CRC16CCITTChecksumGenerator.java

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package project.latex.balloon.writer;

/**
 * ***********************************************************************
 * Compilation: javac CRC16CCITT.java Execution: java CRC16CCITT s Dependencies:
 *
 * Reads in a sequence of bytes and prints out its 16 bit Cylcic Redundancy
 * Check (CRC-CCIIT 0xFFFF).
 *
 * 1 + x + x^5 + x^12 + x^16 is irreducible polynomial.
 *
 * % java CRC16-CCITT 123456789 CRC16-CCITT = 29b1
 * 
 * Courtesy of http://introcs.cs.princeton.edu/java/51data/CRC16CCITT.java.html
 *
 ************************************************************************
 */
public class CRC16CCITTChecksumGenerator implements ChecksumGenerator {

    @Override
    public String generateChecksum(String input) {
        int crc = 0xFFFF;          // initial value
        int polynomial = 0x1021;   // 0001 0000 0010 0001  (0, 5, 12) 

        byte[] bytes = input.getBytes();

        for (byte b : bytes) {
            for (int i = 0; i < 8; i++) {
                boolean bit = ((b >> (7 - i) & 1) == 1);
                boolean c15 = ((crc >> 15 & 1) == 1);
                crc <<= 1;
                if (c15 ^ bit) {
                    crc ^= polynomial;
                }
            }
        }
        crc &= 0xffff;
        return Integer.toHexString(crc);
    }

}