Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

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])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include "bits/stdc++.h"
#include<unordered_map>
#pragma warning(disable:4996)
using namespace std;

const int dx[4] = { -1,0,1,0 };
const int dy[4] = { 0,1,0,-1 };

struct aa {
	int y;
	int x;
	bool black;
};

int main() {
	while (1) {
		int w, h; cin >> w >> h;
		if (!w)break;
		vector<vector<int>>field(h + 2, vector<int>(w + 2, 0));
		int sx, sy;
		for (int i = 0; i < h; ++i) {
			string st; cin >> st;
			for (int j = 0; j < w; ++j) {
				if (st[j] == '.') {
					field[i + 1][j + 1] = 1;
				}
				else {
					if (st[j] == '#') {
						field[i + 1][j + 1] = 2;
					}
					else {
						sx = j + 1;
						sy = i + 1;
						field[i + 1][j + 1] = 1;
					}
				}
			}
		}
		vector<aa>que;
		que.push_back(aa{ sy, sx, true });
		int ans = 1;
		vector<vector<bool>>used(h + 2, vector<bool>(w + 2, false));
		used[sy][sx] = true;

		while (!que.empty()) {
			aa atop(que.back());
			que.pop_back();
			for (int i = 0; i < 4; ++i) {
				const int ny = atop.y + dy[i];
				const int nx = atop.x + dx[i];
				if (atop.black) {
					if (field[ny][nx] == 1) {
						if (!used[ny][nx]) {
							used[ny][nx] = true;
							ans++;
							que.push_back(aa{ ny,nx,!atop.black });
						}
					}
				}
				else {
					if (field[ny][nx] == 1) {
						if (!used[ny][nx]) {
							used[ny][nx] = true;
							ans++;
							que.push_back(aa{ ny,nx,!atop.black });
						}
					}
				}

			}
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <vector>

struct MapHeader {
	int w;
	int h;
};

MapHeader read_header() {
	std::string line;
	std::getline(std::cin, line);
	std::istringstream line_stream(line);
	MapHeader result;
	line_stream >> result.w >> result.h;
	return result;
}

struct SearchContext {
	MapHeader header;
	std::vector<bool> map;
	std::vector<bool> reached_map;

	SearchContext(MapHeader header)
		: header(header),
		reached_map(header.w * header.h)
	{
		map.reserve(header.w * header.h);
	}

	size_t index(int x, int y) {
		return (size_t)header.w * y + x;
	}
};

void search(SearchContext& context, int x, int y) {
	if (x < 0 || x >= context.header.w || y < 0 || y >= context.header.h) return;

	size_t index = context.index(x, y);
	if (context.reached_map[index] || !context.map[index]) return;

	context.reached_map[index] = true;
	
	search(context, x + 1, y);
	search(context, x - 1, y);
	search(context, x, y + 1);
	search(context, x, y - 1);
}

int main()
{
	while (true) {
		MapHeader header = read_header();
		if (header.w == 0 && header.h == 0) break;

		SearchContext context(header);
		int start_x, start_y;

		for (int y = 0; y < header.h; y++) {
			std::string line;
			std::getline(std::cin, line);

			for (int x = 0; x < header.w; x++) {
				switch (line[x]) {
				case '.':
					context.map.push_back(true);
					break;
				case '#':
					context.map.push_back(false);
					break;
				case '@':
					start_x = x;
					start_y = y;
					context.map.push_back(true);
					break;
				}
			}
		}

		search(context, start_x, start_y);

		size_t count = 0;
		for (bool b : context.reached_map) {
			if (b) count++;
		}
		
		std::cout << count << std::endl;
	}

    return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>

using namespace std;

int h,w;
string g[25];

int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};

int dfs(int x, int y){
  g[y][x]='#';
  int ans=0;
  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 && g[ny][nx]!='#'){
      ans+=dfs(nx,ny);
    }
  }
  return ans+1;
}

int main(void){

  while(cin >> w >> h, h|w){
    for(int i=0;i<h;i++)cin >> g[i];
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	if(g[i][j]=='@')cout << dfs(j,i) << endl;
      }
    }
  }
  
  return 0;
}

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

typedef  vector< vector<int> > vector2d;

int main() {
  
  while(1){
    int H,W,count=0,kaisuu=0,kosuu,sum=1,judge;
    cin >>W>>H;
    if (W==0&&H==0)break;
    char map[W][H];
    vector2d nextx,nexty; //nextx[回数][個数]
    nextx.reserve(1000);
    nexty.reserve(1000);
    
    //マップの入力
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cin >> map[cW][cH];
	  if (map[cW][cH]=='@'){
	    nextx[0].push_back(cW);
	    nexty[0].push_back(cH);
	  }
	}
    }
    
    while(1){
      judge = 0;
      for(kosuu=0;kosuu<nextx[kaisuu].size();kosuu++){
	if (nextx[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の左側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]-1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]=='.'){  //@の上側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]-1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nextx[kaisuu][kosuu]+1!=W){
	  if(map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の右側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]+1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]+1!=H){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]=='.'){  //@の下側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]+1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]='@';
	    sum++;
	    judge++;
	  }
	}
      }
      if (judge==0)break;
      kaisuu++;
    }
    
    cout << sum <<endl;
    
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cout << map[cW][cH];
	}
	cout << endl;
    }
  }
  
  return 0;
  
}