Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

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

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

int W, H;
char board[30][30];
bool visited[30][30];

int dx[] = {0, 0, -1, 1};
int dy[] = {-1, 1, 0, 0};

void dfs(int x, int y) {
   for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx < 0 || nx >= H || ny < 0 || ny >= W) continue;
      if (board[nx][ny] == '#') continue;
      if (visited[nx][ny]) continue;
      visited[nx][ny] = true;
      dfs(nx, ny);
   }
}

int main() {
   while (cin >> W >> H, W && H) {
      memset(visited, false, sizeof(visited));
      int sx = 0, sy = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            cin >> board[i][j];
            if (board[i][j] == '@') {
               sx = i, sy = j;
            }
         }
      }

      board[sx][sy] = true;
      dfs(sx, sy);

      int cnt = 0;
      for (int i = 0; i < H; i++) {
         for (int j = 0; j < W; j++) {
            if (visited[i][j]) cnt++;
         }
      }

      cout << cnt << endl;
   }

   return 0;
}

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

int point(int x,int y,int w,int h){
  int s;
  s= x*w*y*h;
  return s;
}

int pointx(int x,int w){
  int s;
  s=x/w;
  return s;
}

int pointy(int x,int w){
  int s;
  s=x%w;
  return s;
}

int main(){
  int w,h;
  w = 1;
  h = 1;
  cin>>w>>h;
  while(w!=0){
    int a[25][25]={};
    int sx,sy;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	char s;
	cin>>s;
	if(s=='.'){
	  a[i][j]=1;
	}else if(s=='#'){
	  a[i][j]=-1;
	}else{
	  a[i][j]=2;
	  sx=i;
	  sy=j;
	}
      }
    }

    int d[21][21];
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	d[i][j]=-1;
      }
    }
    queue<int>q;
    d[sx][sy]=0;
    int str;
    str=point(sx,sy,w,h);
    q.push(str);
    while(! q.empty()){
      int cur =q.front();
      q.pop();
      int x,y;
      x=pointx(cur,w);
      y=pointy(cur,w);
      if(x>=1&&a[x-1][y]==1&&d[x-1][y]==-1){
	d[x-1][y]=0;
	q.push(point(cur-1,y,w,h));
      }
      if(x<=w-2&&a[x+1][y]==1&&d[x+1][y]==-1){
	d[x+1][y]=0;
	q.push(point(cur+1,y,w,h));
      }
      if(y>=1&&a[x][y-1]==1&&d[x][y-1]==-1){
	  d[x][y-1]=0;
	  q.push(point(x,cur-w,w,h));
      }
      if(y<=h-2&&a[x][y+1]==1&&d[x][y+1]==-1){
	d[x][y-1]=0;
	q.push(point(x,cur+w,w,h));
      }
    }

    int t=0;
    for(int i=0;i<w;i++){
      for(int j=0;j<h;j++){
	if(d[i][j]==0){
	  t+=1;
	}
      }
    }
    cout << t<< endl;
    cin>>w>>h;
  }
}

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

char map[20][20];
int W,H;
int cnt;

void fac(int y,int x){
  map[y][x]='#';
  cnt++;
  
  if(map[y+1][x]=='.' && y < H-1)
    fac(y+1,x);
  if(map[y-1][x]=='.' && y > 0)
    fac(y-1,x);
  if(map[y][x+1]=='.' && x < W-1)
    fac(y,x+1);
  if(map[y][x-1]=='.' && x > 0)
    fac(y,x-1);  
}

int main(){
  int i,j;
  int x,y;

  while(1){
    cin >> W >> H;

    if(W+H==0)
      break;
    
    for(i=0;i<H;i++){
      for(j=0;j<W;j++){
	cin >> map[i][j];
	if(map[i][j]=='@'){
	  x=j; y=i; 
	} //@position      
      }
    }
    
    cnt=0;
    fac(y,x); //release cnt
    cout << cnt << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int solve(int W, int H) {
    vector<vector<char>> field(H + 2, vector<char>(W + 2, '#'));
    vector<vector<int>> cnt(H + 2, vector<int>(W + 2, 0));
    pair<int, int> p;
    queue<pair<int, int>> Q;

    for (int i = 1; i <= H; i++) {
        for (int j = 1; j <= W; j++) {
            cin >> field[i][j];
            if (field[i][j] == '@') {
                Q.push(make_pair(i, j));
                cnt[i][j] = 1;
            }
        }
    }

    int dx[] = {1, -1, 0, 0};
    int dy[] = {0, 0, 1, -1};
    while (!Q.empty()) {
        p = Q.front();
        Q.pop();
        for (int i = 0; i < 4; i++) {
            int ny = p.first + dy[i];
            int nx = p.second + dx[i];
            if (field[ny][nx] == '#') continue;
            if (cnt[ny][nx] == 1) continue;
            Q.push(make_pair(ny, nx));
            cnt[ny][nx] = 1;
        }
    }
    int ret = 0;
    for (int i = 0; i < cnt.size(); i++) {
        for (int j = 0; j < cnt[i].size(); j++) {
            if (cnt[i][j] == 1) ret++;
        }
    }

    return ret;
}

int main() {
    int W, H;
    vector<int> ans;
    while (1) {
        cin >> W >> H;
        if (!W && !H) break;
        ans.push_back(solve(W, H));
    }

    for (int r : ans) cout << r << endl;
}