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<stdio.h>
#define SIZE 100000
typedef struct{
  int x;
  int y;
}Zahyo;
Zahyo  z[SIZE];
int dx[]={0,1,0,-1};
int dy[]={-1,0,1,0};
main(){
  int h,w;
  int head=0;
  int tail=0;
  int cnt=0;
  char maps[30][30];
  int i,j;
  scanf("%d %d",&w,&h);
  for(i=0;i<h;i++){
    for(j=0;j<w;j++){
      scanf("%c",&maps[i][j]);
      if(maps[i][j]=='@'){
	z[0]=(Zahyo){j,i};
	tail=(tail+1)%SIZE;
      }
    }
  }
  while(head != tail){
    int x = z[head].x; int y = z[head].y; head = (head+1) % SIZE;
    if(0 <= x && 0 <= y && x < w && y < h && maps[y][x] != '#'){
      maps[y][x] = '#';
      cnt++;
      for(i=0;i<4;i++){
	int nx = x + dx[i]; int ny = y + dy[i];
	z[(tail+i) % SIZE] = (Zahyo){ nx,ny };
      }
      tail = (tail+4) % SIZE;
    }
  }
  printf("%d\n",cnt);
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <queue>
#include <utility>

using namespace std;


char map[30][30];
pair<int,int> s;
int w,h;
int cnt;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
class Node{
public:
  int x,y;
  int cnt;
  Node(int _x,int _y,int _cnt):x(_x),y(_y),cnt(_cnt){

  }

};
int sx,sy;
void show(){
  for(int i=0;i<h;i++){
    cout<<map[i]<<endl;

  }
}
int bfs(){
  queue<Node> q;
  q.push(Node(sx,sy,0));
  map[sy][sx]='#';
  int ans=0;
  while(!q.empty()){
    Node cur=q.front();q.pop();
    //    show();
    ans=max(ans,cur.cnt);
    for(int i=0;i<4;i++){
      int nx=cur.x+dx[i];
      int ny=cur.y+dy[i];
      if(nx<0||nx>=w||ny<0||ny>=h||map[ny][nx]=='#')continue;
      map[ny][nx]='#';
      cnt++;
      q.push(Node(nx,ny,cur.cnt+1));
    }
  }
  return cnt;
    
}void solve(){
  cnt=0;

  cout<<bfs()+1<<endl;
}
int main(){
  while(cin>>w>>h&&(w||h)){
    //    for(int i=0;i<h;i++)for(int j=0;j<w;j++)map[i][j]='.';
    for(int i=0;i<h;i++){
      cin>>map[i];
      for(int j=0;j<w;j++){
	if(map[i][j]=='@'){
	  sx=j;sy=i;
	}
      }
    }
    solve();
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int ans=0;
vector<vector<int>> search(int x,int y,vector<vector<int>> umi){
    umi[y][x]=-1;
    ans++;
    
    if(umi[y+1][x]!=-1){
        umi=search(x,y+1,umi);
    }
    if(umi[y][x-1]!=-1){
        umi=search(x-1,y,umi);
    }
    if(umi[y][x+1]!=-1){
        umi=search(x+1,y,umi);
    }
    if(umi[y-1][x]!=-1){
        umi=search(x,y-1,umi);
    }
    return umi;
}


int main(void){
    int h,w;
    char c;
    int x;
    int y;
    vector<vector<int>> umi;
    while(cin>>w>>h,h){
        vector<vector<int>> umi;
        vector<int> nul;
        for(int j=0;j<w+2;j++){
            nul.push_back(-1);
        }
        umi.push_back(nul);
        for(int i=0;i<h;i++){
            vector<int> tmp;
            tmp.push_back(-1);
            for(int j=0;j<w;j++){
                if(cin>>c,c=='@'){
                    tmp.push_back(1);
                    x=j+1;
                    y=i+1;
                }
                else if(c=='.'){
                    tmp.push_back(0);
                }
                else{
                    tmp.push_back(-1);
                }
            }
            tmp.push_back(-1);
            umi.push_back(tmp);
            getchar();
        }
        umi.push_back(nul);
        for(int i=0;i<h+2;i++){
            for(int j=0;j<w+2;j++){
                //cout<<umi[i][j]+1;
            }
            //cout<<endl;
        }
        search(x,y,umi);

        
        cout<<ans<<endl;
        ans=0;
    }
    
}

C++: 6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.