Knuth–Morris–Pratt Algorithm

Knuth–Morris–Pratt (a.k.a KMP Algorithm) is a string search algorithm. it searches for occurrences of a sub-string within a main-string by employing the observation that when a mismatch occurs, the word itself embodies sufficient information to determine where the next match could begin, thus bypassing re-examination of previously matched characters.

• C#
• VB.Net
• C
• C++
• PHP

									public static int[] SearchString(string str, string pat)
{
	List<int> retVal = new List<int>();
	int M = pat.Length;
	int N = str.Length;
	int i = 0;
	int j = 0;
	int[] lps = new int[M];

	ComputeLPSArray(pat, M, lps);

	while (i < N)
	{
		if (pat[j] == str[i])
		{
			j++;
			i++;
		}

		if (j == M)
		{
			retVal.Add(i - j);
			j = lps[j - 1];
		}

		else if (i < N && pat[j] != str[i])
		{
			if (j != 0)
				j = lps[j - 1];
			else
				i = i + 1;
		}
	}

	return retVal.ToArray();
}

private static void ComputeLPSArray(string pat, int m, int[] lps)
{
	int len = 0;
	int i = 1;

	lps[0] = 0;

	while (i < m)
	{
		if (pat[i] == pat[len])
		{
			len++;
			lps[i] = len;
			i++;
		}
		else
		{
			if (len != 0)
			{
				len = lps[len - 1];
			}
			else
			{
				lps[i] = 0;
				i++;
			}
		}
	}
}
								

Example

									string data = "the quick brown fox jumps over the lazy dog";
int[] value = SearchString(data, "the");
								

Output

									0
31