IsSubstring

IsSubstring

        class SubstringRecognition
        {
            readonly string s;
            int state = 0;
            int pos = 0;

            Func<char, SubstringRecognition>[] accept;

            public bool Found
            {
                get
                {
                    return pos >= s.Length;
                }
            }

            public SubstringRecognition(string s)
            {
                this.s = s;

                accept = new[] { (Func<char, SubstringRecognition>)_Accept0, _Accept1 };
            }

            SubstringRecognition _Accept0(char c)
            {
                if (c == s[0])
                {
                    state = 1;
                    pos++;
                }

                return this;
            }

            SubstringRecognition _Accept1(char c)
            {
                if (pos >= s.Length)
                    return this;
                

                if (c != s[pos])
                {
                    state = 0;
                    pos = 0;

                    return _Accept0(c);
                }
                else
                    pos++;
                
                return this;
            }

            public SubstringRecognition Accept(char c)
            {
                return accept[state](c);
            }
        }


        static class Utils
        {
          static bool IsSubstring(this string s, string t)
          {
              if (s == null || t == null || (s.Length > t.Length))
                  return false;

              var recog = new SubstringRecognition(s);

              for (var i = 0; i < t.Length; i++)
                  recog = recog.Accept(t[i]);

              return recog.Found;

          }
        }