CDDB.java

Below is the syntax highlighted version of CDDB.java from §6.1 Data Representations.

/******************************************************************************
 *  Compilation:  javac CDDB.java
 *  Execution:    java CDDB
 *
 *  % java CDDB
 *  970ada0c
 *
 *  Note: Pearl Jam's album Vs. has N = 12 tracks. The first track
 *  starts at frames[0] =  150, the second at frames[1] = 14672,
 *  the twelfth at frames[11] = 185792, and the disc ends at
 *  frames[N] = 208500. Its disc id is 970ADA0C.
 *
 *  The disc id is a 32-bit integer, which we represent using 8
 *  hex digits XXYYYYZZ.
 *
 *     - XX is the checksum. The checksum is computed as follows:
 *       for each starting frame[i], we convert it to seconds by
 *       dividing by the frame rate 75; then we sum up the decimal
 *       digits. E.g., if frame[i] = 7500600, this corresponds to
 *       100008 seconds whose digit sum is 1 + 8 = 9.
 *       XX is the total sum of all of these digit sums mod 255.
 *     - YYYY is the length of the album tracks in seconds. It is
 *       computed as (frames[N] - frames[0]) / 75 and output in hex.
 *     - ZZ is the number of tracks N expressed in hex.
 *
 ******************************************************************************/

public class CDDB {

    // return sum of decimal digits in n
    public static int sumOfDigits(int n) {
        int sum = 0;
        while (n > 0) {
            sum = sum + (n % 10);
            n = n / 10;
        }
        return sum;
    }



    public static void main(String[] args) {
        int FRAMES_PER_SECOND = 75;
        int[] frames = {
            150, 14672, 27367, 45030, 60545, 76707, 103645,
            116430, 137730, 156887, 171577, 185792, 208500
        };

        int n = frames.length - 1;   // 12

        // a somewhat unusual definition, but consistent with freedb
        int totalLength = frames[n] / FRAMES_PER_SECOND - frames[0] / FRAMES_PER_SECOND;

        // note the integer division before summing up the digits
        int checkSum = 0;
        for (int i = 0; i < n; i++)
            checkSum += sumOfDigits(frames[i] / FRAMES_PER_SECOND);

        int xx = checkSum % 255;
        int yyyy = totalLength;
        int zz = n;

        // xxyyyyzz
        int discID =  ((xx << 24) | (yyyy << 8) | zz);
        StdOut.println(Integer.toHexString(discID));
    }

}