/******************************************************************************
 *  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
 *
 ******************************************************************************/

public class CRC16CCITT {

    public static void main(String[] args) {
        int crc = 0xFFFF;          // initial value
        int polynomial = 0x1021;   // 0001 0000 0010 0001  (0, 5, 12)

        // byte[] testBytes = "123456789".getBytes("ASCII");

        byte[] bytes = args[0].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;
        StdOut.println("CRC16-CCITT = " + Integer.toHexString(crc));
    }

}