xref: /plan9-contrib/acme/bin/source/acd/discid (revision 9a747e4fd48b9f4522c70c07e8f882a15030f964)

CDDB DISCID
-----------

Both forms of CDDB access require that the software compute a "disc
ID" which is an identifier that is used to access the CDDB.  The disc
ID is a 8-digit hexadecimal (base-16) number, computed using data from
a CD's Table-of-Contents (TOC) in MSF (Minute Second Frame) form.  The
algorithm is listed below in Appendix A.

It is crucial that your software compute the disc ID correctly.  If it
does not generate the correct disc ID, it will not be compatible with CDDB.
Moreover, if your software submits CDDB entries with bad disc IDs to the
CDDB archives, it could compromise the integrity of the CDDB.

[...]

APPENDIX A - CDDB DISCID ALGORITHM
----------------------------------

The following is a C code example that illustrates how to generate the
CDDB disc ID. [...] A text description
of the algorithm follows, which should contain the necessary information
to code the algorithm in any programming language.


struct toc {
        int     min;
        int     sec;
        int     frame;
};

struct toc cdtoc[100];

int
read_cdtoc_from_drive(void)
{
        /* Do whatever is appropriate to read the TOC of the CD
         * into the cdtoc[] structure array.
         */
        return (tot_trks);
}

int
cddb_sum(int n)
{
        int     ret;

        /* For backward compatibility this algorithm must not change */

        ret = 0;

        while (n > 0) {
                ret = ret + (n % 10);
                n = n / 10;
        }

        return (ret);
}

unsigned long
cddb_discid(int tot_trks)
{
        int     i,
                t = 0,
                n = 0;

        /* For backward compatibility this algorithm must not change */

        i = 0;

        while (i < tot_trks) {
                n = n + cddb_sum((cdtoc[i].min * 60) + cdtoc[i].sec);
                i++;
        }

        t = ((cdtoc[tot_trks].min * 60) + cdtoc[tot_trks].sec) -
            ((cdtoc[0].min * 60) + cdtoc[0].sec);

        return ((n % 0xff) << 24 | t << 8 | tot_trks);
}

main()
{
        int tot_trks;

        tot_trks = read_cdtoc_from_drive();
        printf("The discid is %08x", cddb_discid(tot_trks));
}


This code assumes that your compiler and architecture support 32-bit
integers.

The cddb_discid function computes the discid based on the CD's TOC data
in MSF form.  The frames are ignored for this purpose.  The function is
passed a parameter of tot_trks (which is the total number of tracks on
the CD), and returns the discid integer number.

It is assumed that cdtoc[] is an array of data structures (records)
containing the fields min, sec and frame, which are the minute, second
and frame offsets (the starting location) of each track.  This
information is read from the TOC of the CD.  There are actually
tot_trks + 1 "active" elements in the array, the last one being the
offset of the lead-out (also known as track 0xAA).

The function loops through each track in the TOC, and for each track
it takes the (M * 60) + S (total offset in seconds) of the track and
feeds it to cddb_sum() function, which simply adds the value of each digit
in the decimal string representation of the number. A running sum of this
result for each track is kept in the variable n.

At the end of the loop:
1. t is calculated by subtracting the (M * 60) + S offset of the lead-out
minus the (M * 60) + S offset of first track (yielding the length of
the disc in seconds).

2. The result of (n modulo FFh) is left-shifted by 24 bits.

3. t is left shifted by 8.

The bitwise-OR operation of result 2., 3. and the tot_trks number is
used as the discid.

The discid is represented in hexadecimal form for the purpose of
xmcd cddb file names and the DISCID= field in the xmcd cddb file itself.
If the hexadecimal string is less than 8 characters long, it is
zero-padded to 8 characters (i.e., 3a8f07 becomes 003a8f07).  All
alpha characters in the string should be in lower case, where
applicable.

Important note for clients using the MS-Windows MCI interface:

The Windows MCI interface does not provide the MSF location of the
lead-out.  Thus, you must compute the lead-out location by taking the
starting position of the last track and add the length of the last track
to it.  However, the MCI interface returns the length of the last track
as ONE FRAME SHORT of the actual length found in the CD's TOC.  In most
cases this does not affect the disc ID generated, because we truncate
the frame count when computing the disc ID anyway.  However, if the
lead-out track has an actual a frame count of 0, the computed quantity
(based on the MSF data returned from the MCI interface) would result in
the seconds being one short and the frame count be 74.  For example,
a CD with the last track at an offset of 48m 32s 12f and having a
track length of 2m 50s 63f has a lead-out offset of 51m 23s 0f. Windows
MCI incorrectly reports the length as 2m 50s 62f, which would yield a
lead-out offset of 51m 22s 74f, which causes the resulting truncated
disc length to be off by one second.  This will cause an incorrect disc
ID to be generated. You should thus add one frame to the length of the
last track when computing the location of the lead-out.

The easiest way for Windows clients to compute the lead-out given information
in MSF format is like this:

(offset_minutes * 60 * 75) + (offset_seconds * 75) + offset_frames +
(length_minutes * 60 * 75) + (length_seconds * 75) + length_frames + 1 = X

Where X is the offset of the lead-out in frames. To find the lead-out in
seconds, simply divide by 75 and discard the remainder.