"""用 Python 标准库 turtle 演示贪吃蛇；方向键移动、空格暂停、R 重开。"""

import random
import turtle

SIZE = 20
CELL = 24
DELAY = 145
screen = turtle.Screen()
screen.setup(540, 600)
screen.title("Learn2Python · 贪吃蛇")
screen.bgcolor("#123c35")
screen.tracer(0)
pen = turtle.Turtle(visible=False)
pen.penup()
pen.speed(0)
snake = []
direction = (1, 0)
pending = (1, 0)
food = None
score = 0
paused = False
over = False
turn_queued = False


def new_food():
    """只从空格中挑果子，棋盘填满后返回 None。"""
    empty = [(x, y) for y in range(SIZE) for x in range(SIZE)
             if (x, y) not in snake]
    return random.choice(empty) if empty else None


def draw():
    """在窗口重画蛇、果子和当前状态。"""
    pen.clear()
    for i, (x, y) in enumerate(snake):
        pen.goto((x - SIZE / 2) * CELL + CELL / 2,
                 (SIZE / 2 - y) * CELL - CELL / 2)
        pen.dot(CELL - 3, "#e2ed8a" if i == 0 else "#b8d76e")
    if food is not None:
        x, y = food
        pen.goto((x - SIZE / 2) * CELL + CELL / 2,
                 (SIZE / 2 - y) * CELL - CELL / 2)
        pen.dot(15, "#ffb17b")
    pen.goto(0, 254)
    pen.color("#f7f7ef")
    state = "GAME OVER / R to restart" if over else "PAUSED" if paused else "Arrows to move / Space to pause"
    pen.write(f"Score: {score}   {state}", align="center", font=("Arial", 11, "normal"))
    screen.update()


def reset():
    """只重置状态，保留唯一的定时循环，防止重开后速度叠加。"""
    global snake, direction, pending, food, score, paused, over, turn_queued
    snake = [(7, 10), (6, 10), (5, 10)]
    direction = pending = (1, 0)
    score = 0
    paused = over = turn_queued = False
    food = new_food()
    draw()


def turn(dx, dy):
    """每步最多接受一次转向，并拒绝直接掉头。"""
    global pending, turn_queued
    if paused or over or turn_queued:
        return
    if (dx, dy) in (direction, (-direction[0], -direction[1])):
        return
    pending = (dx, dy)
    turn_queued = True


def toggle_pause():
    """在游戏进行中切换暂停状态。"""
    global paused
    if not over:
        paused = not paused
        draw()


def tick():
    """先检查碰撞再移动；每次只预约一个后续回调。"""
    global direction, turn_queued, food, score, over
    if not paused and not over:
        direction = pending
        turn_queued = False
        head = (snake[0][0] + direction[0], snake[0][1] + direction[1])
        eating = head == food
        body = snake if eating else snake[:-1]
        if not all(0 <= n < SIZE for n in head) or head in body:
            over = True
        else:
            snake.insert(0, head)
            if eating:
                score += 1
                food = new_food()
                over = food is None
            else:
                snake.pop()
        draw()
    screen.ontimer(tick, DELAY)


screen.onkey(lambda: turn(0, -1), "Up")
screen.onkey(lambda: turn(0, 1), "Down")
screen.onkey(lambda: turn(-1, 0), "Left")
screen.onkey(lambda: turn(1, 0), "Right")
screen.onkey(toggle_pause, "space")
screen.onkey(reset, "r")
screen.onkey(reset, "R")
screen.listen()
reset()
screen.ontimer(tick, DELAY)
screen.mainloop()