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>

#define rep(i, n) for (int i = 0; i < (int)n; ++i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define rer(i, a, b) for (int i = (int)a; i <= (int)b; ++i)
#define each(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define all(v) v.begin(), v.end()
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vector<int> > vvi;
typedef vector<pair<int, int> > vpii;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;

const int inf = 1000000000;
const int mod = 1000000007;
const double eps = 1e-9;
const int dx[] = { -1, 0, 1, 0};
const int dy[] = { 0, -1, 0, 1};

int W, H;
int ans = 0;
string field[100];

void dfs(int y, int x) {
	if (x < 0 || y < 0 || x == W || y == H) return;
	if (field[y][x] == '#') return;
	ans++;
	field[y][x] = '#';
	rep(i, 4) dfs(y + dy[i], x + dx[i]);
}

int main() {
	while (cin >> W >> H, W) {
		rep(i, H) cin >> field[i];
		rep(i, H) rep(j, W) {
			if (field[i][j] == '@') dfs(i, j);
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>

using namespace std;


int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

int w,h;
int map[21][21];

int count;
int init_x,init_y;

void dfs(int x,int y) {
  
  //cout << x << " " << y << endl;

  if(x<0 || x>=w || y<0 || y>=h) return;

  //if(map[x][y]==2) map[x][y]=0;

  if(map[x][y]>0) return;

  map[x][y]=3;

  count++;

  for(int i=0;i<4;i++) dfs(x+dx[i],y+dy[i]);

}

int main() {

  string buf;

  while(cin >> w >> h) {

    if(w==0 && h==0) break;

    getline(cin,buf);

    for(int i=0;i<h;i++) {

      getline(cin,buf);

      for(int j=0;j<w;j++) {

	switch(buf[j]) {

	case '.':
	  map[j][i]=0;
	  break;

	case '#':
	  map[j][i]=1;
	  break;

	case '@':
	  map[j][i]=0;
	  init_x=j;
	  init_y=i;
	  break;

	}

      }

    }

    for(int i=0;i<h;i++) {

      for(int j=0;j<w;j++) {

	cout << map[j][i];

      }

      cout << endl;

    }

    cout << endl;

    cout << map[5][7] << endl;

    count=0;

    dfs(init_x,init_y);

    cout << init_x << " " << init_y << endl;

    for(int i=0;i<h;i++) {

      for(int j=0;j<w;j++) {

	cout << map[j][i];

      }

      cout << endl;

    }

    cout << count << endl;

  }

  return 0;

}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define loop(i,a,b) for(ll i=(a);i<ll(b);i++)
#define rep(i,b) loop(i,0,b)

int h,w;
char g[30][30];

int rec(int ci, int cj){
    g[ci][cj] = '#';
    int ans = 1;
    int di[] = {0,1,0,-1};
    int dj[] = {1,0,-1,0};
    rep(i,4){
        int ni = ci + di[i];
        int nj = cj + dj[i];
        if(ni >= h || nj >= w || ni < 0 || nj < 0 || g[ni][nj] == '#') continue;
        ans += rec(ni,nj);
    }
    return ans;
}

int main(){
    while(scanf("%d%d",&w,&h), w != 0){
        rep(i,h) scanf("%s",g[i]);
        int ans = 0;
        rep(i,h)rep(j,w) if(g[i][j]=='@') ans = rec(i,j);
        printf("%d\n",ans);
    }
}

C++: #include<iostream>
#include<queue>

using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};

int main(){

   while(1){
      int W, H;
      cin>> W>> H;
      if(!(H||W)) break;
      char fld[H][W];
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      bool visited[H][W];
      for(int i=0; i<H; i++)for(int j=0; j<W; j++)visited[i][j]=0;
      queue<pii> Q;
      Q.push(pii(sy, sx));
      while(!Q.empty()){
         pii now=Q.front(); Q.pop();
         visited[now.first][now.second]=1;
         for(pii dd: dxdy){
            int ni=now.first+dd.first;
            int nj=now.second+dd.second;
            if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
            if(fld[ni][nj]=='#') continue;
            if(visited[ni][nj]) continue;
            Q.push(pii(ni, nj));
         }
      }
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=visited[i][j];
      }

      cout<< ans<< endl;
   }

   return 0;
}