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 <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cctype>

#include <iostream>
#include <vector>
#include <deque>
#include <queue>
#include <map>
#include <algorithm>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i,l,n) for(int (i)=(int)(l);(i)<(int)(n);(i)++)
#define d_arr(arr, h, w) rep(i,(h) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}
#define IN(x,s,g) ((x) >= (s) && (x) < (g))
#define ISIN(x,y,w,h) (IN((x),0,(w)) && IN((y),0,(h)))
#define print(x) printf("%d\n",x);
#define fs first
#define sc second

typedef pair<int ,int> P;

char grid[20][20];
bool arrived[20][20];
int dx[4] ={1,0,-1,0};
int dy[4] ={0,1,0,-1};
int w,h;

//i,jから初めて到達可能なタイルの数の最大値(i,j)含む
void rec(int i,int j){
//    printf("i:%d j:%d\n",i,j);
    arrived[i][j] = true;
    grid[i][j] = '#';
    int ret = 0;
    rep(k,4) {
        int next_j = j+dx[k];
        int next_i = i+dy[k];
        if( next_j < w && 0 <= next_j &&
            next_i < h && 0 <= next_i &&
            grid[next_i][next_j] == '.')
        {
            rec(next_i,next_j);
        }
    }
}

int main()
{
    P start;
    while(1){
        cin >> w >> h;
        if( w==0 && h==0 ) {break;}
        rep(i,20) rep(j,20) arrived[i][j] = false;
        rep(i,h){
            rep(j,w){
                cin >> grid[i][j];
                if( grid[i][j] == '@' ){ start.first=i;start.second=j; }
            }
        }
        rec(start.first,start.second);
        int ret=0;
        rep(i,20){
            rep(j,20){
                if(arrived[i][j] == true ){
                    ret+=1;
                }
            }
        }
        cout << ret << endl;
    }
}

C++: #include <cstdio>
#include <cstring>

#define MAP_COLUMNS (20+3) // \r\n\0

//#define dprintf(arg1, ... ) printf( arg1, __VA_ARGS__)
#define dprintf(arg1, ... ) 

void proc( int width, int height, char map[][MAP_COLUMNS], int x, int y );

inline void docheck( int width, int height, char map[][MAP_COLUMNS], int x, int y ) {
	if ( x<0 || x >= width || y<0 || y >= height ) {
		//invalid
		return;
	}
	dprintf("x=%d y=%d  map[y][x]=%c\n",x,y,map[y][x]);
	if ( map[y][x] != '.' ) { return; }
	
	map[y][x] = '@';
	proc(width, height, map, x, y);
}

void proc( int width, int height, char map[][MAP_COLUMNS], int x, int y ) {

	//Left
	docheck(width, height, map, x-1, y);
	
	//Right
	docheck(width, height, map, x+1, y);
	
	//Up
	docheck(width, height, map, x, y-1);
	
	//Down
	docheck(width, height, map, x, y+1);
}

int main(void) {

	char map[20][MAP_COLUMNS];
	
	while (1) {
		int width, height;
		scanf("%d %d\n", &width, &height);
		if ( width==0 && height==0 ) { break; }
		
		memset( map, 0, sizeof(char) * 20 * MAP_COLUMNS );
		
		int startx=-1, starty=-1;
		for ( int i=0; i<height; i++ ) {
			fgets(map[i], MAP_COLUMNS, stdin);
			char* pos = strchr( map[i], '@' );
			if ( pos != NULL ) {
				startx = pos - map[i];
				starty = i;
			}
		}
		dprintf("%d %d\n",startx,starty);
		proc(width, height, map, startx, starty);
		
		int count=0;
		for ( int i=0; i< 20 * MAP_COLUMNS ; i++ ) {
			if( map[0][i] == '@' ) {
				count++;
			}
		}
		printf("%d\n",count);
	}
		
	return 0;
}

C++: #include <iostream>
using namespace std;

int main()
{
	int w, h;
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0) {
			break;
		}
		unsigned next = 1U;
		char** room = new char*[h];
		unsigned** group = new unsigned*[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = 0U;
			}
		}

		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
							}
							if (group[i][j] == 0U) {
								group[i][j] |= next;
								next <<= 1;
							}
						}
					}
				}
			}
		}
		for (int i = h - 1; i >= 0; i--) {
			for (int j = w - 1; j >=0; j--) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		for (int j = 0; j < w; j++) {
			for (int i = 0; i < h; i++) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		for (int j = w - 1; j >= 0; j--) {
			for (int i = h - 1; i >= 0; i--) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if ((group[i][j] & group[y][x]) != 0U) {
					count++;
				}
			}
		}

		cout << count << '\n';

		for (int i = 0; i < h; i++) {
			delete[] room[i];
			delete[] group[i];
		}
		delete[] room;
		delete[] group;
	}

	return 0;
}