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

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

js/solutions/random.js

@@ -0,0 +1,31 @@
+// m04 random: a tiny "state monad" for randomness. A Random<A> is a function seed -> [A, nextSeed],
+// so every draw is a pure function and the seed is passed along explicitly instead of hidden.
+
+/** mulberry32: a small, fast generator. `imul` is 32-bit integer multiplication (plain `*` would lose bits). */
+export const nextUint32 = (seed) => {
+  const s = (seed + 0x6D2B79F5) >>> 0 // `>>> 0` keeps numbers unsigned 32-bit
+  const t1 = Math.imul(s ^ (s >>> 15), s | 1)
+  const t2 = t1 ^ (t1 + Math.imul(t1 ^ (t1 >>> 7), t1 | 61))
+  return [(t2 ^ (t2 >>> 14)) >>> 0, s]
+}
+
+/** A Random that always gives `a` and leaves the seed untouched. */
+export const pure = (a) => (seed) => [a, seed]
+
+/** Transform the value a Random produces; the seed threading is unchanged. */
+export const map = (r, f) => (seed) => {
+  const [a, next] = r(seed)
+  return [f(a), next]
+}
+
+/** Run `r`, let `f` pick the next Random from its value, and run that with the seed `r` left behind. */
+export const flatMap = (r, f) => (seed) => {
+  const [a, next] = r(seed)
+  return f(a)(next)
+}
+
+/** A Random giving a number in `[0, n)`. */
+export const intBelow = (n) => map(nextUint32, (v) => v % n)
+
+/** Run a Random from a seed: `[value, nextSeed]`. */
+export const run = (r, seed) => r(seed)

js/solutions/spawn.js

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+// m04 spawn: placing apples with the Random from random.js.
+import { range, contains } from './util.js'
+import { point, pointEquals, initialState } from './model.js'
+import { advance } from './rules.js'
+import { intBelow, run } from './random.js'
+
+/** Every board cell not covered by the snake, row-major (y outer, x inner). */
+export const freeCells = (state) =>
+  range(state.height)
+    .flatMap((y) => range(state.width).map((x) => point(x, y)))
+    .filter((p) => !contains(state.snake, p, pointEquals))
+
+/** Put an apple on a random free cell; a full board means the player has won. */
+export const spawnApple = (state) => {
+  if (state.apple !== null || state.status !== 'Playing') return state
+  const cells = freeCells(state)
+  if (cells.length === 0) return { ...state, status: 'Won' }
+  const [i, seed] = run(intBelow(cells.length), state.seed)
+  return { ...state, apple: cells[i], seed }
+}
+
+/** One game step: move the snake, then make sure there is an apple. */
+export const tick = (state) => spawnApple(advance(state))
+
+/** A fresh game with its first apple already placed. */
+export const newGame = (seed) => spawnApple(initialState(seed))