I massaged the code a bit to work on the boundary cases, and added a reverse (autoincrement) output to test the input. Unfortunately it seems to crap out on punctuation now.

Here is the output:

--"120" gives "119" reverse "120"
--"abc" gives "abb" reverse "abc"
--"testbaA" gives "testazZ" reverse "testbaA"
--"a" gives "" reverse "1"
--"aaa" gives "zz" reverse "aaa"
--"a01" gives "a00" reverse "a01"
--"aa00" gives "z99" reverse "aa00"


Here is the code:

use strict;
use warnings;

sub predchar {
# modify argument in place.
# return true if no carry (done).
    my $ord= ord($_[0]);
    my $nocarry;
    # define the ranges available
    my @ranges= ([ord('a'),ord('z')], [ord('A'),ord('Z')] );
    foreach my $range (@ranges) {
        my ($first,$last)= @$range;
        next unless $ord >= $first && $ord <= $last;  # my range?
        if ($ord == $first) { $ord= $last; }
        else {
            $nocarry = 1;
            --$ord;
        }
    }
    $_[0]= chr($ord);
    return $nocarry;
}

sub magic_decrement($)
 {
     my $instr = $_;
     my $numpart = $instr;
     my $alphapart = $instr;
     $numpart =~ s/[A-Za-z]+//;
     $alphapart =~ s/[0-9]+//;
     my $numlen = length($numpart);
     $numpart--;
     if($numpart == -1) {
        $numpart = '9' x $numlen;
        my @chars= split ('', $alphapart);
        my @retarr = @chars[0 .. $#chars];
        for (my $loop= $#chars;  $loop>=0;  --$loop) {
            last if predchar ($retarr[$loop]);
        }
        $alphapart = join ('',@retarr);
        $alphapart =~ s/z$// if($alphapart =~ /^z+$/);
     } else {
        $numpart = '0' . $numpart if($numlen != length($numpart));
     }
     return $alphapart . $numpart;
 }

sub test
 {
 my $s= shift || $_;
 my $result= magic_decrement($s);
 my $reverse = $result;
 $reverse++;
 print qq{--"$s" gives "$result" reverse "$reverse"\n};
 }

while (<DATA>) { chomp; test; }

__DATA__
120
abc
testbaA
a
aaa
a01
aa00

[download]

In reply to Re: Magical Auto-Decrement by Anonymous Monk
in thread Magical Auto-Decrement by John M. Dlugosz

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