X-Git-Url: http://git.shiar.nl/perl/list-index.git/blobdiff_plain/e5fa8cd7b5cc84943c5f7c32c4f34974613a235b..36f8fe8df9e85055f0feb31985fe1d07a1f74a33:/lib/List/Index.pm

diff --git a/lib/List/Index.pm b/lib/List/Index.pm
index 339bd7d..b11b5f8 100644
--- a/lib/List/Index.pm
+++ b/lib/List/Index.pm
@@ -24,43 +24,54 @@ sub ranges {
 
 	$pagesize = @$self / $pages;
 	my $offset = $pagesize + .5;
-	my $penalty = 0;
+	my $lookbehind = -$context;
+	my $lookahead  =  $context;
 	my @links = ('');
 	while ($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 + $lookbehind) > 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];
+			$lookbehind = -$context;
+
+			# take a value after the current offset
+			if ((my $after = $offset + $lookahead) < $#$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, $trim;
-					for ($offset .. $#$self) {
-						last if $self->[$offset + $penalty] =~ /^\Q$link/;
-						$penalty++;
+					$link = substr $self->[$after], 0, $trim;
+					# advance lookbehind offset on the next page
+					$penalty = 0;
+					for ($offset .. $after) {
+						last if $self->[$_] =~ /^\Q$link/;
+						$lookbehind++;
 					}
 				}
 			}
+
+			$lookahead = $context - $penalty;
 		}
 
 		push @links, $link;