feat(js): m03 rules with fixtures and tests

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

js/solutions/rules.js

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+// 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),
+      }
+}

js/src/rules.js

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+import { todo } from './todo.js'
+
+/** The cell one step from `p` in direction `dir`. No wrapping: it may land outside the board. */
+export const move = (p, dir) => todo('m03')
+
+/** True if `p` lies on the board: `0 <= x < width` and `0 <= y < height`. */
+export const inBounds = (state, p) => todo('m03')
+
+/**
+ * Ask to turn the snake. Returns `state` unchanged unless the game is 'Playing' and
+ * `dir` is not the opposite of `state.dir` (the direction the snake last *moved*, not `nextDir`).
+ * Otherwise sets `nextDir = dir`. Several turns in one tick: the last allowed one wins.
+ */
+export const turn = (state, dir) => todo('m03')
+
+/**
+ * One step of the game. Returns `state` unchanged unless 'Playing'. Otherwise the head moves in `nextDir`
+ * (which becomes `dir`):
+ * - head lands on the apple: the snake grows (tail kept), score + 1, apple becomes null;
+ * - otherwise the tail cell is vacated, so the head may enter the cell the tail is leaving;
+ * - head off the board, or onto a remaining body cell: status 'GameOver', snake unchanged (but `dir` updated).
+ */
+export const advance = (state) => todo('m03')

js/test/m03-rules.test.js

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+import test from 'node:test'
+import assert from 'node:assert/strict'
+import { move, inBounds, turn, advance } from '../src/rules.js'
+import { loadFixture, freeze } from './helpers.js'
+
+const fixtureTests = (name, f) => {
+  const fx = loadFixture(name)
+  fx.cases.forEach((c) =>
+    test(`m03 ${fx.function}: ${c.name}`, () => assert.deepStrictEqual(f(...freeze(c.args)), c.expected)))
+}
+
+fixtureTests('m03-move', move)
+fixtureTests('m03-in-bounds', inBounds)
+fixtureTests('m03-turn', turn)
+fixtureTests('m03-advance', advance)