BinarySearch

using System;
using System.Collections.Generic;
using System.Linq;

namespace _11.BinarySearch
{
    class BinarySearch
    {
        static int BinaryIndexFinder(int[] array, int key)
        {
            int downBorder = 0;
            int upBorder = array.Length - 1;
            while (true)
            {
                int marker = (upBorder + downBorder) / 2;

                if (marker == downBorder)
                {
                    if (key == array[upBorder])
                    {
                        return upBorder;
                    }
                    else if (key == array[downBorder])
                    {
                        return downBorder;
                    }
                    else
                    {
                        break;
                    }
                }

                if (key > array[marker])
                {
                    downBorder = marker + 1;
                }
                else if (key < array[marker])
                {
                    upBorder = marker - 1;
                }
                else
                {
                    return marker;
                }
            }
            return -1;
        }

        static void Main(string[] args)
        {
            int[] array = {-10, -6, -3, -1, 1, 3, 5, 10, 12, 13, 18, 21};
            Array.Sort(array);
            Console.Write("Enter a number: ");
            int number = int.Parse(Console.ReadLine());
            int index = BinaryIndexFinder(array, number);

            if (index == -1)
            {
                Console.WriteLine("Number is not find in the array");
            }
            else
            {
                Console.WriteLine("Index of number: {0}", index);
            }
        }
    }
}