package List::Merger;
@ISA         = qw(Exporter);
@EXPORT      = qw();
@EXPORT_OK   = qw(gen_merger);

use strict;
use warnings;
use List::Util 'first';

our $VERSION = '0.01';

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, $item[$_]);
            $next = $item[$_] if $result == 1;
        }
        $item[$idx] = $fetch->($list->[$idx]);
        $done = 1 if ! first {$item[$_] ne $finish} $idx .. $#item;
        return $next;
    };
}