118 lines
3.2 KiB
Java
118 lines
3.2 KiB
Java
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/**
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* Java Program to Implement Rolling Hash
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**/
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import java.io.BufferedReader;
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import java.io.InputStreamReader;
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import java.io.IOException;
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import java.util.Random;
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import java.math.BigInteger;
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class RabinKarp
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{
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/** String Pattern **/
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private String pat;
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/** pattern hash value **/
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private long patHash;
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/** pattern length **/
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private int M;
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/** Large prime **/
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private long Q;
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/** radix **/
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private int R;
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/** R^(M-1) % Q **/
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private long RM;
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/** Constructor **/
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public RabinKarp(String txt, String pat)
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{
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this.pat = pat;
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R = 256;
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M = pat.length();
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Q = longRandomPrime();
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/** precompute R^(M-1) % Q for use in removing leading digit **/
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RM = 1;
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for (int i = 1; i <= M-1; i++)
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RM = (R * RM) % Q;
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patHash = hash(pat, M);
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int pos = search(txt);
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if (pos == txt.length())
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System.out.println("\nNo Match\n");
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else
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System.out.println("Pattern found at : "+ pos);
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}
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/** Compute hash **/
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private long hash(String key, int M)
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{
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long h = 0;
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for (int j = 0; j < M; j++)
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h = (R * h + key.charAt(j)) % Q;
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return h;
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}
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/** Funtion check **/
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private boolean check(String txt, int i)
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{
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for (int j = 0; j < M; j++)
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if (pat.charAt(j) != txt.charAt(i + j))
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return false;
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return true;
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}
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/** Funtion to check for exact match**/
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private int search(String txt)
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{
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int N = txt.length();
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if (N < M) return N;
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long txtHash = hash(txt, M);
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/** check for match at start **/
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if ((patHash == txtHash) && check(txt, 0))
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return 0;
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/** check for hash match. if hash match then check for exact match**/
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for (int i = M; i < N; i++)
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{
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// Remove leading digit, add trailing digit, check for match.
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txtHash = (txtHash + Q - RM*txt.charAt(i - M) % Q) % Q;
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txtHash = (txtHash * R + txt.charAt(i)) % Q;
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// match
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int offset = i - M + 1;
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if ((patHash == txtHash) && check(txt, offset))
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return offset;
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}
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/** no match **/
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return N;
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}
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/** generate a random 31 bit prime **/
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private static long longRandomPrime()
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{
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BigInteger prime = BigInteger.probablePrime(31, new Random());
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return prime.longValue();
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}
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}
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/** Class RollingHashTest **/
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public class RollingHashTest
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{
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public static void main(String[] args) throws IOException
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{
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BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
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System.out.println("Rolling Hash Test\n");
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System.out.println("\nEnter Text\n");
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String text = br.readLine();
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System.out.println("\nEnter Pattern\n");
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String pattern = br.readLine();
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System.out.println("\nResults : \n");
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RabinKarp rk = new RabinKarp(text, pattern);
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}
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}
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/*
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Enter Text
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abcdefghijklmnopqrstuvwxyz
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Enter Pattern
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jklmn
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Results :
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Pattern found at : 9
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