Ensure random numbers are correctly distributed (#3734)

.changeset/breezy-falcons-grab.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Ensure random numbers are correctly distributed

packages/effect/src/Utils.ts

@@ -614,21 +614,7 @@ export class PCGRandom {
    * @since 2.0.0
    */
   integer(max: number) {
-    if (!max) {
-      return this._next()
-    }
-    max = max >>> 0
-    if ((max & (max - 1)) === 0) {
-      return this._next() & (max - 1) // fast path for power of 2
-    }
-
-    let num = 0
-    const skew = (-max >>> 0) % max >>> 0
-    for (num = this._next(); num < skew; num = this._next()) {
-      // this loop will rarely execute more than twice,
-      // and is intentionally empty
-    }
-    return num % max
+    return Math.round(this.number() * Number.MAX_SAFE_INTEGER) % max
   }
 
   /**

packages/effect/test/Random.test.ts

@@ -3,6 +3,15 @@ import { expect, it } from "effect/test/utils/extend"
 import { assert, describe } from "vitest"
 
 describe("Random", () => {
+  it.effect("integer is correctly distributed", () =>
+    Effect.gen(function*() {
+      const tenYearsMillis = 10 * 365 * 24 * 60 * 60 * 1000
+      let lastRandom = 0
+      while (lastRandom < tenYearsMillis / 2) {
+        lastRandom = yield* Random.nextIntBetween(0, tenYearsMillis)
+      }
+      assert.isTrue(lastRandom >= tenYearsMillis / 2)
+    }))
   it.effect("shuffle", () =>
     Effect.gen(function*() {
       const start = Array.range(0, 100)