39 lines
1.5 KiB
JavaScript
39 lines
1.5 KiB
JavaScript
// 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),
|
|
}
|
|
}
|