in reply to Re^10: Challenge: Number of unique ways to reach target sum
in thread Challenge: Number of unique ways to reach target sum

Nope, that code is only memoized for generation, the output stage is unmemoized and hence very slow.

The semi-memoized output code is in my followup. Here's the whole thing joined up and ready to run.

use List::Util 'min'; use POSIX 'ceil'; use strict; use warnings; sub make_ways { my ($N, $S, $T) = @_; # one coin left can we do it? if ($S == 1) { if ($T <= $N) { return ["$T"]; } else { return 0; } } my $min = (2*$T-$S+$S**2)/(2*$S); ## more correctly, ceil() of thi +s my $max = min( $T-(($S-1)*$S/2), $N ); my @all_ways; for my $K ($min .. $max) { my $ways = make_ways( $K-1, $S-1, $T-$K ); if ($ways) { push(@all_ways, ["$K", $ways]); } } if (@all_ways) { return \@all_ways; } else { return 0 } } use Memoize; memoize 'make_ways'; my $ways = make_ways(100, 10, 667); my $printed = 0; nasty_print_ways($ways, "", 6); print "$printed\n"; # do my own memoizing as I couldn't get Memoize to work, for this func +tion # possibly to do with the arguments being array refs my %strings; sub string_ways { my $ways = $_[0]; if (my $strings = $strings{$ways}) { #die "already $strings"; return $strings } my @strings; for my $way (@$ways) { if (ref $way) { my ($coin, $more_ways) = @$way; push(@strings, map {"$_+$coin"} (@{string_ways($more_ways)})); } else { push(@strings, $way); } } return $strings{$ways} = \@strings; } sub nasty_print_ways { my ($ways, $base, $depth) = @_; if ($depth == 0) { my $strings = string_ways($ways); for my $string (@$strings) { $printed++; #print STDERR "$printed\n" unless ($printed % 10000000); #print "$string+$base\n"; } } else { $depth--; for my $way (@$ways) { if (ref $way) { my ($coin, $more_ways) = @$way; my $new_base = length($base) ? "$coin+$base" : $coin; nasty_print_ways($more_ways, $new_base, $depth); } else { # print "$base+$way\n"; } } } }

The reason I wanted you to run blokheads quick code was to be able to get a quick comparison of my machine and your machine. It's not important.

I ran your C and it took 2 or 3 hours. I'm running it again with your optimisations, although that fomit-pointer (or whatever it was) make my gcc complain.

By the way, your code printed out -114xxx. That's because j is a double (a kind of float) and you format is "%d" but %d is for int. I've changed j to unsigned long long int and printf("%lld", j). I'm waiting for it to complete.

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Re^12: Challenge: Number of unique ways to reach target sum
by Limbic~Region (Chancellor) on Feb 16, 2006 at 18:14 UTC
    fergal,
    I had a copy/paste error which was the source of a whole lot of problems originally. I left j as type double and updated the printf() format as ".0f" but your way works too. I also had a missing hyphen from -fomit-frame-pointer which I corrected.

    I have run this code on my machine and it finished in 53.5 minutes. I am not really shocked by this but I am impressed. The only optimizations the C code attempts to make is by limiting the number of the 17 trillion different sets it visits. Adding in a cache and some other neat doo-dads could probably blow this 53.5 minutes out of the water - but why bother. That is one of the beauties of Perl. The amount of time it would take to devise a C program to beat the Perl is more than time than it takes the Perl to run. Good Job.

    Cheers - L~R

      Yeah, I thought about rewriting in C, maybe using glib to get nice hashes and strings. Memory usage would be way smaller and it would be way faster but it wouldn't be much fun to write :)

      I just got

      fergal@anacreon:~$ time ./lr 14479062751 real 76m17.183s user 75m32.075s sys 0m0.764s fergal@anacreon:~$ time perl make_ways.pl 14479062752 real 37m46.569s user 37m26.124s sys 0m1.416s

      That's without -march=... and -fomit-doodads.