knuth morris pratt algorithm c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace ClassLibrary1
{
    public class KnutMorrisPratt : ISubstringSearch {
        int[] PrefixFunctionFast(string str)
        {
            int[] result = new int[str.Length];
            result[0] = 0;
            for (int i = 1; i < str.Length; i++)
            {
                int k = result[i - 1];
                while (k > 0 && str[k] != str[i])
                {
                    k = result[k - 1];
                }
                if (str[k] == str[i])
                {
                    k++;
                }
                result[i] = k;
            }
            return result;
        }
        int[] PrefixFunction(string str) {
            int n = str.Length;
            int[] p = new int[n];
            for (int i = 0; i < n; i++)
            {
                for (int k = 0; k < i; k++)
                {
                    bool fl = true;
                    for (int j = 0; j < k + 1; j++)
                    {
                        if (str[j] != str[i - k + j])
                        {
                            fl = false;
                            break;
                        }
                    }
                    if (fl)
                        p[i] = (byte)(k + 1);
                }
            }
            return p;
        }
        public List<int> FindAll(string text, string substring) {
            List<int> res = new List<int>();
            int[] prefix_sub = PrefixFunctionFast(substring);
            int k = 0;
            for(int i = 0; i < text.Length; i++)
            {
                while (k > 0 && substring[k] != text[i]) { k = prefix_sub[k - 1]; }
                if (substring[k] == text[i]) k++;
                if (k == substring.Length)
                {
                    res.Add(i - k + 1);
                    k = prefix_sub[k - 1];
                }
            }
            return res;
        }
    }
}