
=head1 NAME

Mail::Box::Thread::Manager - maintain threads within a set of folders

=head1 CLASS INHERITANCE

 Mail::Box::Thread::Manager
   is a Mail::Reporter

=head1 SYNOPSIS

 my $mgr     = Mail::Box::Thread::Manager->new;
 my $folder  = $mgr->open(folder => '/tmp/inbox');
 my $threads = $mgr->threads(folder => $folder);
 my $threads = $mgr->threads($folder);   # same

 foreach my $thread ($threads->all) {
     $thread->print;
 }

 $threads->includeFolder($folder);
 $threads->removeFolder($folder);

=head1 DESCRIPTION

A (message-)I<thread> is a message with links to messages which followed in
reply of that message.  And then the messages with replied to the messages,
which replied the original message.  And so on.  Some threads are only
one message long (never replied to), some threads are very long.

The Mail::Box::Thread::Manager is very powerful.  Not only is it able to
do a descent job on MH-like folders (makes a trade-off between perfection
and speed), it also can maintain threads from messages residing in different
opened folders.  Both facilities are rare for mail-agents.

=head1 METHODS

=head2 Initiation

=over 4

=item B<new> OPTIONS

(Class method) 
A Mail::Box::Thread::Manager object is created by a Mail::Box::Manager.
One manager can produce more than one of these objects.  One thread manager can
combine messages from a set of folders, which may be partially overlapping
with other objects of the same type.

 OPTION               DEFAULT
 dummy_type           'Mail::Message::Dummy'
 folder               []
 folders              []
 log                  'WARNINGS'
 thread_body          <false>
 thread_type          'Mail::Box::Thread::Node'
 timespan             '3 days'
 trace                'WARNINGS'
 window               10

=over 4

=item B<dummy_type> =E<gt> CLASS

The type of dummy messages.  Dummy messages are used to fill holes in
detected threads: referred to by messages found in the folder, but itself
not in the folder.

=item B<folder> =E<gt> FOLDER | REF-ARRAY-FOLDERS

Specifies which folders are to be covered by the threads.  You can
specify one or more open folders.  When you close a folder, the
manager will automatically remove the messages of that folder from
your threads.

=item B<folders> =E<gt> FOLDER | REF-ARRAY-FOLDERS

Equivalent to the C<folder> option.

=item B<log> =E<gt> LEVEL

See Mail::Reporter::new(log)

=item B<thread_body> =E<gt> BOOLEAN

May thread-detection be based on the content of a message?  This has
a serious performance implication when there are many messages without
C<In-Reply-To> and C<References> headers in the folder, because it
will cause many messages to be parsed. NOT IMPLEMENTED YET.

=item B<thread_type> =E<gt> CLASS

Type of the thread nodes.

=item B<timespan> =E<gt> TIME | 'EVER'

Specify how fast threads usually work: the amount of time between an
answer and a reply.  This is used in combination with the C<window>
option to determine when to give-up filling the holes in threads.

See Mail::Box::timespan2seconds() for the possibilities for TIME.
With 'EVER', the search for messages in a thread
will only be limited by the window-size.

=item B<trace> =E<gt> LEVEL

See Mail::Reporter::new(trace)

=item B<window> =E<gt> INTEGER|'ALL'

The thread-window describes how many messages should be checked at
maximum to fill `holes' in threads for folder which use delay-loading
of message headers.

The constant 'ALL' will cause thread-detection not to stop trying
to fill holes, but continue looking until the first message of the folder
is reached.  Gives the best quality results, but may perform bad.

=back

Examples:

 use Mail::Box::Manager;
 my $mgr     = new Mail::Box::Manager;
 my $inbox   = $mgr->open(folder => $ENV{MAIL});
 my $read    = $mgr->open(folder => 'Mail/read');
 my $threads = $mgr->threads(folders => [$inbox, $read]);

 # longer alternative for last line:
 my $threads = $mgr->threads;
 $threads->includeFolder($inbox);
 $threads->includeFolder($read);

=back

=head2 Grouping Folders

=over 4

=item B<folders> 

Returns the folders as managed by this threader.

