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, 34 insertions, 124 deletions

diff --git a/lib/PublicInbox/SearchThread.pm b/lib/PublicInbox/SearchThread.pm
index 153eef2b..05de9ec5 100644
--- a/lib/PublicInbox/SearchThread.pm
+++ b/lib/PublicInbox/SearchThread.pm
@@ -32,9 +32,8 @@ sub new {
 sub thread {
         my $self = shift;
         _add_message($self, $_) foreach @{$self->{messages}};
-        $self->{rootset} = [
-                        grep { !$_->{parent} } values %{$self->{id_table}} ];
-        delete $self->{id_table};
+        my $id_table = delete $self->{id_table};
+        $self->{rootset} = [ grep { !$_->{parent} } values %$id_table ];
 }
 
 sub _get_cont_for_id ($$) {
@@ -82,156 +81,67 @@ sub _add_message ($$) {
 
 sub order {
         my ($self, $ordersub) = @_;
-
-        # make a fake root
-        my $root = _get_cont_for_id($self, 'fakeroot');
-        $root->add_child( $_ ) for @{ $self->{rootset} };
-
-        # sort it
-        $root->order_children( $ordersub );
-
-        # and untangle it
-        my $kids = $root->children;
-        $self->{rootset} = $kids;
-        $root->remove_child($_) for @$kids;
+        my $rootset = $ordersub->($self->{rootset});
+        $self->{rootset} = $rootset;
+        $_->order_children($ordersub) for @$rootset;
 }
 
 package PublicInbox::SearchThread::Msg;
+use strict;
+use warnings;
 use Carp qw(croak);
 use Scalar::Util qw(weaken);
 
-sub new { my $self = shift; bless { id => shift }, $self; }
+sub new {
+        bless {
+                id => $_[1],
+                children => {}, # becomes an array when sorted by ->order(...)
+        }, $_[0];
+}
 
 sub add_child {
         my ($self, $child) = @_;
         croak "Cowardly refusing to become my own parent: $self"
           if $self == $child;
 
-        if (grep { $_ == $child } @{$self->children}) {
-                # All is potentially correct with the world
-                weaken($child->{parent} = $self);
-                return;
-        }
-
-        my $parent = $child->{parent};
-        remove_child($parent, $child) if $parent;
+        my $cid = $child->{id};
+        $self->{children}->{$cid} = $child;
 
-        $child->{next} = $self->{child};
-        $self->{child} = $child;
-        weaken($child->{parent} = $self);
-}
-
-sub remove_child {
-        my ($self, $child) = @_;
-
-        my $x = $self->{child} or return;
-        if ($x == $child) {  # First one's easy.
-                $self->{child} = $child->{next};
-                $child->{parent} = $child->{next} = undef;
-                return;
+        # reparenting:
+        if (defined(my $parent = $child->{parent})) {
+                delete $parent->{children}->{$cid};
         }
 
-        my $prev = $x;
-        while ($x = $x->{next}) {
-                if ($x == $child) {
-                        $prev->{next} = $x->{next}; # Unlink x
-                        $x->{next} = $x->{parent} = undef; # Deparent it
-                        return;
-                }
-                $prev = $x;
-        }
-        # oddly, we can get here
-        $child->{next} = $child->{parent} = undef;
+        weaken($child->{parent} = $self);
 }
 
 sub has_descendent {
-        my ($self, $child) = @_;
+        my ($cur, $child) = @_;
         my %seen;
-        my @q = ($self);
-        while (my $cont = shift @q) {
-                $seen{$cont} = 1;
+        my @q = ($cur->{parent} || $cur);
 
-                return 1 if $cont == $child;
+        while (defined($cur = shift @q)) {
+                return 1 if $cur == $child;
 
-                if (my $next = $cont->{next}) {
-                        if ($seen{$next}) {
-                                $cont->{next} = undef;
-                        } else {
-                                push @q, $next;
-                        }
-                }
-                if (my $child = $cont->{child}) {
-                        if ($seen{$child}) {
-                                $cont->{child} = undef;
-                        } else {
-                                push @q, $child;
-                        }
+                if (!$seen{$cur}++) {
+                        push @q, values %{$cur->{children}};
                 }
         }
         0;
 }
 
-sub children {
-        my $self = shift;
-        my @children;
-        my $visitor = $self->{child};
-        while ($visitor) {
-                push @children, $visitor;
-                $visitor = $visitor->{next};
-        }
-        \@children;
-}
-
-sub set_children {
-        my ($self, $children) = @_;
-        my $walk = $self->{child} = shift @$children;
-        do {
-                $walk = $walk->{next} = shift @$children;
-        } while ($walk);
-}
-
 sub order_children {
-        my ($walk, $ordersub) = @_;
+        my ($cur, $ordersub) = @_;
 
-        my %seen;
-        my @visited;
-        while ($walk) {
-                push @visited, $walk;
+        my %seen = ($cur => 1);
+        my @q = ($cur);
+        while (defined($cur = shift @q)) {
+                my $c = $cur->{children}; # The hashref here...
 
-                # spot/break loops
-                $seen{$walk} = 1;
-
-                my $child = $walk->{child};
-                if ($child && $seen{$child}) {
-                        $walk->{child} = $child = undef;
-                }
-
-                my $next = $walk->{next};
-                if ($next && $seen{$next}) {
-                        $walk->{next} = $next = undef;
-                }
-
-                # go down, or across
-                $next = $child if $child;
-
-                # no next?  look up
-                if (!$next) {
-                        my $up = $walk;
-                        while ($up && !$next) {
-                                $up = $up->{parent};
-                                $next = $up->{next} if $up;
-                        }
-                }
-                $walk = $next;
-        }
-        foreach my $cont (@visited) {
-                my $children = $cont->children;
-                next if @$children < 2;
-                $children = $ordersub->($children);
-                $cont = $cont->{child} = shift @$children;
-                do {
-                        $cont = $cont->{next} = shift @$children;
-                } while ($cont);
+                $c = [ grep { !$seen{$_}++ } values %$c ]; # spot/break loops
+                $c = $ordersub->($c) if scalar @$c > 1;
+                $cur->{children} = $c; # ...becomes an arrayref
+                push @q, @$c;
         }
 }