Maze Generation

I, personally, like roguelikes that have levels entirely occupied by mazes.

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Here's a simple way to generate mazes:

1. Start from a cell and mark it 'visited'
2. Pick a direction.
3. If the nearest cell in that direction is 'unvisited' or you want to make a loop here, move to it.
4. Mark it 'visited'
5. Move until there is no space left.
6. Pick a random 'visited' cell.
7. Go to step 2 until you're satisfied with the maze.

Now here's the code (written in C):

/* This program is free software. It comes without any warranty, to
 * the extent permitted by applicable law. You can redistribute it
 * and/or modify it under the terms of the Do What The Fuck You Want
 * To Public License, Version 2, as published by Sam Hocevar. See
 * http://sam.zoy.org/wtfpl/COPYING for more details. */ 
 
// NOTE: Since I like the c99 style, I've written this piece of code in it
// so compile with -std=c99!


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


// a magic number!
#define magic 666

// the map's width and height
#define max_x 70
#define max_y 40

// distance from a cell to another (in map blocks)
#define cellrad 2

// max map cells
const int maxc_x=max_x/cellrad-1;
const int maxc_y=max_y/cellrad-1;

// the map and cell arrays
int map[max_x][max_y];
int cell[max_x/cellrad-1][max_y/cellrad-1];

// just some counters
int visited_cells=0,total_cells=0;


// this function returns 1 if (x,y) is in the range 2...max_x/y-2, to prevent some nasty bugs
int inrange(int x,int y)
{
    if (x>2 && y>2 && x<max_x-2 && y<max_y-2) return 1; return 0;
}

// this function links two cells together
void clink(int x1,int y1,int x2,int y2){
    int cx=x1,cy=y1;
    while(cx!=x2){
	if(x1>x2){cx--;}else{cx++;}
	if(inrange(cx,cy)==1) map[cx][cy]=0;}
    while(cy!=y2){
	if(y1>y2){cy--;}else{cy++;}
	if(inrange(cx,cy)==1) map[cx][cy]=0;}
}

// this function counts the visited cells and the total cells
void count_cells(void)
{
    visited_cells=0; total_cells=0;
    for(int ix=0;ix<maxc_x;ix++){
	for(int iy=0;iy<maxc_y;iy++){
	    total_cells++;
	    if (cell[ix][iy]==1) visited_cells++;
	}
    }
}

// this is the main maze-generating function
void mkmaze(void)
{
    // some ints for random x, random y and random direction
    int rx=0,ry=0,rd=0;
    // c is used to count tries -- if there are too many tries, the main loop
    // jumps to the end of the function
    int c=0;

    // we fill the map with 'walls'
    for(int ix=0;ix<max_x;ix++){
	for(int iy=0;iy<max_y;iy++){
	    map[ix][iy]=1;
	}
    }
    
    // and the cell array with 'unvisited' cells
    for(int ix=0;ix<maxc_x;ix++){
	for(int iy=0;iy<maxc_y;iy++){
	    cell[ix][iy]=0;
	}
    }
    
    // we choose a random (x,y) pair, but here we use the center of the screen instead
    rx=maxc_x/2;
    ry=maxc_y/2;
    
    // we mark that cell 'visited'
    cell[rx][ry]=1;
    
    // we count the cells
    count_cells();
    
    //and start the main loop!
    while( visited_cells < total_cells ){
    
	// c is increased, and we check whether it is too big
	c++;
	if (c>magic) goto hell;
	
	// we choose a random direction
	rd=rand()%4;
	
	// and then dig!
	if (rd==0){		    // up
	    if (inrange(rx*cellrad,ry*cellrad-1)==1 && (cell[rx][ry-1]==0 || rand()%7==6)){
		clink(rx*cellrad,ry*cellrad,rx*cellrad,(ry-1)*cellrad);
		ry--;
	    }else{
		while(1){
		    rx=rand()%maxc_x;
		    ry=rand()%maxc_y;
		    if (cell[rx][ry]==1) break;
		}
	    }
	}else if (rd==1) {	    //down
	    if (inrange(rx*cellrad,ry*cellrad+1)==1 && (cell[rx][ry+1]==0 || rand()%7==6)){
		clink(rx*cellrad,ry*cellrad,rx*cellrad,(ry+1)*cellrad);
		ry++;
	    }else{
		while(1){
		    rx=rand()%maxc_x;
		    ry=rand()%maxc_y;
		    if (cell[rx][ry]==1) break;
		}
	    }
	}else if (rd==2) {	    //left
	    if (inrange(rx*cellrad-1,ry*cellrad)==1 && (cell[rx-1][ry]==0 || rand()%7==6)){
		clink((rx-1)*cellrad,ry*cellrad,rx*cellrad,ry*cellrad);
		rx--;
	    }else{
		while(1){
		    rx=rand()%maxc_x;
		    ry=rand()%maxc_y;
		    if (cell[rx][ry]==1) break;
		}
	    }
	}else if (rd==3) {	    //right
	    if (inrange(rx*2+1,ry*2)==1 && (cell[rx+1][ry]==0 || rand()%7==6)){
		clink(rx*cellrad,ry*cellrad,(rx+1)*cellrad,ry*cellrad);
		rx++;
	    }else{
		while(1){
		    rx=rand()%maxc_x;
		    ry=rand()%maxc_y;
		    if (cell[rx][ry]==1) break;
		}
	    }
	}
	
	/* NOTE:    the above code checks whether the to-be-dug cell is free.
	 *	    If it isn't, and rand()%7==6, it links it anyway. This is used
	 *	    to give the maze loops, change 7 and 6 as you want to get different
	 *	    results */
	
	// now we mark the reached cell 'visited'
	cell[rx][ry]=1;
	
	// and we put a 'floor' tile on the map, where the maze cell should be
	map[rx*cellrad][ry*cellrad]=0;
	
	// now we count the cells, so we can know how many cells we've visited.
	count_cells();
    }
    
    // hell is a bad place. but we use it to put 'floor' tiles where we have a 'visited' cell
    hell:
    for(int ix=0;ix<maxc_x;ix++){
	for(int iy=0;iy<maxc_y;iy++){
	    if (cell[ix][iy]==1) map[ix*cellrad][iy*cellrad]=0;
	}
    }
}


// now this is a simple function that shows us what we just dug.
// it uses ANSI codes, so it may or may not work on your system
void showmap(void)
{
    printf("\033[2J\033[1;1f");
    for(int iy=0;iy<max_y;iy++){
	for(int ix=0;ix<max_x;ix++){
	    fprintf(stdout, "\033[%i;%if",iy,ix);
	    switch(map[ix][iy]){
		case 1: 
		    printf("#");
		    break;
		case 0:
		    printf(".");
		    break;
		default: 
		    printf("*");
		    break;
	    }
	printf("\n");
	}
    }
    printf("Done.\n");
}

// and now the main function...
int main(void)
{
    // we get a random seed
    srand(time(0));
    
    // we dig the maze
    mkmaze();
    
    // we show the map
    showmap();
    
    // and say 'goodbye'
    return 0;
}

Feel free to improve it, but please mention what exactly you changed. --Deveah.