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 <algorithm>
#include <cstdio>
#include <stack>

using namespace std;

typedef struct _TAG_POSITION {
	int x;
	int y;
} POSITION;

int main(){
	int W, H;
	stack<POSITION> S;
	POSITION p;
	while(cin >> W >> H, W || H){
		char board[H+2][W+2];
		int ret = 0;
		for(int i = 0; i < H+2; i++){
			for(int j = 0; j < W+2; j++){
				(((i == 0) || (i == H + 1)) || ((j == 0) || (j == W + 1))) && (board[i][j] = '#');
				(0 < i && i < H + 1) && (0 < j && j < W + 1) && cin >> board[i][j];
				if(board[i][j] == '@') { S.push({i,j}); ret++; }
			}
		}

		while(!S.empty()){
			p = S.top();
			S.pop();
			if(board[p.x + 1][p.y] == '.') { board[p.x + 1][p.y] = '@'; ret++; S.push({p.x + 1,p.y}); }
			if(board[p.x - 1][p.y] == '.') { board[p.x - 1][p.y] = '@'; ret++; S.push({p.x - 1,p.y}); }
			if(board[p.x][p.y + 1] == '.') { board[p.x][p.y + 1] = '@'; ret++; S.push({p.x,p.y + 1}); }
			if(board[p.x][p.y - 1] == '.') { board[p.x][p.y - 1] = '@'; ret++; S.push({p.x,p.y - 1}); }
		}
		cout << ret << "\n";
	}
	return 0;
}

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


int main(){
    std::vector<int> row;
    int prev_col, width, height, max = 0, last = 0;
    bool connect_prev_row;
    
    while(true){
        std::cin >> width >> height;
        if( width == 0 && height == 0 ) break;
        row.resize(width+1);
        for( int i = 0; i <= width; i++ ) row[i] = 0;

        std::string line;
        for( int h = 0; h < height; h++ ){
            std::cin >> line;
            connect_prev_row = false;
            for(std::string::size_type i = 0; i < line.size(); i++ ){
                if( line[i] == '.' ){
                    if( connect_prev_row ){
                        if( row[i] > 0 ) row[i+1] = row[i] + 1;
                        else row[i+1] = row[i] - 1;
                    } else {
                        connect_prev_row = true;
                        if( row[i] <= 0 && row[i+1] <= 0 ) row[i+1] = row[i+1] + row[i] - 1;
                        else row[i+1] = std::abs(row[i+1]) + std::abs(row[i]) + 1;
                    }
                } else if( line[i] == '@' ) {
                    if ( connect_prev_row ) row[i+1] = - row[i] + 1;
                    else {
                        connect_prev_row = true;
                        row[i+1] = - row[i+1] - row[i] + 1;
                    }
                } else {
                    row[i+1] = 0;
                    connect_prev_row = false;
                }
                max = max > row[i+1] ? max : row[i+1];
            }
            last = 0;
            for( int i = line.size(); i > 0; i-- ) {
                if( row[i] == 0 ) last = 0;
                else if( row[i] > 0 ){
                    if(  last == 0 ) last = row[i];
                    else row[i] = last;
                } else {
                    if(  last == 0 ) last = row[i];
                    else row[i] = last;
                } 
            }
        }
            
    }
    return 0;
}

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

using namespace std;
#define NINF -999999
typedef pair<int, int> P;
int field[21][21];
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};

void solve()
{
	int W, H;
	while(cin >> W >> H)
	{
		if(!W && !H)
		{
			break;
		}
		string tile[21];
		for(int i = 0; i < H; ++i)
		{
			cin >> tile[i];
		}
		queue<P> Que;
		for(int i = 0; i < H; ++i)
		{
			for(int j = 0; j < W; ++j)
			{
				if(tile[i][j] == '@')
				{
					P p;
					p.first = i;
					p.second = j;
					Que.push(p);
					field[i][j] = 0;
				}
				else
				{
					field[i][j] = NINF;
				}
			}
		}
		int count = 1;
		queue<P> NextQue;
		while(!Que.empty())
		{
			for(int i = 0; i < 4; ++i)
			{
				P p;
				p.first = Que.front().first + dy[i];
				p.second = Que.front().second + dx[i];
				
				if(p.first < 0 || p.first >= H || p.second < 0 || p.second >= W)
				{
					continue;
				}
				else if((tile[p.first][p.second] != '#') && (field[p.first][p.second] == NINF))
				{
					field[p.first][p.second] = field[Que.front().first][Que.front().second] + 1;
					NextQue.push(p);
					count++;
				}
			}
			Que.pop();
			if(Que.empty())
			{
				while(!NextQue.empty())
				{
					Que.push(NextQue.front());
					NextQue.pop();
				}
			}
		}
		cout << count << endl;
	}
}

int main()
{
	solve();
	return(0);
}