BoyerMoore.java


Below is the syntax highlighted version of BoyerMoore.java from §5.3 Substring Search.


/******************************************************************************
 *  Compilation:  javac BoyerMoore.java
 *  Execution:    java BoyerMoore pattern text
 *  Dependencies: StdOut.java
 *
 *  Reads in two strings, the pattern and the input text, and
 *  searches for the pattern in the input text using the
 *  bad-character rule part of the Boyer-Moore algorithm.
 *  (does not implement the strong good suffix rule)
 *
 *  % java BoyerMoore abracadabra abacadabrabracabracadabrabrabracad
 *  text:    abacadabrabracabracadabrabrabracad 
 *  pattern:               abracadabra
 *
 *  % java BoyerMoore rab abacadabrabracabracadabrabrabracad
 *  text:    abacadabrabracabracadabrabrabracad 
 *  pattern:         rab
 *
 *  % java BoyerMoore bcara abacadabrabracabracadabrabrabracad
 *  text:    abacadabrabracabracadabrabrabracad 
 *  pattern:                                   bcara
 *
 *  % java BoyerMoore rabrabracad abacadabrabracabracadabrabrabracad
 *  text:    abacadabrabracabracadabrabrabracad
 *  pattern:                        rabrabracad
 *
 *  % java BoyerMoore abacad abacadabrabracabracadabrabrabracad
 *  text:    abacadabrabracabracadabrabrabracad
 *  pattern: abacad
 *
 ******************************************************************************/

/**
 *  The <tt>BoyerMoore</tt> class finds the first occurrence of a pattern string
 *  in a text string.
 *  <p>
 *  This implementation uses the Boyer-Moore algorithm (with the bad-character
 *  rule, but not the strong good suffix rule).
 *  <p>
 *  For additional documentation,
 *  see <a href="http://algs4.cs.princeton.edu/53substring">Section 5.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 */
public class BoyerMoore {
    private final int R;     // the radix
    private int[] right;     // the bad-character skip array

    private char[] pattern;  // store the pattern as a character array
    private String pat;      // or as a string

    /**
     * Preprocesses the pattern string.
     *
     * @param pat the pattern string
     */
    public BoyerMoore(String pat) {
        this.R = 256;
        this.pat = pat;

        // position of rightmost occurrence of c in the pattern
        right = new int[R];
        for (int c = 0; c < R; c++)
            right[c] = -1;
        for (int j = 0; j < pat.length(); j++)
            right[pat.charAt(j)] = j;
    }

    /**
     * Preprocesses the pattern string.
     *
     * @param pattern the pattern string
     * @param R the alphabet size
     */
    public BoyerMoore(char[] pattern, int R) {
        this.R = R;
        this.pattern = new char[pattern.length];
        for (int j = 0; j < pattern.length; j++)
            this.pattern[j] = pattern[j];

        // position of rightmost occurrence of c in the pattern
        right = new int[R];
        for (int c = 0; c < R; c++)
            right[c] = -1;
        for (int j = 0; j < pattern.length; j++)
            right[pattern[j]] = j;
    }

    /**
     * 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 M = pat.length();
        int N = txt.length();
        int skip;
        for (int i = 0; i <= N - M; i += skip) {
            skip = 0;
            for (int j = M-1; j >= 0; j--) {
                if (pat.charAt(j) != txt.charAt(i+j)) {
                    skip = Math.max(1, j - right[txt.charAt(i+j)]);
                    break;
                }
            }
            if (skip == 0) return i;    // found
        }
        return N;                       // not found
    }


    /**
     * Returns the index of the first occurrrence of the pattern string
     * in the text string.
     *
     * @param  text 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(char[] text) {
        int M = pattern.length;
        int N = text.length;
        int skip;
        for (int i = 0; i <= N - M; i += skip) {
            skip = 0;
            for (int j = M-1; j >= 0; j--) {
                if (pattern[j] != text[i+j]) {
                    skip = Math.max(1, j - right[text[i+j]]);
                    break;
                }
            }
            if (skip == 0) return i;    // found
        }
        return N;                       // not found
    }


    /**
     * 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.
     */
    public static void main(String[] args) {
        String pat = args[0];
        String txt = args[1];
        char[] pattern = pat.toCharArray();
        char[] text    = txt.toCharArray();

        BoyerMoore boyermoore1 = new BoyerMoore(pat);
        BoyerMoore boyermoore2 = new BoyerMoore(pattern, 256);
        int offset1 = boyermoore1.search(txt);
        int offset2 = boyermoore2.search(text);

        // print results
        StdOut.println("text:    " + txt);

        StdOut.print("pattern: ");
        for (int i = 0; i < offset1; i++)
            StdOut.print(" ");
        StdOut.println(pat);

        StdOut.print("pattern: ");
        for (int i = 0; i < offset2; i++)
            StdOut.print(" ");
        StdOut.println(pat);
    }
}


Copyright © 2002–2015, Robert Sedgewick and Kevin Wayne.
Last updated: Sat Aug 29 11:16:30 EDT 2015.