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.

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)
{
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;

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");

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