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

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";
}

[download]
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";
}

[download]
Caution: Contents may have been coded under pressure.

Replies are listed 'Best First'.
Re^4: Algorithm to fit photos into spaces on pages
by SmugX (Beadle) on Nov 16, 2005 at 17:07 UTC

    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";
}

    [download]

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

    Regards
    Neil

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