edu.princeton.cs.algs4.RabinKarp Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac RabinKarp.java
* Execution: java RabinKarp pat txt
* Dependencies: StdOut.java
*
* Reads in two strings, the pattern and the input text, and
* searches for the pattern in the input text using the
* Las Vegas version of the Rabin-Karp algorithm.
*
* % java RabinKarp abracadabra abacadabrabracabracadabrabrabracad
* pattern: abracadabra
* text: abacadabrabracabracadabrabrabracad
* match: abracadabra
*
* % java RabinKarp rab abacadabrabracabracadabrabrabracad
* pattern: rab
* text: abacadabrabracabracadabrabrabracad
* match: rab
*
* % java RabinKarp bcara abacadabrabracabracadabrabrabracad
* pattern: bcara
* text: abacadabrabracabracadabrabrabracad
*
* % java RabinKarp rabrabracad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: rabrabracad
*
* % java RabinKarp abacad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: abacad
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.math.BigInteger;
import java.util.Random;
/**
* The {@code RabinKarp} class finds the first occurrence of a pattern string
* in a text string.
*
* This implementation uses the Rabin-Karp algorithm.
*
* For additional documentation,
* see Section 5.3 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*/
public class RabinKarp {
private String pat; // the pattern // needed only for Las Vegas
private long patHash; // pattern hash value
private int m; // pattern length
private long q; // a large prime, small enough to avoid long overflow
private int R; // radix
private long RM; // R^(M-1) % Q
/**
* Preprocesses the pattern string.
*
* @param pattern the pattern string
* @param R the alphabet size
*/
public RabinKarp(char[] pattern, int R) {
this.pat = String.valueOf(pattern);
this.R = R;
throw new UnsupportedOperationException("Operation not supported yet");
}
/**
* Preprocesses the pattern string.
*
* @param pat the pattern string
*/
public RabinKarp(String pat) {
this.pat = pat; // save pattern (needed only for Las Vegas)
R = 256;
m = pat.length();
q = longRandomPrime();
// precompute R^(m-1) % q for use in removing leading digit
RM = 1;
for (int i = 1; i <= m-1; i++)
RM = (R * RM) % q;
patHash = hash(pat, m);
}
// Compute hash for key[0..m-1].
private long hash(String key, int m) {
long h = 0;
for (int j = 0; j < m; j++)
h = (R * h + key.charAt(j)) % q;
return h;
}
// Las Vegas version: does pat[] match txt[i..i-m+1] ?
private boolean check(String txt, int i) {
for (int j = 0; j < m; j++)
if (pat.charAt(j) != txt.charAt(i + j))
return false;
return true;
}
// Monte Carlo version: always return true
// private boolean check(int i) {
// return true;
//}
/**
* Returns the index of the first occurrrence of the pattern string
* in the text string.
*
* @param txt the text string
* @return the index of the first occurrence of the pattern string
* in the text string; n if no such match
*/
public int search(String txt) {
int n = txt.length();
if (n < m) return n;
long txtHash = hash(txt, m);
// check for match at offset 0
if ((patHash == txtHash) && check(txt, 0))
return 0;
// check for hash match; if hash match, check for exact match
for (int i = m; i < n; i++) {
// Remove leading digit, add trailing digit, check for match.
txtHash = (txtHash + q - RM*txt.charAt(i-m) % q) % q;
txtHash = (txtHash*R + txt.charAt(i)) % q;
// match
int offset = i - m + 1;
if ((patHash == txtHash) && check(txt, offset))
return offset;
}
// no match
return n;
}
// a random 31-bit prime
private static long longRandomPrime() {
BigInteger prime = BigInteger.probablePrime(31, new Random());
return prime.longValue();
}
/**
* Takes a pattern string and an input string as command-line arguments;
* searches for the pattern string in the text string; and prints
* the first occurrence of the pattern string in the text string.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
String pat = args[0];
String txt = args[1];
RabinKarp searcher = new RabinKarp(pat);
int offset = searcher.search(txt);
// print results
StdOut.println("text: " + txt);
// from brute force search method 1
StdOut.print("pattern: ");
for (int i = 0; i < offset; i++)
StdOut.print(" ");
StdOut.println(pat);
}
}
/******************************************************************************
* Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/