using System;

namespace Rextester{

    public class Program{

        public static void Main(string[] args)

        {

            Console.WriteLine(OverlappedStringLength("ab   .  cdefg", " efghijkl"));

        }

    public static int OverlappedStringLength(string s1, string s2) {

        //Trim s1 so it isn't longer than s2

        if (s1.Length > s2.Length) s1 = s1.Substring(s1.Length - s2.Length);

​

        int[] T = ComputeBackTrackTable(s2); //O(n)

​

        int m = 0;

        int i = 0;

        while (m + i < s1.Length) {

            if (s2[i] == s1[m + i]) {

                i += 1;

                //<-- removed the return case here, because |s1| <= |s2|

            } else {

                m += i - T[i];

                if (i > 0) i = T[i];

            }

        }

​

        return i; //<-- changed the return here to return characters matched

    }

​

    public static int[] ComputeBackTrackTable(string s) {

        var T = new int[s.Length];

        int cnd = 0;

        T[0] = -1;

        T[1] = 0;

        int pos = 2;

        while (pos < s.Length) {

            if (s[pos - 1] == s[cnd]) {

                T[pos] = cnd + 1;

                pos += 1;

                cnd += 1;

            } else if (cnd > 0) {

                cnd = T[cnd];

            } else {

                T[pos] = 0;

                pos += 1;

            }

        }

​

        return T;

    }

    }

}