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 <queue>
#include <string>
#include <utility>

using namespace std;

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

int main(){
    int w, h;
    while(cin >> w >> h && w > 0 && h > 0){
        string tmp;
        pair<int, int> init;
        vector<vector <int> > grid(25, vector<int>(25, -1));
        vector<vector <int> > visited(25, vector<int>(25, 0));
        for(int i = 0; i < h; i++){
            cin >> tmp;
            for (int j = 0; j < w; j++){
                if(tmp[j] == '.'){
                    grid[i][j] = 0;
                }else if(tmp[j] == '#'){
                    grid[i][j] = -1;
                }else if(tmp[j] == '@'){
                    grid[i][j] = 0;
                    init = make_pair(i, j);
                }
            }
        }
        queue<pair<int, int> > Q;
        Q.push(init);
        visited[init.first][init.second] = 1; 
        while(!Q.empty()){
            pair<int, int> cur = Q.front(); 
            Q.pop();
            int cur_x = cur.first;
            int cur_y = cur.second;
            for(int i = 0; i < 4; i++){
                pair<int, int> dst = make_pair(cur_x + dx[i], cur_y + dy[i]);
                int x = dst.first;
                int y = dst.second;
                if(0 <= x && 0 <= y && grid[x][y] == 0 && visited[x][y] == 0){
                    Q.push(dst);
                    visited[x][y] = 1;
                }
            }
        }
        int ans = 0;
        for(int i = 0; i < h; i++){
            for (int j = 0; j < w; j++){
                ans += visited[i][j];
            }
        }
        cout << ans << endl;
    }
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <functional>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <deque>
#include <ctime>

using namespace std;

#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define pb push_back
#define mp make_pair
#define all(r) (r).begin(),(r).end()
#define rall(r) (r).rbegin(),(r).rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int INF = 1e9;
const ll LINF = 1e18;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;
const double PI = acos(-1);

int dx[]={-1,1,0,0};
int dy[]={0,0,-1,1};
int w,h;
int sx,sy;
int a[30][30];
string str[30];
queue<int> q;

int main(){
	while(cin >> w >> h){
		if(w == 0 && h == 0){
			break;
		}

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				a[i][j] = 0;
			}
		}

		for(int i=0; i<h; i++){
			cin >> str[i];
		}

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(str[i][j] == '@'){
					sx = j;
					sy = i;
				}
			}
		}
		q.push(sy*30+sx);
		a[sy][sx] = 1;

		while(!q.empty()){
			int tmp = q.front();
			q.pop();

			for(int i=0; i<4; i++){
				int nx = tmp%30 + dx[i];
				int ny = tmp/30 + dy[i];

				if(ny <0 || nx <0 || ny >= h || nx >= w || str[ny][nx] == '#' || a[ny][nx] == 1){
					continue;
				}

				q.push(ny*30+nx);

				a[ny][nx] = 1;
			}
		}

		int count=0;

		for(int i=0; i<h; i++){
			for(int j=0; j<w; j++){
				if(a[i][j] == 1){
					count++;
				}
			}
		}

		cout << count << endl;
	}


}

C++: #include "bits/stdc++.h"

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back

string fie[30]={};
int w,h;
int c;
bool visited[30][30];

void dfs(int y,int x){
    visited[y][x]=true;
    c++;
    int dx[]={1,0,-1,0};
    int dy[]={0,1,0,-1};
    rep(i,4){
        if(0<=x+dx[i] && x+dx[i]<w  &&  0<=y+dy[i] && y+dy[i]<h  &&  fie[y+dy[i]][x+dx[i]]=='.'  &&  visited[y+dy[i]][x+dx[i]]==false){
            dfs(y+dy[i],x+dx[i]);
        }
    }
}


int main(){
    while(cin>>w>>h){
        c=0;
        if(w==0 && h==0)break;
        rep(i,30)rep(j,30)visited[i][j]=false;
        int sy,sx;
        rep(i,h){
            cin>>fie[i];
        }
        rep(i,h)rep(j,w){
            if(fie[i][j]=='@') sy=i,sx=j;
        }
        
        dfs(sy,sx);
             
        cout<<c<<endl;
    }
}