test(spec): assert pcg.ts matches the PCG golden fixture

pkg/spec/test/pcg.test.ts

@@ -0,0 +1,58 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+import { Pcg } from "../src/pcg.ts";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const fixturePath = join(here, "fixtures", "pcg-golden.json");
+
+interface GoldenCase {
+  seed: { hi: string; lo: string };
+  uint64: string[];
+  float64: number[];
+  intN: Record<string, number[]>;
+}
+
+const cases: GoldenCase[] = JSON.parse(readFileSync(fixturePath, "utf8"));
+
+function seedLabel(c: GoldenCase): string {
+  return `seed(${c.seed.hi}, ${c.seed.lo})`;
+}
+
+test("golden fixture has the expected shape", () => {
+  assert.ok(cases.length >= 5, "expected several seed cases");
+  for (const c of cases) {
+    assert.equal(c.uint64.length, 40);
+    assert.equal(c.float64.length, 40);
+    for (const draws of Object.values(c.intN)) {
+      assert.equal(draws.length, 40);
+    }
+  }
+});
+
+for (const c of cases) {
+  test(`${seedLabel(c)} uint64 sequence is bit-exact`, () => {
+    const pcg = new Pcg(BigInt(c.seed.hi), BigInt(c.seed.lo));
+    c.uint64.forEach((want, i) => {
+      assert.equal(pcg.uint64(), BigInt(want), `uint64 draw ${i}`);
+    });
+  });
+
+  test(`${seedLabel(c)} float64 sequence is bit-exact`, () => {
+    const pcg = new Pcg(BigInt(c.seed.hi), BigInt(c.seed.lo));
+    c.float64.forEach((want, i) => {
+      assert.equal(pcg.float64(), want, `float64 draw ${i}`);
+    });
+  });
+
+  for (const [n, expected] of Object.entries(c.intN)) {
+    test(`${seedLabel(c)} intN(${n}) sequence is bit-exact`, () => {
+      const pcg = new Pcg(BigInt(c.seed.hi), BigInt(c.seed.lo));
+      expected.forEach((want, i) => {
+        assert.equal(pcg.intN(Number(n)), want, `intN(${n}) draw ${i}`);
+      });
+    });
+  }
+}