Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include <stdio.h>
#include <string.h>
#include <queue>

using namespace std;
const int dx[4] = {1,0,-1,0},dy[4] = {0,1,0,-1};

int main(void){
	queue<int> qx, qy;
	int x, y, i, j, w, h, flag[20][20], count;
	char field[20][21];
	while(1){
		scanf("%d%d%*c",&w,&h);
		if(w == 0 && h == 0)
			break;
		memset(flag,0,sizeof(flag));
		for(i = 0;i < h;i++){
			for(j = 0;j < w;j++){
				field[i][j] = getchar();
				if(field[i][j] == '@')
					y = i, x = j;
			}
			getchar();
		}
		qx.push(x);
		qy.push(y);
		while(qx.size()){
			x = qx.front();
			y = qy.front();
			qx.pop();
			qy.pop();
			flag[y][x] = 1;
			for(i = 0;i < 4;i++){
				if(x + dx[i] >= 0 && x + dx[i] < w && y + dy[i] >= 0 && y + dy[i] < h && !flag[y + dy[i]][x + dx[i]] && field[y + dy[i]][x + dx[i]] == '.')
					qx.push(x + dx[i]), qy.push(y + dy[i]);
			}
		}
		count = 0;
		for(i = 0;i < h;i++){
			for(j = 0;j < w;j++)
				if(flag[i][j]) count++;
		}
		printf("%d\n",count);
	}
	return 0;
}

C++: #include <cstdio>

int W, H;
char tile[21][21];

int search(int y, int x) {
  if (tile[y][x] == '#') return 0;
  int cnt = 1;
  tile[y][x] = '#';
  if (y > 0)   cnt += search(y-1, x);
  if (y < H-1) cnt += search(y+1, x);
  if (x > 0)   cnt += search(y, x-1);
  if (x < W-1) cnt += search(y, x+1);
  return cnt;
}

int main() {
  while (true) {
    scanf("%d %d", &W, &H);
    if (W == 0 && H == 0) break;
    for (int i=0;i<H;i++) scanf("%s", tile[i]);
    int cnt = 0;
    for (int i=0;i<H;i++) {
      for (int j=0;j<W;j++) {
        if (tile[i][j] == '@') {
          cnt = search(i, j);
          break;
        }
      }
    }
    printf("%d\n", cnt);
  }
  return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#include <tuple>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> plglg;
typedef tuple<int, int, int> tiii;
typedef tuple<ll, ll, ll> tlglglg;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775807;

struct edge {
    int to, cost;
};

int H, W;
bool marked[100][100];
int sum;

void BFS(pii st) {
    queue<pii> que;
    que.push(st);
    sum++;
    marked[st.first][st.second] = false;
    while (que.size() > 0) {
        pii base = que.front();
        que.pop();
        int basex = base.first;
        int basey = base.second;
        for (int i = 0; i < 4; i++) {
            int x = basex + dx[i];
            int y = basey + dy[i];
            if (x >= 0 && x < H && y >= 0 && y < W) {
                if (marked[x][y]) {
                    que.push(pii(x, y));
                    sum++;
                    marked[x][y] = false;
                }
            }
        }
    }
}

int main() {
    while (true) {
        cin >> W >> H;
        if (W == 0) {
            break;
        }
        for (int i = 0; i < H; i++) {
            fill(marked[i], marked[i] + W, true);
        }
        pii st;
        for (int i = 0; i < H; i++) {
            string s;
            cin >> s;
            for (int j = 0; j < W; j++) {
                if (s[j] == '#') {
                    marked[i][j] = false;
                } else if (s[j] == '@') {
                    st = pii(i, j);
                }
            }
        }
        sum = 0;
        BFS(st);
        cout << sum << endl;
    }
}

C++: #include<iostream>

using namespace std;

char field[21][21];
int direction_x[] = {1, -1, 0, 0};
int direction_y[] = {0, 0, 1, -1};
int w, h;
int lx = 0, ly = 0;
int cnt = 0;

void set_data(int w, int h){
    for(int i=0; i < h; i++){
        for(int j=0; j < w; j++){
            cin >> field[i][j];
            if(field[i][j] == '@'){
                ly = i;
                lx = j;
            }
        }
    }
}

void count(int x, int y){
    field[y][x] ='#';
    cnt++;
    int nx = 0, ny = 0;

    for(int i=0; i<4; i++){
        nx = x + direction_x[i];
        ny = y + direction_y[i];

        if(nx < 0 || nx >= w){
            continue;
        }
        if(ny < 0 || ny >= h){
            continue;
        }
        if(field[ny][nx] != '.'){
            continue;
        }
        count(nx,ny);
    }

}

int main(void){

    while(1){
        cnt = 0;
        cin >> w >> h;
        if(w==0 || h==0){
            break;
        }

        set_data(w,h);
        count(lx,ly);

        cout << cnt << endl;
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int ans;
char room[22][22] = { '0' };

void judge(int x, int y) {
	ans++;
	room[x][y] = '#';
	if (room[x - 1][y] != '#') {
		judge(x - 1, y);
	}
	if (room[x + 1][y] != '#') {
		judge(x + 1, y);
	}
	if (room[x][y - 1] != '#') {
		judge(x, y - 1);
	}
	if (room[x][y + 1] != '#') {
		judge(x, y + 1);
	}
	return;
}

int main(void) {
	while (1) {
		int w, h, human_x, human_y;
		for (int i = 0; i <= 22; i++) {
			for (int j = 0; j <= 22; j++) {
				room[j][i] = { '#' };
			}
		}
		cin >> w >> h;
		if (w == 0) return 0;
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				cin >> room[j][i];
				if (room[j][i] == '@') {
					human_x = j;
					human_y = i;
				}
			}
		}
		ans = 0;
		judge(human_x, human_y);

		cout << ans << endl;
	}

}