85 lines
2.6 KiB
Java
85 lines
2.6 KiB
Java
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/**
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* @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
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*/
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import java.util.Scanner;
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import java.lang.Math;
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//An implementation of Rabin-Karp string matching algorithm
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//Program will simply end if there is no match
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public class RabinKarp {
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public static Scanner scanner = null;
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public final static int d = 256;
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public static void main(String[] args){
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scanner = new Scanner(System.in);
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System.out.println("Enter String");
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String text = scanner.nextLine();
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System.out.println("Enter pattern");
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String pattern = scanner.nextLine();
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int q = 101;
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searchPat(text,pattern,q);
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}
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private static void searchPat(String text, String pattern, int q) {
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int m = pattern.length();
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int n = text.length();
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int t = 0;
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int p = 0;
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int h = 1;
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int j = 0;
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int i = 0;
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h = (int)Math.pow(d,m-1)%q;
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for(i =0 ; i< m; i++){
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//hash value is calculated for each character and then added with the hash value of the next character for pattern
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// as well as the text for length equal to the length of pattern
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p = (d*p + pattern.charAt(i))%q;
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t = (d*t + text.charAt(i))%q;
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}
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for(i=0; i<=n-m;i++){
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//if the calculated hash value of the pattern and text matches then
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//all the characters of the pattern is matched with the text of length equal to length of the pattern
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//if all matches then pattern exist in string
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//if not then the hash value of the first character of the text is subtracted and hash value of the next character after the end
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//of the evaluated characters is added
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if(p==t){
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//if hash value matches then the individual characters are matched
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for(j=0;j<m;j++){
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//if not matched then break out of the loop
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if(text.charAt(i+j) != pattern.charAt(j))
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break;
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}
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//if all characters are matched then pattern exist in the string
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if (j==m){
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System.out.println("Pattern found at index " + i);
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}
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}
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//if i<n-m then hash value of the first character of the text is subtracted and hash value of the next character after the end
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//of the evaluated characters is added to get the hash value of the next window of characters in the text
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if ( i < n-m )
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{
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t = (d*(t - text.charAt(i)*h) + text.charAt(i+m))%q;
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//if hash value becomes less than zero than q is added to make it positive
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if (t < 0)
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t = (t + q);
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}
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}
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}
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}
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