Here's an iterative solution, similar in spirit to the algorithm discussed in HOP, flip, and swap:
use strict;
use warnings;
my $base = 4;
my $lvl = shift || 2;
my @indices = indices( $lvl );
print "$_\n" for @indices;
sub indices {
my $lvl = shift;
die if $lvl < 0;
my $first = 'A1';
my @index = map $first, 1..$lvl;
my @ret;
my $n = 1;
{
push @ret, join '', @index;
my $p = $n;
my $j = $#index;
$index[ $j--] = $first, $p /= $base until $p % $base;
last if $j < 0;
$index[ $j ] =
chr( ord( $index[ $j ] ) + 1 ) . substr( $index[ $j ], 1 );
++$n;
redo;
}
return @ret;
}
But it's much easier to do this with globs:
my $glob_string = ( '{' . join( ',', 'A'..'D' ) . '}1' ) x $level;
my @indices;
while ( < $glob_string > ) { push @indices, $_ }
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