#!/usr/bin/perl
use strict;
use warnings;
use List::Util 'first';

sub gen_merger {
    my ($list, $fetch, $compare, $finish) = @_;
    my @item = map $fetch->($_), @$list;
    
    my $done;
    return sub {
        return $finish if $done;        
        my $idx  = first {$item[$_] ne $finish} 0 .. $#item;
        my $next = $item[$idx];
        for ($idx + 1 .. $#item) {
            next if $item[$_] eq $finish;
            my $result = $compare->($next, $item[$_]);
            if ($result == 1) {
                $next = $item[$_];
                $idx = $_;
            }
        }
        $item[$idx] = $fetch->($list->[$idx]);
        $done = 1 if ! first {$_ ne $finish} @item;
        return $next;
    };
}

sub gen_sum_iter {
    my ($num, $list) = @_;
    my $idx = 0;
    return sub {
        return undef if $idx > $#$list;
        return $num + $list->[$idx++];
    };
};

my @list1 = (1 .. 5);
my @list2 = (1 .. 5);
my $finish = 'A val that is guaranteed not to be present in any list';
my @merge_list = map gen_sum_iter($_, \@list2), @list1; # Map over smaller list if possible
my $fetch = sub {my $sum = $_[0]->(); return defined $sum ? $sum : $finish;};
my $compare  = sub {$_[0] <=> $_[1]};
my $next = gen_merger(\@merge_list, $fetch, $compare, $finish);
while (1) {
    my $item = $next->();
    last if defined $item && $item eq $finish;
    print "$item\n";
}