Flood fill algorithm maze

Question

I ma trying to make my program read a maze like this:

    #.#######
    #.......#
    ####.####
    #....#..#
    #.####.##

and print out the reachable zones and non-reachable zones in the maze, which should look like this:

    #+#######
    #+++++++#
    ####+####
    #++++#--#
    #+####-##

Walls are represented with "#", and passable cells are represented with ".".

The "+" that replaces the cells means that those cells are reachable from the top entry point of the maze. The "-" symbol are the cells that can't be reached if you enter the top of the maze.

For example, in the above maze, all the cells are reachable except for the cells in the bottom right-hand corner. This is because those cells cannot be reached from the entry point at the top of the maze.

I am trying to use some recursion to flood fill the maze, and determine the reachable zones, but I am having trouble with that.

This is what I have so far:

    int
    flood_fill(m_t * maze, int row, int col) {

        int direction;

        direction = flood_fill(maze, row+1, col); /* down */
        if (!direction) {
            direction = flood_fill(maze, row, col+1); /* right */
        }

        if (!direction) {
            direction = flood_fill(maze, row-1, col); /* up */
        }

        if (!direction) {
            direction = flood_fill(maze, row, col-1); /* left */
        }

        if (direction) {
            maze->M[row][col].type = path;
        }

        return direction;
    }

I know that my flood_fill function is not doing the right thing, and I'm having difficulty getting it right. Can anyone help me please me get my flood filling part of the code right, so I can call the function elsewhere in the code and determine which cells can be reached.

score 2 · Accepted Answer · answered May 15 '16 at 20:34

try this

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

#define MAX_HEIGHT 100
#define MAX_WIDTH  100
#define wall '#'
#define path_cell '.'

typedef struct {
    char type;
    int reachable;
    int visited;
} mazecells_t;

typedef struct {
    int height;
    int width;
    mazecells_t M[MAX_HEIGHT][MAX_WIDTH];
} m_t;

void readmaze(m_t *maze);
void print(m_t *m);
void print_reachable(m_t *m);

int main(void) {
    m_t MAZE;

    readmaze(&MAZE);
    print(&MAZE);
    puts("");
    print_reachable(&MAZE);

    return 0;
}

void readmaze(m_t *maze) {
    int row=0, col=0;
    char ch;
    FILE *fp = stdin;//fopen("map.txt", "r");
    while(EOF != fscanf(fp, "%c", &ch)){
        if(ch == wall || ch == path_cell){
            maze->M[row][col].type = ch;
            maze->M[row][col].reachable = 0;
            maze->M[row][col].visited = 0;
            ++col;
        } else if(ch == '\n'){
            maze->width = col;
            col = 0;
            ++row;
        }
    }
    if(col != 0)
        ++row;
    //fclose(fp);
    maze->height = row;
}

void print(m_t *m){
    for(int r = 0; r < m->height; ++r){
        for(int c = 0; c < m->width; ++c){
            putchar(m->M[r][c].type);
        }
        putchar('\n');
    }
}

typedef enum dir {
    UP, RIGHT, DOWN, LEFT, FORWARD
} DIR;

typedef struct pos {
    int r, c;
    DIR dir;
} Pos;

typedef struct node {
    Pos pos;
    struct node *next;
} Node;

typedef struct queque {
    Node *head, *tail;
} Queque;

Queque *new_queque(void){
    Queque *q = malloc(sizeof(*q));
    q->head = q->tail = NULL;
    return q;
}

void enque(Queque *q, Pos pos){
    Node *node = malloc(sizeof(*node));
    node->pos = pos;
    node->next = NULL;
    if(q->head == NULL){
        q->head = q->tail = node;
    } else {
        q->tail = q->tail->next = node;
    }
}

Pos deque(Queque *q){
    Pos pos = q->head->pos;
    Node *temp = q->head;
    if((q->head = q->head->next)==NULL)
        q->tail = NULL;
    free(temp);
    return pos;
}

bool isEmpty_que(Queque *q){
    return q->head == NULL;
}

Pos dxdy(DIR curr, DIR next){
    Pos d = { 0, 0, 0};
    switch(curr){
    case UP:
        switch(next){
        case LEFT:
            d.c -= 1;
            break;
        case FORWARD:
            d.r -= 1;
            break;
        case RIGHT:
            d.c += 1;
            break;
        }
        break;
    case RIGHT:
        switch(next){
        case LEFT:
            d.r -= 1;
            break;
        case FORWARD:
            d.c += 1;
            break;
        case RIGHT:
            d.r += 1;
            break;
        }
        break;
    case DOWN:
        switch(next){
        case LEFT:
            d.c += 1;
            break;
        case FORWARD:
            d.r += 1;
            break;
        case RIGHT:
            d.c -= 1;
            break;
        }
        break;
    case LEFT:
        switch(next){
        case LEFT:
            d.r += 1;
            break;
        case FORWARD:
            d.c -= 1;
            break;
        case RIGHT:
            d.r -= 1;
            break;
        }
        break;
    }
    return d;
}

