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packages/gaming/games/game-server/src/plugins/ab-test-plugin/ab-test-plugin.md

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+# ABTestPlugin
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+`ABTestPlugin` registers weighted experiments and assigns players to variants
+deterministically via an injected `Rng`. It records conversion events per
+(experiment, player) pair and exposes per-variant summary statistics so the
+game engine can drive data-informed balance decisions without taking a
+runtime dep on a separate experiments service.
+
+The plugin is intentionally self-contained: it owns the experiment catalog,
+the assignment map, and the conversion log in memory. Persistence and
+statistical tests (z-test, chi-squared) are out of scope — callers compose
+the plugin with their preferred storage and analysis tooling.
+
+## Features
+
+- Weighted variant bucketing via `Rng.nextFloat()` (deterministic with a
+  seeded RNG).
+- Idempotent assignment: re-calling `assignPlayer` returns the same variant.
+- Per-experiment conversion log with arbitrary `goalId` and numeric value.
+- Per-variant summary: assignments, conversions, total value, conversion rate.
+- Zod-validated experiment input (id, name, ≥1 variant with positive weight).
+- Lifecycle-safe: clears every experiment on `onUninstall`.
+
+## Usage: Basic
+
+```ts
+import { ABTestPlugin } from "@suaveplan/game-server/plugins/ab-test-plugin";
+
+const rng = { nextFloat: () => Math.random(), nextUint32: () => 0 };
+const plugin = new ABTestPlugin({ rng });
+plugin.createExperiment({
+    id: "exp-1",
+    name: "Drop rate test",
+    variants: [
+        { id: "control", weight: 1 },
+        { id: "treatment", weight: 1 },
+    ],
+});
+const a = plugin.assignPlayer("exp-1", "player-1");
+```
+
+## Usage: Advanced
+
+```ts
+import { ABTestPlugin } from "@suaveplan/game-server/plugins/ab-test-plugin";
+
+// Seeded RNG for deterministic test runs.
+let s = 1;
+const rng = {
+    nextUint32: () => (s = (s * 9301 + 49297) % 233280),
+    nextFloat: () => {
+        s = (s * 9301 + 49297) % 233280;
+        return s / 233280;
+    },
+};
+
+const plugin = new ABTestPlugin({ rng });
+plugin.createExperiment({
+    id: "drop",
+    name: "Drop rate test",
+    variants: [
+        { id: "low", weight: 3 },
+        { id: "high", weight: 1 },
+    ],
+});
+plugin.assignPlayer("drop", "p1");
+plugin.recordConversion("drop", "p1", "first-kill", 1);
+const summary = plugin.getSummary("drop");
+```
+
+## API Reference
+
+- `class ABTestPlugin extends BasePlugin`
+- `constructor(options: ABTestPluginOptions)`
+- `createExperiment(input: ExperimentInput): void`
+- `hasExperiment(id: string): boolean`
+- `assignPlayer(experimentId: string, playerId: string): Assignment`
+- `getAssignment(experimentId: string, playerId: string): Assignment | undefined`
+- `recordConversion(experimentId: string, playerId: string, goalId: string, value?: number): Conversion`
+- `getSummary(experimentId: string): readonly VariantSummary[]`
+- `listExperiments(): readonly string[]`
+
+## Implementation Notes
+
+- Bucketing accumulates variant weights and compares against
+  `nextFloat() * totalWeight`. The implementation falls back to the last
+  variant if the accumulator never exceeds the threshold (a defensive
+  guard against floating-point ulp drift when weights are tiny).
+- Recording a conversion for an unassigned player throws
+  `GAME_SERVER.AB_TEST_INVALID_VARIANT` so callers can route the error to
+  their analytics dead-letter queue rather than corrupting the report.
+- The summary recomputes from raw records on each call. The data sets are
+  small (tens of variants, thousands of players); pre-aggregation would
+  add latency on a rare path. Callers needing high-frequency dashboards
+  should snapshot the summary at a checkpoint instead of polling.
+- The plugin uses `BasePlugin.assertLive` on every public method so a
+  destroyed plugin cannot accidentally serve stale data.