FloodFill

using System;
using System.Diagnostics;
using System.Linq;

namespace FloodFill
{
    internal class Program
    {
        private const int MAP_SIZE = 1024; // only powers of two are supported

        private static void Main()
        {
            var map = new byte[MAP_SIZE * MAP_SIZE];

            var stopwatch = Stopwatch.StartNew();
            {
                FloodFill(map, startX: 123, startY: 123, fromValue: 0, toValue: 1);
            }
            stopwatch.Stop();

            Console.WriteLine(stopwatch.ElapsedMilliseconds);

            // Test results
            if (map.Any(item => item == 0))
            {
                Console.WriteLine("Error");
            }
            else
            {
                Console.WriteLine("Ok");
            }

            Console.ReadLine();
        }

        private static void FloodFill(byte[] map, int startX, int startY, byte fromValue, byte toValue)
        {
            int shift = (int) Math.Round(Math.Log(map.Length, 4)); // if the array's length is (2^x * 2^x), then shift = x
            int startIndex = startX + (startY << shift);

            if (map[startIndex] != fromValue)
            {
                return;
            }

            // initialize flood fill
            int size = 1 << shift;
            int sizeMinusOne = size - 1;
            int xMask = size - 1;
            int minIndexForVerticalCheck = size;
            int maxIndexForVerticalCheck = map.Length - size - 1;

            // initialize queue
            int capacity = size * 2;
            int mask = capacity - 1;
            uint tail = 0;
            uint head = 0;
            var queue = new int[capacity];

            map[startIndex] = toValue;
            queue[tail++ & mask] = startIndex;

            while (tail - head > 0)
            {
                int index = queue[head++ & mask];
                int x = index & xMask;

                if (x > 0 && map[index - 1] == fromValue)
                {
                    map[index - 1] = toValue;
                    queue[tail++ & mask] = index - 1;
                }
                if (x < sizeMinusOne && map[index + 1] == fromValue)
                {
                    map[index + 1] = toValue;
                    queue[tail++ & mask] = index + 1;
                }
                if (index >= minIndexForVerticalCheck && map[index - size] == fromValue)
                {
                    map[index - size] = toValue;
                    queue[tail++ & mask] = index - size;
                }
                if (index <= maxIndexForVerticalCheck && map[index + size] == fromValue)
                {
                    map[index + size] = toValue;
                    queue[tail++ & mask] = index + size;
                }
            }
        }
    }
}