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 <vector>
#include <string>
#include <stack>
using namespace std;

int main(){
  int W, H;
  while(cin >> W >> H && W>0 && H>0){
    int map[20][20];
    for(int i=0;i<20;i++)
      for(int j=0;j<20;j++)
        map[i][j]=0;
    int at[2];

    for(int i=0;i<H;i++){
      string s;
      cin >> s;
      for(int j=0;j<W;j++){
        map[i][j]=(s[j]=='.'?1:(s[j]=='@'?2:0));
        if(s[j]=='@'){
          at[0]=i; at[1]=j;
        }
      }
    }

    vector<int> list[400];  //??£??\?????????????????????????????????20*i+j?????????
    for(int i=0;i<400;i++){
      if(i>=20) //???
        if(map[i/20-1][i%20]>0) list[i].push_back(i-20);
      if(i%20>0) //???
        if(map[i/20][(i-1)%20]>0) list[i].push_back(i-1);
      if(i%20<19) //???
        if(map[i/20][(i+1)%20]>0) list[i].push_back(i+1);
      if(i<180) //???
        if(map[i/20+1][i%20]>0) list[i].push_back(i+20);
    }

    bool d[400];
    for(int i=0;i<400;i++) d[i]=false;

    stack<int> cur;
    cur.push(at[0]*20+at[1]);
    d[at[0]*20+at[1]]=true;

    while(!cur.empty()){
      bool flg=false;
      for(vector<int>::iterator it=list[cur.top()].begin();it!=list[cur.top()].end();it++){
        if(!d[*it]){
          flg=true;
          cur.push(*it);
          d[*it]=true;
          break;
        }
      }
      if(flg) continue;
      cur.pop();
    }

    int cnt=0;
    for(int i=0;i<400;i++)
      if(d[i]) cnt++;

    cout << cnt << endl;
  }

  return 0;  
}

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

char fe[20][20];
int flag[20][20];
int W, H;

void dfs(int x, int y){

  if(flag[y][x] == 1) return;

  //  cout << x << " " << y << endl;

  flag[y][x] = 1;

  if(fe[y+1][x] == '.' && y < H-1) dfs(x, y+1);
  if(fe[y-1][x] == '.' && y > 0) dfs(x, y-1);
  if(fe[y][x+1] == '.' && x < W-1) dfs(x+1, y);
  if(fe[y][x-1] == '.' && x > 0) dfs(x-1, y);


}

int main(){

  while(cin >> W >> H && (W || H)){

    memset(flag,0,sizeof(flag));

    int x, y, cnt = 0;

    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
	cin >> fe[i][j];
	if(fe[i][j] == '@'){
	  x = j; 
	  y = i;
	}
      }
    }

    dfs(x, y);

    for(int i = 0; i < H; i++){
      for(int j = 0; j < W; j++){
	if(flag[i][j] == 1) cnt++;
      }
    }
    //  cout << x << " " << y << endl;
    cout << cnt << endl;
  }


  return 0;
}

C++: #include <iostream>
#include <string>
using namespace std;
int main(){
	int w,h,ct;
	string s;
	while(cin>>w>>h){
		if(w==0&&h==0) break;
		int a[w+2][h+2];
		for(int i=0;i<h+2;i++){
			for(int j=0;j<w+2;j++){
				a[j][i]=0;
			}
		}
		int num=1;
		for(int i=1;i<h+1;i++){
			cin>>s;
			for(int j=1;j<w+1;j++){
				if(s[j-1]=='.'){
					a[j][i]=1;
				}
				else if(s[j-1]=='#'){
					a[j][i]=0;
				}
				else if(s[j-1]=='@'){
					a[j][i]=2;
				}
			}
		}
		do{
			ct=0;
			for(int i=1;i<h+1;i++){
				for(int j=1;j<w+1;j++){
					if(a[j][i]==2&&a[j-1][i]==1){
						a[j-1][i]=2;
						ct++;
						num++;
					}
					else if(a[j][i]==2&&a[j+1][i]==1){
						a[j+1][i]=2;
						ct++;
						num++;
					}
					else if(a[j][i]==2&&a[j][i-1]==1){
						a[j][i-1]=2;
						ct++;
						num++;
					}
					else if(a[j][i]==2&&a[j][i+1]==1){
						a[j][i+1]=2;
						ct++;
						num++;
					}
				}
			}
		} while(ct>0);
		cout<<num<<endl;
	}
    return 0;
}

C++: // ICPC 2004 à\I B
// Red and Black
// C++ queue Å

#include <cstdio>
#include <queue>

using namespace std;

struct Coord {
  int col;
  int row;
};

int solve (int W, int H);
  
int main () {
  while (1) {
    int W, H;
    scanf("%d%d", &W, &H);
    if (W==0 && H==0) break;
    printf("%d\n", solve(W,H));
  }

  return 0;
}

int solve (int W, int H) {
  char M[H+2][W+2];
  Coord start;
  //  ÇiÔºjÌÝè 
  for (int i=0; i<=W+1; i++) { M[0][i] = '#'; M[H+1][i] = '#'; }
  for (int i=1; i<=H; i++) { M[i][0] = '#'; M[i][W+1] = '#'; }
  //  f[^ÌÇÝÝ
  for (int i=1; i<=H; i++) {
    for (int j=1; j<=W; j++) {
      scanf(" %c", &M[i][j]);
      if (M[i][j]=='@') {
	start.row = i;	start.col = j;
      }
    }
  }


  // X^[gn_ÌÝè
  queue<Coord> Q;                              // BZðÛ·éL[
  Q.push(start);                               // X^[gn_ð enqueue
  int c=1;                                     // BZJE^

  // ÈºTõi±ê©çìéj
	while( !Q.empty()){
		Coord current = Q.front();Q.pop();
		int D1[] ={1,0,-1,0};int D2[]={0,-1,0,1};
		for ( int d=0;d<4;d++){
			Coord nc = current;
			nc.row += D1[d]; nc.col += D2[d];
//		printf("r:%d , c:%d\n",nc.row ,nc.col);
			if (M[ nc.row ][ nc.col ]=='.'){
				M[ nc.row ][ nc.col ]='*';
				Q.push(nc);
				c++;
			}
		}
	}

  return c;
}