Hi

I'm trying to create an iterator which takes a list of subs and creates the cross product of the returned lists:

the expected result is

for my $x ( $sub_x->() ) {
    for my $y ( $sub_y->() ) {
         return [ $x,$y ];
    }
}
[download]

Please note that an iterator can be interrupted and resumed, and that the numbers of subs (=nesting levels) should be free.

Since function calls are expensive in Perl I tried to implement it as a big while loop instead of recursive calls.

Now I'm wondering if there is a more elegant or faster way to code this:

use 5.12.0;

use strict;
use warnings;
use Data::Dump qw/pp dd/;


my $DBG;

;;; # skipped

$DBG = 0;
my $i_cross = cross(
                    sub {"A".."B"},
                    sub {1..2},
                    sub {"a".."b"},
                   );


my $count = 4;

while ( my ($v) = $i_cross->() ) {
    warn  "*** First 5 RESULTs: ", pp $v;
    last unless $count--;
}

warn "stopped";

while ( my ($v) = $i_cross->() ) {
    warn  "*** Remaining RESULT: ", pp $v;
}


sub cross {
    my @stack  = @_;
    my $level  = 0;
    my @ranges = ();
    my @res;

    return sub {

        while ( $level >= 0 ) {

            my $ra_range = \ $ranges[$level];

            # ENTER LOOP
            unless (defined $$ra_range ) {
                $$ra_range = [ $stack[$level]->() ];
                warn "ENTER level=$level left \@ranges=" ,pp \@ranges 
+ if $DBG;
            }

            # LOOP OVER
            if ( @$$ra_range  ) {
                $res[$level] = shift @$$ra_range ;

                # deepest level?
                if ( $level == $#stack ) {
                    return [@res];
                }
                # go deeper
                else {
                    $level++;
                }

            }

            #EXIT LOOP:
            else {
                $level--;
                $$ra_range = undef;
            }

        }
        return;
    }
}
[download]

RESULT

*** First 5 RESULTs: ["A", 1, "a"] at d:/exp/iter/cross.pl line 92.
*** First 5 RESULTs: ["A", 1, "b"] at d:/exp/iter/cross.pl line 92.
*** First 5 RESULTs: ["A", 2, "a"] at d:/exp/iter/cross.pl line 92.
*** First 5 RESULTs: ["A", 2, "b"] at d:/exp/iter/cross.pl line 92.
*** First 5 RESULTs: ["B", 1, "a"] at d:/exp/iter/cross.pl line 92.
stopped at d:/exp/iter/cross.pl line 96.
*** Remaining RESULT: ["B", 1, "b"] at d:/exp/iter/cross.pl line 99.
*** Remaining RESULT: ["B", 2, "a"] at d:/exp/iter/cross.pl line 99.
*** Remaining RESULT: ["B", 2, "b"] at d:/exp/iter/cross.pl line 99.
[download]

(it's part of a bigger file, so don't worry about the line numbers)

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