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>
using namespace std;

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

int dfs(int y, int x, vector<string> &g){
  if(g[y][x] == '#')return 0;
  g[y][x] = '#';

  int res = 1;
  for(int i=0;i<4;i++){
    int ny = y + dy[i], nx = x + dx[i];
    if(ny<0 || nx<0 || ny>=(int)g.size() || nx>=(int)g[0].size())continue;
    res += dfs(ny,nx,g);
  }
  return res;
}

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

    vector<string> g(h);
    int y=0,x=0;
    for(int i=0;i<h;i++){
      cin >> g[i];
      for(int j=0;j<w;j++){
	if(g[i][j] == '@')y = i, x = j;
      }
    }

    cout << dfs(y,x,g) << endl;
  }
}

C++: #include <iostream>
using namespace std;

int main()
{
	int w, h;
	while (true) {
		cin >> w >> h;
		if (w == 0 && h == 0) {
			break;
		}
		unsigned next = 1U;
		char** room = new char*[h];
		unsigned** group = new unsigned*[h];
		int x, y;
		for (int i = 0; i < h; i++) {
			room[i] = new char[w];
			group[i] = new unsigned[w];
			for (int j = 0; j < w; j++) {
				cin >> room[i][j];
				if (room[i][j] == '@') {
					x = j;
					y = i;
				}
				group[i][j] = 0U;
			}
		}

		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
							}
							if (group[i][j] == 0U) {
								group[i][j] |= next;
								next <<= 1;
							}
						}
					}
				}
			}
		}
		for (int i = h - 1; i >= 0; i--) {
			for (int j = w - 1; j >=0; j--) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		for (int j = 0; j < w; j++) {
			for (int i = 0; i < h; i++) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		for (int j = w - 1; j >= 0; j--) {
			for (int i = h - 1; i >= 0; i--) {
				if (room[i][j] != '#') {
					for (int a = i - 1; a <= i + 1; a++) {
						for (int b = j - 1; b <= j + 1; b++) {
							if (a < 0 || b < 0 || a >= h || b >= w) {
								continue;
							}
							if ((a == i && b != j) || (a != i && b == j)) {
								group[i][j] |= group[a][b];
								if (group[i][j] == 0U) {
									group[i][j] |= next;
									next <<= 1;
								}
							}
						}
					}
				}
			}
		}
		int count = 0;
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				if (group[i][j] & group[y][x]) {
					count++;
				}
			}
		}

		cout << count << '\n';

		for (int i = 0; i < h; i++) {
			delete[] room[i];
			delete[] group[i];
		}
		delete[] room;
		delete[] group;
	}

	return 0;
}

C++: #include <iostream>
using namespace std;

int W, H;
char b[20][20];
int ans;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

void dfs(int x, int y) {
	ans++;
	b[y][x] = '#';
	for (int i = 0; i < 4; i++) {
		int nx = x + dx[i], ny = y + dy[i];
		if (0 <= nx && nx < W && 0 <= ny && ny < H && b[ny][nx] == '.') {
			dfs(nx, ny);
		}
	}
}

int main() {
	while (cin >> W >> H , W) {
		for (int y = 0; y < H; y++) {
			for (int x = 0; x < W; x++) {
				cin >> b[y][x];
			}
		}

		for (int y = 0; y < H; y++) {
			for (int x = 0; x < W; x++) {
				if (b[y][x] == '@') {
					ans = 0;
					dfs(x, y);
				}
			}
		}

		cout << ans << endl;
	}
}