From e4a88469783de3376b2531ecd735982e466c0725 Mon Sep 17 00:00:00 2001
From: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>
Date: Fri, 20 Sep 2024 10:33:05 +0200
Subject: [PATCH] feat: add RandomMocker class to mock Math.random

---
 lib/__tests__/curriculum-helper.test.ts | 45 +++++++++++++++++++++++++
 lib/index.ts                            | 31 +++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/lib/__tests__/curriculum-helper.test.ts b/lib/__tests__/curriculum-helper.test.ts
index 34b08d9..fec7014 100644
--- a/lib/__tests__/curriculum-helper.test.ts
+++ b/lib/__tests__/curriculum-helper.test.ts
@@ -22,6 +22,51 @@ const {
   jsCodeWithCommentedCall,
 } = jsTestValues;
 
+describe("RandomMocker", () => {
+  let random: () => number;
+
+  beforeEach(() => {
+    random = Math.random;
+  });
+
+  afterEach(() => {
+    Math.random = random;
+  });
+
+  describe("mock", () => {
+    it('should replace "Math.random" with a mock function', () => {
+      const mocker = new helper.RandomMocker();
+      mocker.mock();
+      expect(Math.random).not.toBe(random);
+    });
+
+    it('should mock "Math.random" with a pseudorandom function', () => {
+      const mocker = new helper.RandomMocker();
+      mocker.mock();
+      // Predictable random values:
+      expect(Math.random()).toBe(0.2523451747838408);
+      expect(Math.random()).toBe(0.08812504541128874);
+    });
+
+    it("should reset the pseudorandom function when called multiple times", () => {
+      const mocker = new helper.RandomMocker();
+      mocker.mock();
+      expect(Math.random()).toBe(0.2523451747838408);
+      mocker.mock();
+      expect(Math.random()).toBe(0.2523451747838408);
+    });
+  });
+
+  describe("restore", () => {
+    it('should restore "Math.random" to its original function', () => {
+      const mocker = new helper.RandomMocker();
+      mocker.mock();
+      mocker.restore();
+      expect(Math.random).toBe(random);
+    });
+  });
+});
+
 describe("removeWhiteSpace", () => {
   const { removeWhiteSpace } = helper;
   it("returns a string", () => {
diff --git a/lib/index.ts b/lib/index.ts
index 4cfd8d9..e1a47ff 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -1,6 +1,37 @@
 import { strip } from "./strip";
 import astHelpers from "../python/py_helpers.py";
 
+/**
+ * The `RandomMocker` class provides functionality to mock and restore the global `Math.random` function.
+ * It replaces the default random number generator with a deterministic pseudo-random number generator.
+ */
+export class RandomMocker {
+  private random: () => number;
+
+  constructor() {
+    this.random = Math.random;
+  }
+
+  private createRandom() {
+    let seed = 42;
+    const a = 1664525;
+    const c = 1013904223;
+    const mod = 2 ** 32;
+    return () => {
+      seed = (a * seed + c) % mod;
+      return seed / mod;
+    };
+  }
+
+  mock(): void {
+    globalThis.Math.random = this.createRandom();
+  }
+
+  restore(): void {
+    globalThis.Math.random = this.random;
+  }
+}
+
 /**
  * Removes every HTML-comment from the string that is provided
  * @param {String} str a HTML-string where the comments need to be removed of