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packages/foundation/data/web-workers/src/worker-pool/scheduler.md

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+# `scheduler`
+
+## Purpose
+
+`scheduler.ts` exposes `pickSlot`, the pure function `WorkerPool` calls to
+decide which slot receives the next submit. The function takes a
+projection of the pool's slots (only the data the scheduler needs:
+`name` + `pendingCount`) and the strategy name, and returns the chosen
+slot plus the next round-robin index.
+
+Isolating the scheduling decision in a pure function makes it independently
+testable, swappable (a future scheduler can be added without touching the
+pool), and free of side effects.
+
+## Features
+
+- Two scheduling strategies: `round-robin` and `least-busy`.
+- `SchedulerSlot` projection — lightweight view of the pool's slots so the
+  scheduler doesn't depend on `WorkerPool`'s internals.
+- Returns `undefined` when the slot list is empty, letting the caller
+  decide what to do (typically reject with `RESTART_EXHAUSTED`).
+- Round-robin returns the next index so the caller can persist it.
+
+## Usage: Basic Example
+
+```ts
+import { pickSlot } from "@suaveplan/web-workers";
+
+const slots = [
+    { name: "w0", pendingCount: 0 },
+    { name: "w1", pendingCount: 0 },
+];
+const pick = pickSlot("round-robin", slots, 0);
+// pick.slot.name === "w0", pick.nextIndex === 1
+```
+
+## Usage: Advanced Example
+
+```ts
+import {
+    pickSlot,
+    type SchedulerSlot,
+    type SchedulerStrategy,
+} from "@suaveplan/web-workers";
+
+class CustomPool {
+    private slots: Array<{ name: string; pendingCount: number }> = [];
+    private nextRR = 0;
+    pick(strategy: SchedulerStrategy): SchedulerSlot | undefined {
+        const projection = this.slots.map((s) => ({
+            name: s.name,
+            pendingCount: s.pendingCount,
+        }));
+        const result = pickSlot(strategy, projection, this.nextRR);
+        if (result === undefined) return undefined;
+        this.nextRR = result.nextIndex;
+        return result.slot;
+    }
+}
+```
+
+## API Reference
+
+- `type SchedulerStrategy = "round-robin" | "least-busy"`.
+- `interface SchedulerSlot { name: string; pendingCount: number }`.
+- `pickSlot(strategy, slots, nextRoundRobinIndex): { slot, nextIndex } | undefined`.
+
+## Implementation Notes
+
+`round-robin` uses `nextRoundRobinIndex % slots.length` so the caller can
+keep a monotonically-increasing counter without worrying about wraparound.
+`least-busy` linear-scans the slot list; ties go to the lowest-index slot
+(deterministic). The pure-function shape means the same slot projection
+always returns the same result — no hidden state, no randomness. For
+large pools (> 100 slots) a heap-based least-busy could be faster, but
+the typical pool size (4–16 slots) makes the linear scan negligible.