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 <string>
#include <vector>

using namespace std;

const int dm[] = {-1, 0, 1, 0}, dn[] = {0, -1, 0, 1};
int h, w, count;
vector< vector<int> > field;

bool valid(int m, int n) {
    return 0 <= m && m < h && 0 <= n && n < w && field[m][n] == 1;
}

void dfs(int m, int n) {
    field[m][n] = 0;
    count++;

    for(int i = 0; i < 4; i++) {
        if(valid(m + dm[i], n + dn[i])) {
            dfs(m + dm[i], n + dn[i]);
        }
    }
}

int main() {
    while(1) {
        cin >> w >> h;
        if (w == 0 && h == 0) {
            break;
        }

        field.resize(h);
        int m, n;
        for (int i = 0; i < h; i++) {
            string input;
            cin >> input;

            field[i].resize(w);

            for (int j = 0; j < w; j++) {
                if(input[j] == '@') {
                    m = i;
                    n = j;
                }
                if(input[j] == '#') {
                    field[i][j] = 0;
                } else {
                    field[i][j] = 1;
                }
            }
        }

        count = 0;
        dfs(m, n);

        cout << count << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
int H,W;
char map[21][21];
int dist[21][21];
using namespace std;

void solve(){
  int nx,ny,sx,sy;
  const int dx[] = {1,0,-1,0};
  const int dy[] = {0,1,0,-1};
  for(int height = 0; height < H; height++){
    for(int width = 0; width < W ; width++){
      dist[height][width] = 0;
      if(map[height][width] == '@'){
        sx = width;
        sy = height;
      }
    }
  }

  dist[sy][sx] = 1;
  for(int d = 1; d < H * W; d++){
    for(int height = 0; height < H; height++){
      for(int width = 0; width < W ; width++){
        if(d == dist[height][width]){
          for(int dir = 0 ; dir < 4; dir++){
            nx = width + dx[dir];
            ny = height + dy[dir];
            if(nx >= 0 && nx < W && ny >= 0 && ny < H &&
               map[ny][nx] != '#' && dist[ny][nx] == 0){

              dist[ny][nx] = dist[height][width] + 1;

            }
          }
        }
      }
    }
  }
  int res = 0;
  for(int height = 0; height < H; height++){
    for(int width = 0; width < W ; width++){
      //cout << dist[height][width] ;
      if(dist[height][width] != 0) res++;
    }
    //cout << endl;
  }
  cout << res << endl;
}

int main(){
  while(true){
    cin >> W >> H ;
    if(W + H == 0) break;
    for(int height = 0; height < H; height++){
      for(int width = 0; width < W ; width++){
        cin >> map[height][width];
      }
    }
    solve();
  }
  return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
bool c[20][20] = {} ;
int cnt,w, h, p[20][20] = {};

int main() {
	int j, f,cx,cy;
	char in[20];
	while (1) {
		for (int i = 0; i < 400; i++) {
			c[i / 20][i % 20] = p[i / 20][i % 20] = 0;
		}
		cnt = 0;
		f = 1;
		scanf("%d %d", &w, &h);
		if (w ==0&& h == 0)
			break;
		scanf("\n", &j);
		for (int i = 0; i < h; i++) {
			scanf("%s", &in);
			for (j = 0; j < w; j++) {
				switch (in[j]) {
				case '#':
					c[j][i] = 0;
					p[j][i] = -1;
					break;
				case '@':
					cx = j;
					cy = i;
					p[j][i] = 1;
				case '.':
					c[j][i] = 1;
				}
			}
		}

		/*for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++)
				printf("%3d", c[j][i]);
			printf("\n");
		}*/

		while (f == 1) {
			f = 0;
			if (w != 1 && h != 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h][i%h] == 0 && (p[i / h + 1][i % h] == 1 || p[i / h - 1][i % h] == 1 || p[i / h][i % h + 1] == 1 || p[i / h][i % h - 1] == 1)) {
						p[i / h][i%h] = 1;
						f = 1;
					}
				}
			}
			if (w = 1 && h != 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h][i%h] == 0 && (p[i / h][i % h + 1] == 1 || p[i / h][i % h - 1] == 1)) {
						p[i / h][i%h] = 1;
						f = 1;
					}
				}
			}
			if (w != 1 && h = 1) {
				for (int i = 0; i < w*h; i++) {
					if (p[i / h][i%h] == 0 && (p[i / h + 1][i % h] == 1 || p[i / h - 1][i % h] == 1)) {
						p[i / h][i%h] = 1;
						f = 1;
					}
				}
			}


			/*for (int i = 0; i < h; i++) {
				for (int j = 0; j < w; j++)
					printf("%3d", p[j][i]);
				printf("\n");
			}
			printf("\n");*/
		}

		for (int i = 0; i < w*h; i++) {
			if (p[i / h][i%h] == 1)
				cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}