我怎样才能让蛇长大并链接蛇身体的运动？

问题描述：

我想实现一个蛇游戏。蛇在操场上蜿蜒前行。每当蛇吃到一些食物时，蛇的长度就会增加一个元素。蛇身体的元素像链条一样跟随它的头部。

snake_x, snake_y = WIDTH//2, HEIGHT//2
body = []
move_x, move_y = (1, 0)
food_x, food_y = new_food(body)

run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT: move_x, move_y = (-1, 0)
            elif event.key == pygame.K_RIGHT: move_x, move_y = (1, 0)
            elif event.key == pygame.K_UP: move_x, move_y = (0, -1)
            elif event.key == pygame.K_DOWN: move_x, move_y = (0, 1)
 
    snake_x = (snake_x + move_x) % WIDTH
    snake_y = (snake_y + move_y) % HEIGHT 
    if snake_x == food_x and snake_y == food_y:
        food_x, food_y = new_food(body)
        body.append((snake_x, snake_x))

    # [...]

当蛇头向前移动时，我怎样才能使身体部位跟随蛇头移动？

解决方案 1：

一般来说，你必须区分两种不同类型的蛇。在第一种情况下，蛇在网格中移动，每次移动时，它都会在网格中向前跨出一个区域。在另一种类型中，蛇的位置不在光栅中，也不在网格区域上，位置是自由的，蛇在区域间平滑滑动。

在前一种情况下，身体的每个元素都与网格区域对齐，就像头部一样。另一种则更复杂，因为身体元素的位置取决于元素的大小和蛇头的动态先前位置。

首先是蛇，它被捕捉到网格中。

蛇的元素可以存储在元组列表中。每个元组包含网格中蛇元素的列和行。列表中项目的更改直接跟随蛇的移动。如果蛇移动，则将新位置添加到列表的头部并删除列表的尾部。

例如，我们有一条具有以下元素的蛇：

body = [(3, 3), (3, 4), (4, 4), (5, 4), (6, 4)]

当蛇头从 移动(3, 3)到 ( 3, 2) 时，新的头部位置被添加到列表的头部（body.insert(0, (3, 2)）：

body = [(3, 2), (3, 3), (3, 4), (4, 4), (5, 4), (6, 4)]

最后删除 ist 的尾部( del body[-1])：

body = [(3, 2), (3, 3), (3, 4), (4, 4), (5, 4)]

最小示例：![IT科技](https://i.sstatic.net/5jD0C.png) repl.it/@Rabbid76/PyGame-SnakeMoveInGrid

import pygame
import random

pygame.init()
COLUMNS, ROWS, SIZE = 10, 10, 20
screen = pygame.display.set_mode((COLUMNS*SIZE, ROWS*SIZE))
clock  = pygame.time.Clock()

background = pygame.Surface((COLUMNS*SIZE, ROWS*SIZE))
background.fill((255, 255, 255))
for i in range(1, COLUMNS):
    pygame.draw.line(background, (128, 128, 128), (i*SIZE-1, 0), (i*SIZE-1, ROWS*SIZE), 2)
for i in range(1, ROWS):
    pygame.draw.line(background, (128, 128, 128), (0, i*SIZE-1), (COLUMNS*SIZE, i*SIZE-1), 2)

def random_pos(body):
    while True:
        pos = random.randrange(COLUMNS), random.randrange(ROWS)
        if pos not in body:
            break     
    return pos

length = 1
body = [(COLUMNS//2, ROWS//2)]
dir = (1, 0)
food = random_pos(body)

run = True
while run:
    clock.tick(5)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT: dir = (-1, 0)
            elif event.key == pygame.K_RIGHT: dir = (1, 0)
            elif event.key == pygame.K_UP: dir = (0, -1)
            elif event.key == pygame.K_DOWN: dir = (0, 1)

    body.insert(0, body[0][:])    
    body[0] = (body[0][0] + dir[0]) % COLUMNS, (body[0][1] + dir[1]) % ROWS
    if body[0] == food:
        food = random_pos(body)
        length += 1
    while len(body) > length:
        del body[-1] 
    
    screen.blit(background, (0, 0))
    pygame.draw.rect(screen, (255, 0, 255), (food[0]*SIZE, food[1]*SIZE, SIZE, SIZE))
    for i, pos in enumerate(body):
        color = (255, 0, 0) if i==0 else (0, 192, 0) if (i%2)==0 else (255, 128, 0)
        pygame.draw.rect(screen, color, (pos[0]*SIZE, pos[1]*SIZE, SIZE, SIZE))
    pygame.display.flip()

