphonic sampler. `loadSample(note, url)` fetches a raw `Float32Array` PCM buffer from a URL and registers it under a MIDI note key; `triggerAttack(note, velocity?)` finds the nearest loaded base sample and plays it back at the pitch ratio `2^((note − baseNote) / 12)` using linear interpolation. Concurrent voices are summed into the mono output channel, with deterministic voice stealing when the configured pool size is exceeded.
+
+The plugin is a source (`io.inputs[0].count === 0`), so it slots into chains at position 0 — the chain's IO validator rejects placing it elsewhere. The processing path is allocation-free; only the voice pool array is mutated per block.
+
+## Features
+
+- Source plugin (`io.inputs[0].count === 0`), mono output (`io.outputs[0].count === 1`).
+- One automatable parameter: `gain` (0..4, default 1) for master-out level.
+- `loadSample(note, url, { signal? })` — fetches `url` as a raw Float32Array buffer; `signal` aborts pre-fetch (`AUDIO_FX.ABORTED`) or mid-fetch.
+- `triggerAttack(note, velocity?)` — throws `AUDIO_FX.SAMPLE_NOT_LOADED` if no samples are registered; throws `AUDIO_FX.INVALID_NOTE` for `note ∉ [0, 127]` integer.
+- `triggerRelease(note)` — switches the voice into release mode; one-pole exponential decay ends the voice cleanly.
+- Voice pool (default 32, configurable via `initialMaxVoices` up to 128). Stealing is deterministic: the lowest-sequence voice is replaced.
+- `getLoadedSamples()`, `getActiveVoiceCount()` accessors for diagnostics and meters.
+
+## Usage: Basic Example
+
+```ts
+import { SamplerPlugin } from "@suaveplan/audio-fx-plugin-sampler";
+
+const sampler = new SamplerPlugin();
+await sampler.loadSample(60, "/sounds/piano-c4.raw");
+chain.add(sampler);
+sampler.triggerAttack(60); // play at native pitch
+sampler.triggerAttack(72); // play one octave up
+```
+
+## Usage: Advanced Example
+
+```ts
+import { SamplerPlugin } from "@suaveplan/audio-fx-plugin-sampler";
+
+const sampler = new SamplerPlugin({
+    initialMaxVoices: 16,
+    initialReleaseMs: 40,
+});
+await sampler.setup(
+    { sampleRate: 48_000, channels: 1, blockSize: 512 },
+    ctx
+);
+
+// Load a piano sample bank, key-mapped across the keyboard.
+const ac = new AbortController();
+try {
+    await Promise.all([
+        sampler.loadSample(48, "/sounds/piano-c3.raw", { signal: ac.signal }),
+        sampler.loadSample(60, "/sounds/piano-c4.raw", { signal: ac.signal }),
+        sampler.loadSample(72, "/sounds/piano-c5.raw", { signal: ac.signal }),
+    ]);
+} catch (error) {
+    if (error instanceof AudioFxError && error.code === "AUDIO_FX.ABORTED") {
+        console.warn("sample bank load aborted");
+    } else {
+        throw error;
+    }
+}
+
+// triggerAttack(65) picks the nearest base (60), pitch-shifts +5 semitones.
+sampler.triggerAttack(65, 0.8);
+```
+
+## API Reference
+
+- `SamplerPlugin` (class extends `AbstractAudioFxPlugin`)
+  - `new SamplerPlugin(options?: SamplerPluginOptions)` — throws `AUDIO_FX.INVALID_OPTIONS` on malformed options.
+  - `loadSample(note, url, { signal? }): Promise<void>` — fetches and registers a base sample.
+  - `triggerAttack(note, velocity?: number): void`
+  - `triggerRelease(note: number): void`
+  - `getLoadedSamples(): ReadonlyArray<SamplerLoadedSample>`
+  - `getActiveVoiceCount(): number`
+  - Inherited accessor for `gain`.
+
+## Implementation Notes
+
+The pitch ratio is `2^((note − baseNote) / 12)` — the standard equal-temperament pitch-shifting formula. We use linear interpolation between adjacent samples in the source buffer, which is the cheapest interpolator that avoids audible quantisation artefacts. For high-fidelity pitch shifts (e.g., spanning more than an octave from the base), callers should load multiple base samples spaced an octave apart so the nearest-neighbour lookup keeps the interpolation distance short.
+
+Voice stealing uses a monotonic `sequence` counter on each voice. When the pool fills, we scan for the lowest sequence and replace it. The cost is O(maxVoices) per attack — for the default pool size of 32, this is 32 comparisons, well under the per-attack budget of a real-time engine. Faster algorithms (heap, ring buffer) would lower the constant factor but add bookkeeping complexity; the linear scan is the right MVP.
+
+Release uses a one-pole exponential decay: `releaseGain *= 1 − releaseCoef` per sample with `releaseCoef = 1 − exp(-1/releaseSamples)`. When the gain falls below `1e-4`, the voice is removed from the pool. This is the same envelope-decay form used by the gate plugin and gives a perceptually-correct "soft release" rather than a click-prone hard cut.
+
+`loadSample` mirrors the reverb's `loadImpulseResponse` pattern: pre-aborted signal throws `AUDIO_FX.ABORTED` before any fetch; signal aborted mid-fetch surfaces as the same error, leaving the existing sample map untouched. The signal field is built conditionally in `fetchInit` to satisfy `exactOptionalPropertyTypes` — `RequestInit.signal` is `AbortSignal | null`, and assigning `undefined` would be a type error.
+
+The plugin output is mono (`io.outputs[0].count === 1`) so callers can chain a `MixerPlugin` or `StereoWidenerPlugin` downstream to place the sampler in stereo space. Spatial positioning is the consumer's responsibility — the sampler intentionally does not pan internally so a single instance can drive multiple voices that are panned differently downstream.