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

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

js/solutions/spawn.js

@@ -0,0 +1,26 @@
+// 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))