feat(js): m03 rules with fixtures and tests

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

js/solutions/rules.js

@@ -0,0 +1,38 @@
+// m03 rules: the pure game rules. Each function takes a state and returns a new one.
+import { contains, dropLast } from './util.js'
+import { pointEquals, point, delta, opposite } from './model.js'
+
+/** The cell one step from `p` in direction `dir`. No wrapping: it may land outside the board. */
+export const move = (p, dir) => {
+  const d = delta(dir)
+  return point(p.x + d.x, p.y + d.y)
+}
+
+/** True if `p` lies on the board. */
+export const inBounds = (state, p) => p.x >= 0 && p.x < state.width && p.y >= 0 && p.y < state.height
+
+/**
+ * Ask to turn. We compare with `dir` (where the snake last moved), not `nextDir`:
+ * otherwise Up-then-Left within one tick would turn a Right-moving snake back onto itself.
+ */
+export const turn = (state, dir) =>
+  state.status === 'Playing' && dir !== opposite(state.dir) ? { ...state, nextDir: dir } : state
+
+/** One step of the game. */
+export const advance = (state) => {
+  if (state.status !== 'Playing') return state
+  const d = state.nextDir
+  const head = move(state.snake[0], d)
+  const eating = state.apple !== null && pointEquals(head, state.apple)
+  // When not eating, the tail leaves its cell this step, so the head may move into it.
+  const body = eating ? state.snake : dropLast(state.snake)
+  return !inBounds(state, head) || contains(body, head, pointEquals)
+    ? { ...state, dir: d, status: 'GameOver' }
+    : {
+        ...state,
+        snake: [head, ...body],
+        dir: d,
+        apple: eating ? null : state.apple,
+        score: state.score + (eating ? 1 : 0),
+      }
+}