feat(js): m04 random and spawn with fixtures and tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

js/test/m04-random.test.js

@@ -0,0 +1,52 @@
+import test from 'node:test'
+import assert from 'node:assert/strict'
+import { nextUint32, pure, map, flatMap, intBelow, run } from '../src/random.js'
+import { loadFixture, freeze } from './helpers.js'
+
+const fixtureTests = (name, f) => {
+  const fx = loadFixture(name)
+  fx.cases.forEach((c) =>
+    test(`m04 ${fx.function}: ${c.name}`, () => assert.deepStrictEqual(f(...freeze(c.args)), c.expected)))
+}
+
+fixtureTests('m04-random', (seed) => run(nextUint32, seed))
+fixtureTests('m04-int-below', (n, seed) => run(intBelow(n), seed))
+
+test('m04 the recorded steps chain: each step starts from the previous step\'s seed', () => {
+  const cases = loadFixture('m04-random').cases
+  cases.forEach((c, i) => {
+    if (!c.name.endsWith('-step-1')) assert.equal(c.args[0], cases[i - 1].expected[1])
+  })
+})
+
+test('m04 pure returns its value and leaves the seed alone', () =>
+  assert.deepStrictEqual(run(pure('x'), 99), ['x', 99]))
+
+test('m04 map applies f to the value and keeps the seed from the underlying step', () => {
+  const [v, s] = run(nextUint32, 5)
+  assert.deepStrictEqual(run(map(nextUint32, (x) => x + 1), 5), [v + 1, s])
+})
+
+test('m04 map with the identity function changes nothing', () =>
+  assert.deepStrictEqual(run(map(nextUint32, (x) => x), 5), run(nextUint32, 5)))
+
+test('m04 flatMap threads the seed through both steps', () => {
+  const [a, s1] = run(nextUint32, 5)
+  const [b, s2] = run(nextUint32, s1)
+  assert.deepStrictEqual(run(flatMap(nextUint32, (x) => map(nextUint32, (y) => [x, y])), 5), [[a, b], s2])
+})
+
+test('m04 flatMap with pure on the left or right changes nothing', () => {
+  assert.deepStrictEqual(run(flatMap(pure(3), (x) => map(nextUint32, (v) => v + x)), 5), run(map(nextUint32, (v) => v + 3), 5))
+  assert.deepStrictEqual(run(flatMap(nextUint32, pure), 5), run(nextUint32, 5))
+})
+
+test('m04 intBelow always lands in [0, n)', () => {
+  const draws = (n, seed, count) =>
+    count === 0 ? [] : ((([v, s]) => [v, ...draws(n, s, count - 1)])(run(intBelow(n), seed)))
+  ;[1, 2, 7, 400].forEach((n) =>
+    draws(n, 123, 50).forEach((v) => assert.ok(Number.isInteger(v) && v >= 0 && v < n, `${v} not in [0, ${n})`)))
+})
+
+test('m04 running a Random does not mutate anything: same seed, same answer', () =>
+  assert.deepStrictEqual(run(nextUint32, 8), run(nextUint32, 8)))