=item B<includeFolder> FOLDERS

Add one or more folders to the list of folders whose messages are
organized in the threads maintained by this object.  Duplicated
inclusions will not cause any problems.

From the folders, the messages which have their header lines parsed
(see Mail::Box about lazy extracting) will be immediately scanned.  Messages
of which the header is known only later will have to report this
(see Mail::Box::toBeThreaded()).

Examples:

 $threads->includeFolder($inbox, $draft);

=item B<removeFolder> FOLDERS

Remove one or more folders from the list of folders whose messages are
organized in the threads maintained by this object.

Examples:

 $threads->removeFolder($draft);

=back

=head2 The Threads

=over 4

=item B<all> 

Returns all messages which start a thread.  The list may contain dummy
messages and messages which are scheduled for deletion.

To be able to return all threads, thread construction on each
message is performed first, which may be slow for some folder-types
because is will enforce parsing of message-bodies.

=item B<known> 

Returns the list of all messages which are known to be the start of
a thread.  Threads containing messages which where not read from their
folder (like often happens MH-folder messages) are not yet known, and
hence will not be returned.

The list may contain dummy messages, and messages which are scheduled
for deletion.  Threads are detected based on explicitly calling
inThread() and thread() with a messages from the folder.

Be warned that, each time a message's header is read from the folder,
the return of the method can change.

=item B<sortedAll> [PREPARE [COMPARE]]

Returns all() the threads by default, but sorted on timestamp.

=item B<sortedKnown> [PREPARE [,COMPARE]]

Returns all known() threads, in sorted order.  By default, the threads
will be sorted on timestamp, But a different COMPARE method can be
specified.

=item B<thread> MESSAGE

Returns the thread where this MESSAGE is the start of.  However, there
is a possibility that this message is a reply itself.

Usually, all messages which are in reply of this message are dated later
than the specified one.  All headers of messages later than this one are
are getting parsed first, for each folder in this threads-object.

Examples:

 my $threads = $mgr->threads(folder => $inbox);
 my $thread  = $threads->thread($inbox->message(3));
 print $thread->string;

=item B<threadStart> MESSAGE

Based on a message, and facts from previously detected threads, try
to build solid knowledge about the thread where this message is in.

=back

=head2 Keeping Thread Information [internals]

=over 4

=item B<createDummy> MESSAGE-ID

Get a replacement message to be used in threads.  Be warned that a
dummy is not a member of any folder, so the program working with
threads must test with C<< $msg->isDummy >> before trying things only
available to real messages.

=item B<inThread> MESSAGE

Collect the thread-information of one message.  The `In-Reply-To' and
`Reference' header-fields are processed.  If this method is called on
a message whose header was not read yet (as usual for MH-folders,
for instance) the reading of that header will be triggered here.

=item B<outThread> MESSAGE

Remove the message from the thread-infrastructure.  A message is
replaced by a dummy.

=item B<toBeThreaded> FOLDER, MESSAGES

Include the specified messages in/from the threads managed by
this object, if this folder is maintained by this thread-manager.

=item B<toBeUnthreaded> FOLDER, MESSAGES

Remove the specified messages in/from the threads managed by
this object, if this folder is maintained by this thread-manager.

=back

=head2 Logging and Tracing

=over 4

=item B<defaultTrace> [LEVEL, [LEVEL]

See Mail::Reporter::defaultTrace()

=item B<errors> 

See Mail::Reporter::errors()

=item B<log> [LEVEL [,STRINGS]]

See Mail::Reporter::log()

=item B<report> [LEVEL]

See Mail::Reporter::report()

=item B<reportAll> [LEVEL]

See Mail::Reporter::reportAll()

=item B<trace> [LEVEL]

See Mail::Reporter::trace()

=item B<warnings> 

See Mail::Reporter::warnings()

