#  You may distribute under the terms of either the GNU General Public License
#  or the Artistic License (the same terms as Perl itself)
#
#  (C) Paul Evans, 2007-2009 -- leonerd@leonerd.org.uk

package Async::MergePoint;

use strict;

our $VERSION = '0.02';

use Carp;

=head1 NAME

C<Async::MergePoint> - resynchronise diverged control flow

=head1 SYNOPSIS

 use Async::MergePoint;

 my $merge = Async::MergePoint->new(
    needs => [ "leaves", "water" ],

    on_finished => sub {
       my %items = @_;
       # Make tea using $items{leaves} and $items{water}
    }
 );

 Kettle->boil(
    on_boiled => sub { $merge->done( "water", $_[0] ) }
 );

 Cupboard->get_tea_leaves(
    on_fetched => sub { $merge->done( "leaves", $_[0] ) }
 );

=head1 DESCRIPTION

Often in program logic, multiple different steps need to be taken that are
independent of each other, but their total result is needed before the next
step can be taken. In synchonous code, the usual approach is to do them
sequentially. 

An asynchronous or event-based program could do this, but if each step
involves some IO idle time, better overall performance can often be gained by
running the steps in parallel. A C<Async::MergePoint> object can then be used
to wait for all of the steps to complete, before passing the combined result
of each step on to the next stage.

A merge point maintains a set of outstanding operations it is waiting on;
these are arbitrary string values provided at the object's construction. Each
time the C<done()> method is called, the named item is marked as being
complete. When all of the required items are so marked, the C<on_finished>
continuation is invoked.

When an item is marked as complete, a value can also be provided, which would
contain the results of that step. The C<on_finished> callback is passed a hash
(in list form, rather than by reference) of the collected item values.

This module was originally part of the L<IO::Async> distribution, but was
removed under the inspiration of Pedro Melo's L<Async::Hooks> distribution,
because it doesn't itself contain anything IO-specific.

=cut

=head1 CONSTRUCTOR

=cut

=head2 $merge = Async::MergePoint->new( %params )

This function returns a new instance of a C<Async::MergePoint> object. The
C<%params> hash takes the following keys:

=over 8

=item needs => ARRAY

An array containing unique item names to wait on. The order of this array is
not significant.

=item on_finished => CODE

CODE reference to the continuation for when the merge point becomes ready.

=back

The C<on_finished> continuation will be called when every key in the C<needs>
list has been notified by the C<done()> method. It will be called as

 $on_finished->( %items )

where the C<%items> hash will contain the item names that were waited on, and
the values passed to the C<done()> method for each one. Note that this is
passed as a list, not as a HASH reference.

=cut

sub new
{
   my $class = shift;
   my ( %params ) = @_;

   ref $params{needs} eq 'ARRAY' or croak "Expected 'needs' to be an ARRAY ref";
   ref $params{on_finished} eq 'CODE' or croak "Expected 'on_finished' to be a CODE ref";

   # Store these as a hash for ease of deletion
   my %needs = map { $_ => 1 } @{ $params{needs} };

   my $self = bless {
      needs => \%needs,
      items => {},
      on_finished => $params{on_finished},
   }, $class;

   return $self;
}

=head1 METHODS

=cut

=head2 $merge->done( $item, $value )

This method informs the merge point that the C<$item> is now ready, and
passes it a value to store, to be passed into the C<on_finished> continuation.
If this call gives the final remaining item being waited for, the
C<on_finished> continuation is called within it, and the method will not
return until it has completed.

=cut

sub done
{
   my $self = shift;
   my ( $item, $value ) = @_;

   exists $self->{needs}->{$item} or croak "$self does not need $item";

   delete $self->{needs}->{$item};
   $self->{items}->{$item} = $value;

   if( !keys %{ $self->{needs} } ) {
      $self->{on_finished}->( %{$self->{items}} );
   }
}

# Keep perl happy; keep Britain tidy
1;

__END__

=head1 AUTHOR

Paul Evans E<lt>leonerd@leonerd.org.ukE<gt>
