From: Mischa POSLAWSKY <perl@shiar.org>
Date: Sat, 14 Nov 2009 22:08:45 +0000 (+0100)
Subject: XXX: lookahead penalty
X-Git-Url: http://git.shiar.nl/perl/list-index.git/commitdiff_plain/2af0622fdc633320a7172164ddb9c7436e41998f

XXX: lookahead penalty
---

diff --git a/lib/List/Index.pm b/lib/List/Index.pm
index 339bd7d..4ce9c25 100644
--- a/lib/List/Index.pm
+++ b/lib/List/Index.pm
@@ -24,43 +24,52 @@ sub ranges {
 
 	$pagesize = @$self / $pages;
 	my $offset = $pagesize + .5;
-	my $penalty = 0;
+	my $shrunk = 0;
+	my $enlarged = 0;
 	my @links = ('');
 	while ($offset < @$self) {
 		my $link = substr $self->[$offset], 0, $length;
 		if ($context) {
-			if ($offset > $context - 1 + $penalty) {
-				# take a value slightly before the current offset
-				my $before = $self->[$offset - $context - 1 + $penalty];
+			my $penalty = 0;
+			# take a value slightly before the current offset
+			if ((my $before = $offset - $context + $shrunk) > 0) {
 				# see how much of it matches the current link
 				my $trim = 1;
-				for my $match (split //, $before) {
+				for my $match (split //, $self->[$before - 1]) {
 					scalar $link =~ /\G\Q$match/g or last;
 					$trim++;
 				}
 				# truncate link upto where the earlier value starts to differ
-				substr($link, $trim) = '' unless $trim > length $link;
+				if ($trim < length $link) {
+					substr($link, $trim) = '';
+					for (reverse $before .. $offset) {
+						$self->[$offset - $penalty] =~ /^\Q$link/ or last;
+						$penalty++;
+					}
+				}
 			}
 
-			$penalty = 0;
-			if ($offset + $context < $#$self) {
-				# take a value after the current offset
-				my $after = $self->[$offset + $context];
+			$shrunk = 0;
+			# take a value after the current offset
+			if ((my $after = $offset + $context - $enlarged) < $#$self) {
 				# see how much of it matches the current link
 				my $trim = 1;
-				for my $match (split //, $after) {
+				for my $match (split //, $self->[$after]) {
 					scalar $link =~ /\G\Q$match/g or last;
 					$trim++;
 				}
 				# use this link if it's shorter
 				if ($trim < length $link) {
-					$link = substr $after, 0, $trim;
+					$link = substr $self->[$after], 0, $trim;
+					# advance lookbehind offset on the next page
+					$penalty = 0;
 					for ($offset .. $#$self) {
-						last if $self->[$offset + $penalty] =~ /^\Q$link/;
-						$penalty++;
+						last if $self->[$offset + $shrunk] =~ /^\Q$link/;
+						$shrunk++;
 					}
 				}
 			}
+			$enlarged = $penalty;
 		}
 
 		push @links, $link;
diff --git a/t/10-ranges.t b/t/10-ranges.t
index cf3b0a4..9cfce11 100644
--- a/t/10-ranges.t
+++ b/t/10-ranges.t
@@ -37,7 +37,7 @@ subtest 'context' => sub {
 	my $index = List::Index->new([qw(
 		kkeg kl km kmlu knsy    koxb kpeo kuaa kuab kuac
 		kuapa kuq kur kux kzb   lc lg lgu lgua lguc
-		lguq lgur lgus lgx lka  lkq lks lln llq llx
+		lguq lgur lgws lgx lka  lkq lks lln llq llx
 	)]) or return;
 	is_deeply(
 		$index->ranges({ pagesize=>10, context=>0, length=>5 }),
@@ -109,13 +109,12 @@ subtest 'distribution' => sub {
 		hnvtvpievbdlkrmb hs hvdvcqn hvn hyrybeur iaiaab ib ibavqyar idfniqvxpohbk idh
 	)]) or return;
 	is_deeply(
-		$index->ranges({ pagesize=>10, context=>6 }),
+		$index->ranges({ pagesize=>10, context=>8 }),
 		[qw(-g h i-)],
 		'large context'
 	);
-{ local $TODO = '?';
 	is_deeply(
-		$index->ranges({ pagesize=>10, context=>5 }),
+		$index->ranges({ pagesize=>10, context=>7 }),
 		# after 2nd page is enlarged by lookbehind to 'h', limit subsequent lookahead
 		# to prevent the page from getting too large (17 entries if forwarded to 'i')
 		[qw(-g h-hm hn-)],
@@ -124,7 +123,6 @@ subtest 'distribution' => sub {
 	# page #14 [gn-g] (8): gnihka gniub go gsearnrqns gtdvcxyt gwawkvmueovdjtfj gwoufolwcvmtueyg gysgphci
 	# page #15 [h] (17): h habkdgifjfxoh hbbvjf hbqleexnqts hccgszftbaymfu hdaqzkow hdoeqwdmgqwaoya hfbegicieu hfmlpzzioqjbthz hj hkoysmws hmylu hnvtvpievbdlkrmb hsodfpkatk hvdvcqn hvn hyrybeurqtevjfmi
 	# page #16 [i-ie] (5): i iaab ibiavqyar idfniqvxpohbk idh
-}
 };
 
 subtest 'context' => sub {