=back

=head2 Other Methods

=over 4

=item B<AUTOLOAD> 

See Mail::Reporter::AUTOLOAD()

=item B<DESTROY> 

See Mail::Reporter::DESTROY()

=item B<inGlobalDestruction> 

See Mail::Reporter::inGlobalDestruction()

=item B<logPriority> LEVEL

See Mail::Reporter::logPriority()

=item B<logSettings> 

See Mail::Reporter::logSettings()

=item B<notImplemented> 

See Mail::Reporter::notImplemented()

=back

=head1 IMPLEMENTATION

This module implements thread-detection on a folder.  Messages created
by the better mailers will include C<In-Reply-To> and C<References>
lines, which are used to figure out how messages are related.  If you
prefer a better thread detection, they are implementable, but there
may be a serious performance hit (depends on the type of folder used).

=head2 Maintaining threads

A Mail::Box::Thread::Manager object is created by the Mail::Box::Manager,
using its threads() method.  Each object can monitor the thread-relations
between messages in one or more folders.  When more than one folder
is specified, the messages are merged while reading the threads, although
nothing changes in the folder-structure.  Adding and removing folders
which have to be maintained is permitted at any moment, although may
be quite costly in performance.

An example of the maintained structure is shown below.  The
Mail::Box::Manager has two open folders, and a thread-builder which
monitors them both.  The combined folders have two threads, the second
is two long (msg3 is a reply on msg2).  Msg2 is in two folders at once.

       manager
        |    \
        |     `----------- threads
        |                  |     |
        |                thread thread---thread
        |                  |    /|        /
        |                  |   //        /
        +---- folder1      |  //        /
        |       |         /  //        /
        |       `-----msg1  //        /
        |       `-----msg2-'/        /
        |                  /        /
        `-----folder2     /        /
                |        /        /
                `-----msg2       /
                `-----msg3------'

=head2 Delayed thread detection

With all() you get the start-messages of each thread of this folder.
When that message was not found in the folder (not saved or already
removed), you get a message of the dummy-type.  These thread descriptions
are in perfect state: all messages of the folder are included somewhere,
and each missing message of the threads (`holes') are filled by dummies.

However, to be able to detect all threads it is required to have the
headers of all messages, which is very slow for some types of folders,
especially MH and IMAP folders.

For interactive mail-readers, it is preferred to detect threads only
on messages which are in the viewport of the user.  This may be sloppy
in some situations, but everything is preferable over reading an MH
mailbox with 10k e-mails to read only the see most recent messages.

In this object, we take special care not to cause unnecessary parsing
(loading) of messages.  Threads will only be detected on command, and
by default only the message headers are used.

The following reports the Mail::Box::Thread::Node object which is
related to a message:

 my $thread = $message->thread;

When the message was not put in a thread yet, it is done now.  But, more
work is done to return the best thread.  Based on various parameters,
which where specified when the folder was created, the method walks
through the folder to fill the holes which are in this thread.

Walking from back to front (recently arrived messages are usually in the back
of the folder), message after message are triggered to be included in their
thread.  At a certain moment, the whole thread of the requested method
is found, a certain maximum number of messages was tried, but that
didn't help (search window bound reached), or the messages within the
folder are getting too old.  Then the search to complete the thread will
end, although more messages of them might have been in the folder: we
don't scan the whole folder for performance reasons.

Finally, for each message where the head is known, for instance for
all messages in mbox-folders, the correct thread is determined
immediately.  Also, all messages where the head get loaded later, are
automatically included.

=head1 SEE ALSO

A good start to read is Mail::Box-Overview.
More documentation and a mailinglist are available from the project's
website at L<http://perl.overmeer.net/mailbox/>.

=head1 AUTHOR

Written by Mark Overmeer (L<mark@overmeer.net|mailto:mark@overmeer.net>)
with the help of many.  See the ChangeLog for details.

=head1 VERSION

This code is beta, version 2.038.

Copyright (c) 2001-2003 by the authors. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
