// m02 model: the data the whole game is built from. Plain objects, never mutated.

/** @typedef {{x: number, y: number}} Point */
/** @typedef {'Up'|'Down'|'Left'|'Right'} Direction */
/** @typedef {'Playing'|'Paused'|'GameOver'|'Won'} Status */
/** @typedef {{width: number, height: number, snake: Point[], dir: Direction, nextDir: Direction,
 *   apple: Point|null, seed: number, score: number, status: Status, frame: number}} GameState */

/** A grid cell. */
export const point = (x, y) => ({ x, y })

/** Structural equality: same x and same y. */
export const pointEquals = (a, b) => a.x === b.x && a.y === b.y

const deltas = {
  Up: point(0, -1),
  Down: point(0, 1),
  Left: point(-1, 0),
  Right: point(1, 0),
}

/** The unit step for a direction (y grows downward). */
export const delta = (dir) => deltas[dir]

const opposites = { Up: 'Down', Down: 'Up', Left: 'Right', Right: 'Left' }

/** The direction pointing the other way. */
export const opposite = (dir) => opposites[dir]

/** Everything that can happen to the game, as data. `Turn` is a function because it carries a direction. */
export const Action = {
  Tick: { tag: 'Tick' },
  Turn: (dir) => ({ tag: 'Turn', dir }),
  TogglePause: { tag: 'TogglePause' },
  Restart: { tag: 'Restart' },
  Frame: { tag: 'Frame' },
}

/** A fresh 20x20 game: 3-segment snake heading Right, no apple yet. */
export const initialState = (seed) => ({
  width: 20,
  height: 20,
  snake: [point(10, 10), point(9, 10), point(8, 10)],
  dir: 'Right',
  nextDir: 'Right',
  apple: null,
  seed,
  score: 0,
  status: 'Playing',
  frame: 0,
})