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<iostream>
#include<stdio.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<deque>
#include<stack>
#include<string>
#include<string.h>
#include<vector>
#include<set>
#include<map>
#include<bitset>
#include<stdlib.h>
#include<cassert>
#include<time.h>
#include<bitset>
using namespace std;

int main(){
  while(1){
    int h,w;
    cin>>w>>h;
    if(h==0&&w==0) break;
    char a[h][w];
    queue<pair<int,int>> q;
    bool used[h][w];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
        cin>>a[i][j];
        used[i][j]=0;
        if(a[i][j]=='@') q.push(make_pair(i,j));
      }
    }
    int ans=0;
    while(!q.empty()){
      pair<int,int> p=q.front();
      q.pop();
      int x=p.first, y=p.second;
      if(used[x][y]) continue;
      used[x][y]=1;
      ans++;
      if(x>0){
        if(!used[x-1][y]&&a[x-1][y]=='.'){
          q.push(make_pair(x-1,y));
        }
      }
      if(x<h-1){
        if(!used[x+1][y]&&a[x+1][y]=='.'){
          q.push(make_pair(x+1,y));
        }
      }
      if(y>0){
        if(!used[x][y-1]&&a[x][y-1]=='.'){
          q.push(make_pair(x,y-1));
        }
      }
      if(y<w-1){
        if(!used[x][y+1]&&a[x][y+1]=='.'){
          q.push(make_pair(x,y+1));
        }
      }
    }
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include<stdio.h>

int main(void){
  int W,H;
  char room[20][20];
  int i,j,k;
  int count;
  scanf("%d %d",&W,&H);
  while(!(W==0&&H==0)){
    for(i=0;i<H;i++){
      scanf("%s",room[i]);
    }
    for(i=0;i<W*H;i++){
      for(j=1;j<H;j++){
	for(k=0;k<W;k++){
	  if(room[j][k]!='#' && room[j-1][k]=='@'){
	    room[j][k]='@';
	  }
	}
            }
      for(j=0;j<H;j++){
	for(k=1;k<W;k++){
	  if(room[j][k]!='#' && room[j][k-1]=='@'){
	    room[j][k]='@';
	  }
	}
      }
      for(j=0;j<H-1;j++){
	for(k=0;k<W;k++){
	  if(room[j][k]!='#' && room[j+1][k]=='@'){
	    room[j][k]='@';
	  }
	}
      }
      for(j=0;j<H;j++){
	for(k=0;k<W-1;k++){
	  if(room[j][k]!='#' && room[j][k+1]=='@'){
	    room[j][k]='@';
	  }
	}
      }
    }
    count=0;
    for(i=0;i<H;i++){
      for(j=0;j<W;j++){
	if(room[i][j]=='@'){
	  count++;
	}
      }
    }
    printf("%d\n",count);
    scanf("%d %d", &W,&H);
  }
  return 0;
}

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

#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))

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

int W, H;
char B[30][30];

int dfs(int y, int x) {
   int ret = 1;
   B[y][x] = '#';
   rep (i, 4) {
      int nx = x + dx[i], ny = y + dy[i];
      if (nx >= 0 && nx < W && ny >= 0 && ny < H && B[ny][nx] == '.') ret += dfs(ny, nx);
   }
   return ret;
}

int main() {
   while (cin >> W >> H, W) {
      rep (i, H) cin >> B[i];
      int sy = 0, sx = 0;
      rep (i, H) rep (j, W) if (B[i][j] == '@') sy = i, sx = j;
      cout << dfs(sy, sx) << endl;
   }
   
   return 0;
}

C++: #include <iostream>
using namespace std;
int cnt,w,h;
string s[21];
void check(int i,int j){
	if(s[i][j]=='#')return;
	cnt++;
	s[i][j]='#';
	if(i>0)check(i-1,j);
	if(i<h-1)check(i+1,j);
	if(j>0)check(i,j-1);
	if(j<w-1)check(i,j+1);
}


int main(){
	while(1){
		cin >>w >>h;
		if(!(w||h))break;
		for(int i=0;i<h;i++){
			cin >>s[i];
		}
		cnt=0;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(s[i][j]=='@')check(i,j);
			}
		}
	cout << cnt<<endl; 

	}
}