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From: shirriff@sprite.berkeley.edu (Ken Shirriff)
Newsgroups: sci.electronics,rec.video
Subject: VCR+: algorithm for first 100
Message-ID: <1991May14.074739.3646@agate.berkeley.edu>
Date: 14 May 91 07:47:39 GMT
Sender: root@agate.berkeley.edu (Charlie Root)
Distribution: usa
Organization: University of California, Berkeley
Lines: 84

I've determined the algorithm for the first 100 codes.  I wrote a C program
to decode a code from 1 to 99.  To use this, run "code 42 3" to determine
what happens with code 42 in March.

The first 100 codes correspond to channels 1-4 recording 30 minutes on
a fixed set of 8 times for the 1st to the 3rd of the month.

The first 1000 codes extend the same times and channels for the rest of
the month, but I haven't decoded that yet.

Let me know if you find any errors with this program.
Also, if there's a VCR+ mailing list, can someone add me to it?

/*
 * Copyright 1991 Ken Shirriff   shirriff@sprite.Berkeley.EDU
 */

char *tn[8] = { "6:30", "4", "7:30", "4:30", "3:30", "5:30", "6", "2:30"};

main(argc,argv)
int argc;
char **argv;
{
    int num, month;
    int result, day;
    int time, chan;
    if (argc != 3) {
	printf("usage: code num, month\n");
	exit(-1);
    }
    num = atoi(argv[1]);
    month = atoi(argv[2]);
    code(num, &result, &day);
    result = (result+day*month)&31;
    time = ((result&16)>>2) | ((result&4)>>1) | (result&1);
    chan = (((result&8)>>2) | ((result&2)>>1))+1;
    printf("Code %d in month %d = %s, ch %d on day %d\n", num, month,
	    tn[time], chan, day);
}

code(num,result,day)
int num, *result, *day;
{
    int row, col, rem, div, d;
    int res;

    /*
     * Break up into 7 rows of 5 columns on 3 days.
     * The numbers are broken mod 29 and then broken in half again unevenly.
     */
    rem = num%29;
    div = num/29;
    if (rem<16-div) {
	row = 3-div;
    } else {
	row = 6-div;
	rem -= 13;
    }
    col = 4-(rem-1)/3;
    d = ((rem-1)%3)+1;

    /*
     * The numbers are then assigned consecutively down the columns.
     */
    res = col*7+row;

    /*
     * Swap numbers 1-9 with row 3
     */
    if (num<=9) {
	res = num;
	d = 1;
    } else if (d==1 && res<=9) {
	col = 4-(res-1)/3;
	d = ((res-1)%3)+1;
	row = 3;
	res = col*7+row;
    }

    *day = d;
    *result = res;
}

Ken Shirriff			shirriff@sprite.Berkeley.EDU