现在蛇可以完全自由定位。

我们必须在一个列表中跟踪蛇头访问过的所有位置。我们必须将蛇身的元素放置在列表中的位置上，就像链条上的珍珠一样。

关键在于计算链中主体的最后一个元素与轨道上后续位置之间的欧几里德距离。当找到距离足够大的新点时，就会将新的珍珠（元素）添加到链（主体）中。

dx, dy = body[-1][0]-pos[0], body[-1][1]-pos[1]
if math.sqrt(dx*dx + dy*dy) >= distance:
    body.append(pos)

以下函数有 3 个参数。track是蛇头位置的列表。no_pearls是蛇身元素的数量，distance是元素之间的欧几里得距离。该函数创建并返回蛇身位置的列表。

def create_body(track, no_pearls, distance):
    body = [(track[0])]
    track_i = 1
    for i in range(1, no_pearls):
        while track_i < len(track):
            pos = track[track_i]
            track_i += 1
            dx, dy = body[-1][0]-pos[0], body[-1][1]-pos[1]
            if math.sqrt(dx*dx + dy*dy) >= distance:
                body.append(pos)
                break
    while len(body) < no_pearls:
        body.append(track[-1])
    del track[track_i:]
    return body

最小示例：![IT科技](https://i.sstatic.net/5jD0C.png) repl.it/@Rabbid76/PyGame-SnakeMoveFree

import pygame
import random
import math

pygame.init()
COLUMNS, ROWS, SIZE = 10, 10, 20
WIDTH, HEIGHT = COLUMNS*SIZE, ROWS*SIZE
screen = pygame.display.set_mode((WIDTH, HEIGHT))
clock  = pygame.time.Clock()

background = pygame.Surface((WIDTH, HEIGHT))
background.fill((255, 255, 255))
for i in range(1, COLUMNS):
    pygame.draw.line(background, (128, 128, 128), (i*SIZE-1, 0), (i*SIZE-1, ROWS*SIZE), 2)
for i in range(1, ROWS):
    pygame.draw.line(background, (128, 128, 128), (0, i*SIZE-1), (COLUMNS*SIZE, i*SIZE-1), 2)

def hit(pos_a, pos_b, distance):
    dx, dy = pos_a[0]-pos_b[0], pos_a[1]-pos_b[1]
    return math.sqrt(dx*dx + dy*dy) < distance

def random_pos(body):
    pos = None
    while True:
        pos = random.randint(SIZE//2, WIDTH-SIZE//2), random.randint(SIZE//2, HEIGHT-SIZE//2)
        if not any([hit(pos, bpos, 20) for bpos in body]):
            break    
    return pos

def create_body(track, no_pearls, distance):
    body = [(track[0])]
    track_i = 1
    for i in range(1, no_pearls):
        while track_i < len(track):
            pos = track[track_i]
            track_i += 1
            dx, dy = body[-1][0]-pos[0], body[-1][1]-pos[1]
            if math.sqrt(dx*dx + dy*dy) >= distance:
                body.append(pos)
                break
    while len(body) < no_pearls:
        body.append(track[-1])
    del track[track_i:]
    return body

length = 1
track = [(WIDTH//2, HEIGHT//2)]
dir = (1, 0)
food = random_pos(track)

run = True
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT: dir = (-1, 0)
            elif event.key == pygame.K_RIGHT: dir = (1, 0)
            elif event.key == pygame.K_UP: dir = (0, -1)
            elif event.key == pygame.K_DOWN: dir = (0, 1)

    track.insert(0, track[0][:])    
    track[0] = (track[0][0] + dir[0]) % WIDTH, (track[0][1] + dir[1]) % HEIGHT

    body = create_body(track, length, 20)

    if hit(body[0], food, 20):
        food = random_pos(body)
        length += 1 
        
    screen.blit(background, (0, 0))
    pygame.draw.circle(screen, (255, 0, 255), food, SIZE//2)
    for i, pos in enumerate(body):
        color = (255, 0, 0) if i==0 else (0, 192, 0) if (i%2)==0 else (255, 128, 0)
        pygame.draw.circle(screen, color, pos, SIZE//2)
    pygame.display.flip()