thread: use hash + array instead of hand-rolled linked list

This starts to show noticeable performance improvements when
attempting to thread over 400 messages; but the improvement
may not be measurable with less.

However, the resulting code is much shorter and (IMHO)
much easier to understand.

1 files changed, 17 insertions, 11 deletions

diff --git a/lib/PublicInbox/View.pm b/lib/PublicInbox/View.pm
index c09b4a22..d0c6d339 100644
--- a/lib/PublicInbox/View.pm
+++ b/lib/PublicInbox/View.pm
@@ -203,13 +203,15 @@ sub _th_index_lite {
         my $pad = '  ';
         # map = [children, attr, node, idx, level]
         my $map = $mapping->{$mid_raw};
-        my $nr_c = scalar @{$map->[0]};
+        my $children = $map->[0];
+        my $nr_c = scalar @$children;
         my $nr_s = 0;
         my $level = $map->[4];
         my $idx = $map->[3];
+        my $siblings;
         my $irt_map = $mapping->{$irt} if defined $irt;
         if (defined $irt_map) {
-                my $siblings = $irt_map->[0];
+                $siblings = $irt_map->[0];
                 $nr_s = scalar(@$siblings) - 1;
                 $rv .= $pad . $irt_map->[1];
                 if ($idx > 0) {
@@ -233,24 +235,26 @@ sub _th_index_lite {
         $this =~ s!<a\nhref=[^>]+>([^<]+)</a>!$1!s; # no point linking to self
         $rv .= "<b>@ $this";
         my $node = $map->[2];
-        if (my $child = $node->{child}) {
-                my $cmid = $child->{id};
+        if ($nr_c) {
+                my $cmid = $children->[0]->{id};
                 $rv .= $pad . $mapping->{$cmid}->[1];
                 if ($nr_c > 2) {
                         my $s = ($nr_c - 1). ' more replies';
                         $rv .= pad_link($cmid, $level + 1, $s);
-                } elsif (my $cn = $child->{next}) {
+                } elsif (my $cn = $children->[1]) {
                         $rv .= $pad . $mapping->{$cn->{id}}->[1];
                 }
         }
-        if (my $next = $node->{next}) {
+
+        my $next = $siblings->[$idx+1] if $siblings && $idx >= 0;
+        if ($next) {
                 my $nmid = $next->{id};
                 $rv .= $pad . $mapping->{$nmid}->[1];
                 my $nnext = $nr_s - $idx;
                 if ($nnext > 2) {
                         my $s = ($nnext - 1).' subsequent siblings';
                         $rv .= pad_link($nmid, $level, $s);
-                } elsif (my $nn = $next->{next}) {
+                } elsif (my $nn = $siblings->[$idx + 2]) {
                         $rv .= $pad . $mapping->{$nn->{id}}->[1];
                 }
         }
@@ -264,7 +268,8 @@ sub walk_thread {
                 my $level = shift @q;
                 my $node = shift @q or next;
                 $cb->($ctx, $level, $node);
-                unshift @q, $level+1, $node->{child}, $level, $node->{next};
+                ++$level;
+                unshift @q, map { ($level, $_) } @{$node->{children}};
         }
 }
 
@@ -296,7 +301,8 @@ sub stream_thread ($$) {
         while (@q) {
                 $level = shift @q;
                 my $node = shift @q or next;
-                unshift @q, $level+1, $node->{child}, $level, $node->{next};
+                my $cl = $level + 1;
+                unshift @q, map { ($cl, $_) } @{$node->{children}};
                 $mime = $inbox->msg_by_smsg($node->{smsg}) and last;
         }
         return missing_thread($ctx) unless $mime;
@@ -309,8 +315,8 @@ sub stream_thread ($$) {
                 while (@q) {
                         $level = shift @q;
                         my $node = shift @q or next;
-                        unshift @q, $level+1, $node->{child},
-                                        $level, $node->{next};
+                        my $cl = $level + 1;
+                        unshift @q, map { ($cl, $_) } @{$node->{children}};
                         my $mid = $node->{id};
                         if ($mime = $inbox->msg_by_smsg($node->{smsg})) {
                                 $mime = Email::MIME->new($mime);