Created
July 15, 2019 21:43
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// Following program is a Java implementation | |
// of Rabin Karp Algorithm given in the CLRS book | |
public class Main | |
{ | |
// d is the number of characters in the input alphabet | |
public final static int d = 256; | |
/* pat -> pattern | |
txt -> text | |
q -> A prime number | |
*/ | |
static void search(String pat, String txt, int q) | |
{ | |
int M = pat.length(); | |
int N = txt.length(); | |
int i, j; | |
int p = 0; // hash value for pattern | |
int t = 0; // hash value for txt | |
int h = 1; | |
// The value of h would be "pow(d, M-1)%q" | |
for (i = 0; i < M-1; i++) | |
h = (h*d)%q; | |
// Calculate the hash value of pattern and first | |
// window of text | |
for (i = 0; i < M; i++) | |
{ | |
p = (d*p + pat.charAt(i))%q; | |
t = (d*t + txt.charAt(i))%q; | |
} | |
// Slide the pattern over text one by one | |
for (i = 0; i <= N - M; i++) | |
{ | |
// Check the hash values of current window of text | |
// and pattern. If the hash values match then only | |
// check for characters on by one | |
if ( p == t ) | |
{ | |
/* Check for characters one by one */ | |
for (j = 0; j < M; j++) | |
{ | |
if (txt.charAt(i+j) != pat.charAt(j)) | |
break; | |
} | |
// if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1] | |
if (j == M) | |
System.out.println("Pattern found at index " + i); | |
} | |
// Calculate hash value for next window of text: Remove | |
// leading digit, add trailing digit | |
if ( i < N-M ) | |
{ | |
t = (d*(t - txt.charAt(i)*h) + txt.charAt(i+M))%q; | |
// We might get negative value of t, converting it | |
// to positive | |
if (t < 0) | |
t = (t + q); | |
} | |
} | |
} | |
/* Driver program to test above function */ | |
public static void main(String[] args) | |
{ | |
String txt = "GEEKS FOR GEEKS"; | |
String pat = "GEEK"; | |
int q = 101; // A prime number | |
search(pat, txt, q); | |
} | |
} | |
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