Pos next_pos(Pos pos, DIR dir){
    Pos dxy = dxdy(pos.dir, dir);
    switch(dir){
    case RIGHT:
        pos.dir = (pos.dir + 1) % 4;
        break;
    case LEFT:
        if((pos.dir = (pos.dir - 1)) < 0)
            pos.dir += 4;
        break;
    case FORWARD:
        break;
    }
    pos.r += dxy.r;
    pos.c += dxy.c;
    return pos;
}
static inline bool isValid(m_t *m, Pos pos){
    if(pos.r < 0 || pos.r >= m->height || pos.c < 0 || pos.c >= m->width || m->M[pos.r][pos.c].type == wall)
        return false;
    return true;
}
static inline bool isValidAndUnvisit(m_t *m, Pos pos){
    return isValid(m, pos) && !m->M[pos.r][pos.c].reachable;
}

void print_reachable(m_t *m){
    int i;
    for(i = 0; i < m->width; ++i)
        if(m->M[0][i].type == path_cell)
            break;
    Pos pos = { 0, i, DOWN};
    Queque *q = new_queque();
    enque(q, pos);
    while(!isEmpty_que(q)){
        pos = deque(q);
        if(!m->M[pos.r][pos.c].reachable){
            m->M[pos.r][pos.c].reachable = 1;

            Pos next = next_pos(pos, LEFT);
            if(isValidAndUnvisit(m, next))
                enque(q, next);
             next = next_pos(pos, FORWARD);
            if(isValidAndUnvisit(m, next))
                enque(q, next);
             next = next_pos(pos, RIGHT);
            if(isValidAndUnvisit(m, next))
                enque(q, next);
        }
    }
    free(q);
    for(int r = 0; r < m->height; ++r){
        for(int c = 0; c < m->width; ++c){
            if(m->M[r][c].reachable)
                putchar('+');
            else if(m->M[r][c].type == path_cell)
                putchar('-');
            else
                putchar(m->M[r][c].type);
        }
        putchar('\n');
    }
}

Klas Lindbäck · Answer 2 · 2016-05-16T08:55:52.473

You need to check if the current position is a path, a visited path or a wall.

If it is a path, change it to reachable and visited and then call flood_fill in each direction.

If it is a wall or a visited path then return without further calls to flood_fill.

After flood_fill has returned, any position that is an unvisited path is unreachable.

Edit:

The code for flood_fill could look something like this:

void
flood_fill(m_t * maze, int row, int col) {

    if (row < 0 || col < 0 || row >= MAX_HEIGHT || col >= MAX_WIDTH) {
        /* Out of bounds */
        return;
    }
    if (maze->M[row][col].type != path_cell) {
        /* Not a path cell */
        return;
    }
    if (maze->M[row][col].visited) {
        /* We have already processed this cell */
        return;
    }

    /* We have now established that the cell is a reachable path cell */
    maze->M[row][col].visited = 1;
    maze->M[row][col].reachable = 1;
    maze->M[row][col].type = '+';  /* Not sure if you want this line or if you exchange the symbol in the presentation */

    /* Check the surrounding cells */
    flood_fill(maze, row+1, col); /* down */
    flood_fill(maze, row, col+1); /* right */
    flood_fill(maze, row-1, col); /* up */
    flood_fill(maze, row, col-1); /* left */
}

A. Sarid · Answer 3 · 2016-05-15T23:51:54.957

First, you can find a very nice explanation for the maze recursion here.

In your case the function should look like this:

void
flood_fill(m_t * maze, int row, int col)
{
    // If row,col is outside maze
    if ( row < 0 || row >= maze->height || col < 0 || col >= maze->width) return;
    // If row,col is not open
    if (maze->M[row][col].type != '.') return;
    // Mark row,col as part of path.
    maze->M[row][col].type = '+';
    // Go Up
    flood_fill(maze, row, col - 1);
    // Go Right
    flood_fill(maze, row + 1, col);
    // Go Down
    flood_fill(maze, row, col + 1);
    // Go Left
    flood_fill(maze, row - 1, col);

    return;
}

Result after calling this function with your example matrix will be:

#+#######
#+++++++#
####+####
#++++#..#
#+####.##

After running this you can just go over the matrix again and mark every . cell with - and you're done.

Note: You don't need to change your matrix before calling this function. You only need to call this function with the index of the beginning of the maze when you find it. In your example it will be flood_fill(maze,0,1).

Flood fill algorithm maze

3 Answers3