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Re^2: Algorithm to fit photos into spaces on pages

by SmugX (Beadle)
on Nov 16, 2005 at 12:21 UTC ( #508996=note: print w/ replies, xml ) Need Help??


in reply to Re: Algorithm to fit photos into spaces on pages
in thread Algorithm to fit photos into spaces on pages

Thanks for your reply, Roy.

This is a really good solution, and I like it a lot. It's also _really_ fast.

However, I was feeding a couple of "real life" books through it, and curiously, it does seem a little "keen" to use the same layouts consecutively.

If I feed in a book of "llpllplppllpplplpplpplpllplplp" with $desired_page_count = 13, your regexp routine returns a book featuring layouts "12,01,07,06,02,08,02,03,04,04,03,03,03" - valid, certainly, but the same layouts repeated at the end.

However, my sloooooow routine returns "01,04,03,05,01,02,08,11,02,03,01,07,03", without repeating layouts.

I assume (guess alert!) it's because the regexp routine is happy to proceed along a certain route as long as solutions exist - not necessarily "good" (by my definition!) solutions. Also, as you get nearer the end of the book, there are fewer valid layout combinations.

Does anyone have any suggestions for tweaking this behaviour?

Thanks again,
Neil.


Comment on Re^2: Algorithm to fit photos into spaces on pages
Re^3: Algorithm to fit photos into spaces on pages
by Roy Johnson (Monsignor) on Nov 16, 2005 at 14:15 UTC
    Couldn't think how to do this yesterday, but it came to me this morning. Generate a regex for the whole thing at once, with the insurance that there be no two-in-a-row. The code actually gets simpler and faster, but on some very long random layouts, it does get hung up (meaning it's doing a lot of backtracking and probably not going to find a solution).

    Update: The reason it's prone not to find a solution is that the no-two-in-a-row check is more strict than it needs to be. It will not allow 'pl' to be used if the next two chars are 'pl', even if you have a 'pll' layout available. The fix for this is to use an assertion ((??{ })) to generate each match as "any available pattern except the previous match". The 2nd code block below implements that.

    my $desired_page_count = 10; # Structure holding all the different page layouts. ('l' = Landscape, +'p' = Portrait) my $pages = { ll => '01', pp => '02', lp => '03', pl => '04', lpp => '05', pll => '06', plp => '07', lpl => '08', lll => '09', ppp => '10', ppl => '11', llp => '12', ppll => '13', llpp => '14', }; my $photos = "llppllpplpppllplpplpplpllplpll"; # Make a really long layout -- at much over 100, it tends to hang up $photos .= substr 'lp', rand 2 for 1..100; $desired_page_count = int( length($photos)/ 2.7 ); my $page_regex = join '|', keys %$pages; my $regex = "($page_regex)"; $regex .= "(?!\\$_)($page_regex)" for 1..($desired_page_count - 1); print "\$regex is $regex\n"; if (my @pages = $photos =~ /^$regex$/) { # Added the anchors as per S +mugX's reply print "$pages->{$_}\n" for @pages; } else { print "Could not come up with no-repeat layout in $desired_page_cou +nt pages\n"; }
    use warnings; use strict; my $desired_page_count = 10; # Structure holding all the different page layouts. ('l' = Landscape, +'p' = Portrait) my $pages = { ll => '01', pp => '02', lp => '03', pl => '04', lpp => '05', pll => '06', plp => '07', lpl => '08', lll => '09', ppp => '10', ppl => '11', llp => '12', ppll => '13', llpp => '14', }; # Construct a hash of regexen: each value is all the patterns except f +or the # key. This way, they don't need to be constructed repeatedly during t +he matching my @patterns = keys %$pages; my %all_except = map {my $k=$_; ($k => join '|', grep {$_ ne $k} @patt +erns)} @patterns; my $photos = "llppllpplpppllplpplpplpllplpll"; # Make a really long layout $photos .= substr 'lp', rand 2 for 1..100; $desired_page_count = int( length($photos)/ 2.7 ); my $page_regex = join '|', @patterns; my $regex = "($page_regex)"; $regex .= "((??{\$all_except{\$$_}}))" for 1..($desired_page_count - 1 +); print "\$regex is $regex\n"; use re 'eval'; if (my @pages = $photos =~ /$regex/) { print "$pages->{$_}\n" for @pages; } else { print "Could not come up with no-repeat layout in $desired_page_cou +nt pages\n"; }

    Caution: Contents may have been coded under pressure.

      Excellent!

      I was just playing around with your first lot of code when you posted your second update! :-) This works really well, and I can definitely take this idea and run with it.

      I noticed that it seemed to have a tendency to use the earlier layouts - most books seemed to often use layouts 1-6, but rarely the later layouts.

      I assumed this was because the layouts were always being presented to the regex engine in the same order. Indeed, if I tweak your second example to randomise the order, it seems to use a more general spread of layouts, with the (interesting?) side effect of generating a different valid book each time.

      Then I noticed that in your second example at least, the photos aren't necessarily all used. I assumed I can prepend a '^' and append a '$' to the regexp, and this seems to resolve the issue.

      I'm hence left with:

      use warnings; use strict; my $desired_page_count = 10; # Structure holding all the different page layouts. ('l' = Landscape, +'p' = Portrait) my $pages = { ll => '01', pp => '02', lp => '03', pl => '04', lpp => '05', pll => '06', plp => '07', lpl => '08', lll => '09', ppp => '10', ppl => '11', llp => '12', ppll => '13', llpp => '14', }; # Construct a hash of regexen: each value is all the patterns except f +or the # key. This way, they don't need to be constructed repeatedly during t +he matching my @patterns = keys %$pages; my %all_except = map {my $k=$_; ($k => join '|', sort {rand(1) <=> ran +d(1)} grep {$_ ne $k} @patterns)} @patterns; my $photos = "llppllpplpppllplpplpplpllplpll"; # Make a really long layout $photos .= substr 'lp', rand 2 for 1..50; $desired_page_count = int( length($photos)/ 2.7 ); my $page_regex = join '|', sort {rand(1) <=> rand(1)} @patterns; my $regex = "^($page_regex)"; $regex .= "((??{\$all_except{\$$_}}))" for 1..($desired_page_count - 1 +); $regex .= "\$"; print "\$regex is $regex\n"; use re 'eval'; if (my @pages = $photos =~ /$regex/) { print "$pages->{$_}\n" for @pages; } else { print "Could not come up with no-repeat layout in $desired_page_cou +nt pages\n"; }

      Thanks for all your help, Roy. ++ your posts.

      Regards
      Neil

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