ODS2: Does not calculate interleave correctly

[al20878]

The formula in phyvirt.c for the maplbn() function is wrong:

    if( s >= dp->sectors / dp->interleave )
        s = (s * dp->interleave) - (dp->sectors -1);  /* <<< BUG HERE! */
    else
        s = s * dp->interleave;

    s = (s + (dp->skew * c)) % dp->sectors;

First, since the last line shown in the snippet above operates with the reminders modulo dp->sectors, then subtracting dp->sectors -1 on the line marked "BUG HERE" is the same as just adding 1. Thus, the formula on that line can be simplified as:

s = (s * dp->interleave) +1;

That, however, is a problem for dp->sectors not being a multiple of dp->interleave (given, DEC did not use those values in practice, so it may not "trigger" the bug, but still). An example: if sectors:=10 and interleave:=3, the mapping sequence of sectors, which the code calculates, yields:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9 -> 0, 3, 6, 0, 3, 6, 9, 2, 5, 8

The problem is quite obvious -- there cannot be any repeated sectors! Instead, the correct output sector sequence in these conditions would be:

0, 3, 6, 9, 2, 5, 8, 1, 4, 7

The correct interleave formula can be found in the lbn2pbn tool, which was added recently to the simtools/converters.