rabin karp substring search algo O(m+n)

namespace RabinKarpSubstringSearch
{
    using System;

    class Program
    {

        private const int Prime = 31;

        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            string pattern = "foo";
            int patternHash = CalcHash(pattern);

            int pos = IsMatch("hello _foobar", patternHash, pattern.Length);
            if (pos != 1)
            {
                Console.WriteLine("Tadaaa! " + pos);
            }

            Console.ReadLine();
        }

        private static int CalcHash(string pattern)
        {
            int hashValue = 0;
            int len = pattern.Length;
            //for (int i = 0; i < len; i++)
            //{
            //    hashValue += Convert.ToInt32(pattern[i]) * (int)Math.Pow(Prime, i);
            //}
            int i = 0;
            while (i < len)
            {
                hashValue += Convert.ToInt32(pattern[i]) * (int)Math.Pow(Prime, i);
                i++;
            }
            return hashValue;
        }

        private static int IsMatch(string text, int patternHash, int lenPattern)
        {
            int i = 0;
            // compute 1st hash from 0 to lenghth of pattern -1
            int textHash = 0;
            while (i < lenPattern)
            {
                textHash += Convert.ToInt32(text[i]) * (int)Math.Pow(Prime, i);
                i++;
            }

            // check match found at position 0
            if (patternHash == textHash)
            {
                return 0;
            }

            // compute and comparing the the hashes
            int len = text.Length;
            while (i < len)
            {
                int k = i;

                // note: this is quite easy to understand
                // https://www.youtube.com/watch?v=H4VrKHVG5qI&t=324s
                textHash = (textHash - Convert.ToInt32(text[i - lenPattern])) / Prime + Convert.ToInt32(text[i]) * (int)Math.Pow(Prime, lenPattern - 1);
                if (patternHash == textHash)
                {
                    // return the index. k is index of last letter in pattern so return back length of pattern - 1 (-1 is that we are already in the last letter of the pattern)
                    return k - lenPattern - 1;
                }
                i++;
            }

            return -1;
        }
    }
}

// explanation: https://www.youtube.com/watch?v=H4VrKHVG5qI&t=324s