using System;
using System.Diagnostics;

namespace ConsoleApplication1 {

  internal class Program {

    /// <summary>Looks for the next occurrence of a sequence in a byte array</summary>
    /// <param name="array">Array that will be scanned</param>
    /// <param name="start">Index in the array at which scanning will begin</param>
    /// <param name="sequence">Sequence the array will be scanned for</param>
    /// <returns>
    ///   The index of the next occurrence of the sequence of -1 if not found
    /// </returns>
    private static int findSequence(byte[] array, int start, byte[] sequence) {
      int end = array.Length - sequence.Length; // past here no match is possible
      byte firstByte = sequence[0]; // cached to tell compiler there's no aliasing

      while(start <= end) {
        // scan for first byte only. compiler-friendly.
        if(array[start] == firstByte) {
          // scan for rest of sequence
          for (int offset = 1;; ++offset) {
            if(offset == sequence.Length) { // full sequence matched?
              return start;
            } else if(array[start + offset] != sequence[offset]) {
              break;
            }
          }
        }
        ++start;
      }

      // end of array reached without match
      return -1;
    }

    private static void checkSingleByteSequence() {
      byte[] sequence = new byte[] { 123 };

      Debug.Assert(findSequence(new byte[] { 123 }, 0, sequence) == 0);
      Debug.Assert(findSequence(new byte[] { 100 }, 0, sequence) == -1);

      Debug.Assert(findSequence(new byte[] { 123, 124, 125 }, 0, sequence) == 0);
      Debug.Assert(findSequence(new byte[] { 122, 123, 124 }, 0, sequence) == 1);
      Debug.Assert(findSequence(new byte[] { 121, 122, 123 }, 0, sequence) == 2);

      Debug.Assert(findSequence(new byte[] { 123, 124, 125, 126 }, 1, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 122, 123, 124, 125 }, 2, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 123, 124, 123, 124 }, 1, sequence) == 2);
      Debug.Assert(findSequence(new byte[] { 120, 121, 122, 123 }, 3, sequence) == 3);
      Debug.Assert(findSequence(new byte[] { 121, 122, 123, 124 }, 3, sequence) == -1);
    }

    private static void checkThreeByteSequence() {
      byte[] sequence = new byte[] { 150, 150, 160 };

      Debug.Assert(findSequence(new byte[] { 100 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 200 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 110 }, 1, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 110, 120 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 110, 120, 130 }, 3, sequence) == -1);

      Debug.Assert(findSequence(new byte[] { 150 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 150 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 150, 150 }, 1, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 100, 150, 150, 150 }, 0, sequence) == -1);
      Debug.Assert(findSequence(new byte[] { 150, 150, 120, 150 }, 3, sequence) == -1);

      Debug.Assert(findSequence(new byte[] { 150, 150, 160, 170 }, 0, sequence) == 0);
      Debug.Assert(findSequence(new byte[] { 150, 150, 160, 170 }, 1, sequence) == -1);

      Debug.Assert(findSequence(new byte[] { 140, 150, 150, 160 }, 0, sequence) == 1);
      Debug.Assert(findSequence(new byte[] { 140, 150, 150, 160 }, 1, sequence) == 1);
      Debug.Assert(findSequence(new byte[] { 140, 150, 150, 160 }, 2, sequence) == -1);
    }

    public static void Main(string[] args) {
      checkSingleByteSequence();
      checkThreeByteSequence();
    }

  }
  
}