Add a simple linear congruential PRNG.

lib/random.ts

@@ -0,0 +1,45 @@
+export type RandomParameters = {
+  modulus: number;
+  multiplier: number;
+  increment: number;
+};
+
+const NUMERICAL_RECIPES_PARAMETERS: RandomParameters = {
+  modulus: Math.pow(2, 32),
+  multiplier: 1664525,
+  increment: 1013904223,
+};
+
+/**
+ * A simple linear congruential random number generator, as described in
+ * https://en.wikipedia.org/wiki/Linear_congruential_generator.
+ */
+export class Random {
+  private latestSeed: number;
+
+  constructor(
+    readonly seed: number = Date.now(),
+    readonly params: RandomParameters = NUMERICAL_RECIPES_PARAMETERS
+  ) {
+    this.latestSeed = seed;
+  }
+
+  /**
+   * Return a random number that is greater than or equal to zero, and less
+   * than one.
+   */
+  next(): number {
+    this.latestSeed =
+      (this.params.multiplier * this.latestSeed + this.params.increment) %
+      this.params.modulus;
+    return this.latestSeed / this.params.modulus;
+  }
+
+  /**
+   * Return a random item from the given array.
+   */
+  choice<T>(array: T[]): T {
+    const idx = Math.floor(this.next() * array.length);
+    return array[idx];
+  }
+}