貪吃蛇，是一款經典的益智遊戲。我們可以通過演算法捕捉和傳遞智慧，設計出能自動跑著吃食物的智慧蛇。

有不少人對此研究，設計出了很棒的演算法，如圖：

歸納一些比較高階的演算法為三類：
1.寬度優先搜尋最短路徑
2.走哈密頓迴路
3.特殊決策

這裡我主要介紹最簡單的貪婪演算法及整個遊戲的設計。

首先是自動尋路函式whereGoNext(void)。決策思想是走曼哈頓距離fabs(snakeX[0]-foody)+fabs(snakeY[0]-foodx)最小且不會死掉的方向。當然，這很不智慧，容易死掉，但容易實現。

char whereGoNext() {
    char movable[4
 
]= {'D','A','S','W'};
    int min=9999,minDir;
    for(int i=0; i<4; i++) {
        snakeX[0]+=direx[i];
        snakeY[0]+=direy[i];

        if(!gameover()) {
            int cost=fabs(snakeX[0]-foody)+fabs(snakeY[0]-foodx);
            if(min>cost) {
                min=cost;
                minDir=i;
            }
        }

        snakeX[0
 
]-=direx[i];
        snakeY[0]-=direy[i];
    }
    if(min<9999)
        return movable[minDir];
    else return 0;
}

接著是如何製作平滑的輸出介面。我的實現方法是：

每走一步
   掩蓋蛇尾
   輸出蛇頭
   輸出蛇頸

完整的程式碼如下，供大家參考。程式碼不長，但實現了：
1.貪吃蛇每100ms行走一步；
2.平滑的輸出介面；
3.地圖中有一些設定的障礙物（牆），並且蛇會繞開障礙物；
4.貪婪演算法。
實驗環境：Ubuntu。

// snake_smart.cpp
 

// Kuncheng Xie 2017-12-26
// All rights reserved.

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<time.h>
#include<unistd.h>

#define SNAKE_MAX_LENGTH 150
#define SNAKE_HEAD 'H'
#define SNAKE_BODY 'X'
#define BLANK_CELL ' '
#define SNAKE_FOOD '$'
#define WALL_CELL '*'
#define MAP_LENGTH 12
#define DELAY_TIME  100000  //delay 100ms

//move snake
void snakeMove(int dir);
//output cells of the grid
void show(void);
//output snake
void output(void);
//put a food randomized on a blank cell,1 for put food successfully,0 for fail
int putFood(void);
//judge whether gameover;0 for not over,1 for game over
int gameover(void);
//decide the smart snake's direction
char whereGoNext(void);

char map[MAP_LENGTH][MAP_LENGTH+1]= {//initial status
    "************",
    "*XXXXH     *",
    "*      *   *",
    "*      *   *",
    "*     **   *",
    "*      *   *",
    "*          *",
    "*          *",
    "*          *",
    "*          *",
    "*          *",
    "************"
};
const int direx[4]= {0,0,1,-1};
const int direy[4]= {1,-1,0,0};
int snakeX[SNAKE_MAX_LENGTH]= {5,4,3,2,1};
int snakeY[SNAKE_MAX_LENGTH]= {1,1,1,1,1};
int snakeLength=5;
int foodx,foody,eatFood=1;

int main(void) {
    char s;
    srand(time(NULL));
    printf("\033[2J");//clear the screen
    printf("\033[1;1H");
    show();
    do {
        s=whereGoNext();
        if(s==0)break;
        switch(s) {
            case 'D':
                snakeMove(0);
                break;
            case 'A':
                snakeMove(1);
                break;
            case 'S':
                snakeMove(2);
                break;
            case 'W':
                snakeMove(3);
                break;
        }
        output();
        usleep(DELAY_TIME);
    } while(!gameover());
    printf("\033[13;1HGame over!!!\n");
    return 0;
}

void snakeMove(int dir) {
    //first clear the tail
    printf("\033[%d;%dH%c\n",snakeY[snakeLength-1]+1,snakeX[snakeLength-1]+1,BLANK_CELL);
    map[snakeY[snakeLength-1]][snakeX[snakeLength-1]]=BLANK_CELL;

    if(snakeX[0]+direx[dir]==foody&&snakeY[0]+direy[dir]==foodx) {//eat food
        eatFood=1;
        snakeLength++;//increase length
    }

    for(int i=snakeLength-1; i>0; i--) {//body move
        snakeX[i]=snakeX[i-1];
        snakeY[i]=snakeY[i-1];
    }

    snakeX[0]+=direx[dir];//head move
    snakeY[0]+=direy[dir];
}

void show() {
    for(int i=0; i<MAP_LENGTH; i++) {//output map
        for(int j=0; j<MAP_LENGTH; j++) {
            printf("%c",map[i][j]);
        }
        printf("\n");
    }
}

void output(void) {
    printf("\033[%d;%dH%c\n",snakeY[0]+1,snakeX[0]+1,SNAKE_HEAD);//print snake
    printf("\033[%d;%dH%c\n",snakeY[1]+1,snakeX[1]+1,SNAKE_BODY);
    printf("\033[%d;%dH%c\n",snakeY[snakeLength-1]+1,snakeX[snakeLength-1]+1,SNAKE_BODY);
    //must empty buffers: printf+'\n' or fflush(stdout);

    map[snakeY[0]][snakeX[0]]=SNAKE_HEAD;
    map[snakeY[snakeLength-1]][snakeX[snakeLength-1]]=SNAKE_BODY;

    if(eatFood) {
        eatFood=0;
        if(putFood()) { //put food
            printf("\033[%d;%dH%c\n",foodx+1,foody+1,SNAKE_FOOD);
        }
    } else {
        printf("\033[%d;%dH%c\n",foodx+1,foody+1,SNAKE_FOOD);
    }
    printf("\033[13;1H\n");
}

int gameover(void) {
    if(snakeLength>=(MAP_LENGTH-2)*(MAP_LENGTH-2))return 1;//the map is full

    if(map[snakeY[0]][snakeX[0]]==WALL_CELL)return 1;//not touch wall

    for(int j=1; j<snakeLength; j++) {//doesn't eat itself
        if(snakeX[0]==snakeX[j]&&snakeY[0]==snakeY[j])return 1;
    }
    return 0;
}

int putFood(void) {
    int foodMap[MAP_LENGTH*MAP_LENGTH][2],len,ran;
    len=0;
    for(int i=1; i<MAP_LENGTH-1; i++) {//put empty place into foodMap
        for(int j=1; j<MAP_LENGTH-1; j++) {
            if(map[i][j]==BLANK_CELL) {
                foodMap[len][0]=i;
                foodMap[len][1]=j;
                len++;
            }
        }
    }

    if(len>1)ran=rand()%len;//place food randomly
    else if(len==0)return 0;
    else ran=0;

    foodx=foodMap[ran][0];
    foody=foodMap[ran][1];
    return 1;
}

char whereGoNext() {
    char movable[4]= {'D','A','S','W'};
    int min=9999,minDir;
    for(int i=0; i<4; i++) {
        snakeX[0]+=direx[i];
        snakeY[0]+=direy[i];

        if(!gameover()) {
            int cost=fabs(snakeX[0]-foody)+fabs(snakeY[0]-foodx);
            if(min>cost) {
                min=cost;
                minDir=i;
            }
        }

        snakeX[0]-=direx[i];
        snakeY[0]-=direy[i];
    }
    if(min<9999)
        return movable[minDir];
    else return 0;
}

最終介面如圖：

這裡大力推薦一個同學的部落格：設計簡單的貪吃蛇AI 。身為同學，不免自慚形穢。