# $Id: Stream.pm,v 1.7 2010/10/15 15:55:34 Paulo Exp $

package Asm::Preproc::Stream;

#------------------------------------------------------------------------------

=head1 NAME

Asm::Preproc::Stream - Object to encapsulate an iterator that is able to unget

=cut

#------------------------------------------------------------------------------

use strict;
use warnings;

our $VERSION = '0.06';

#------------------------------------------------------------------------------

=head1 SYNOPSIS

  use Asm::Preproc::Stream;
  my $stream = Asm::Preproc::Stream->new(@input)
  my $head = $stream->head;
  my $next = $stream->get;
  $stream->unget(@items);
  my $it = $stream->iterator; my $next = $it->();

=head1 DESCRIPTION

This module encapsulates an iterator function. An iterator function returns the
next element in the stream on each call, and returns undef on end of input.

The iterator can return a code reference - this new iterator is inserted at the 
head of the queue.

The object allows the user to peek the head element of the stream without
consuming it, or to get it and remove from the stream.

A list of items can also be pushed back to the stream, to be retrieved in the
subsequent get() calls.

The input list to the constructor and to unget() contains either items to be
retireved on each get() call, or code references to be called to extract the
next item from the stream.

=head1 FUNCTIONS

=head2 new

Creates a new object ready to retrieve elements from the given input list.

=head2 head

Retrieves the element at the head of the stream, but keeps it in the stream to
be retrieved by the next get().

Returns undef if the stream is empty.

=cut

#------------------------------------------------------------------------------
# attributes
sub head { $_[0][0] }			# get head element

sub new {
    my($class, @input) = @_;
    my $self = bless [@input], $class;
    $self->_check_head();
    $self;
}

#------------------------------------------------------------------------------
# check if the head is an iterator, if yes extract the item and put it at head
# of queue
sub _check_head {
	my($self) = @_;
    while (@$self && ref(my $head = $self->[0]) eq 'CODE') {
		if (defined(my $element = $head->())) {
			unshift @$self, $element;		# insert at head of list
		}
		else {								# iterator exhausted, drop it
			shift @$self;
		}
	}
}
				
#------------------------------------------------------------------------------

=head2 get

Retrieves the element at the head of the stream and removes it from the stream.

Returns undef if the stream is empty

=cut

#------------------------------------------------------------------------------

sub get {
    my($self) = @_;
    my $head = shift @$self;
    $self->_check_head;
    return $head;
}

#------------------------------------------------------------------------------

=head2 unget

Pushes back a list of values and/or iterators to the stream, that will be
retrieved on the subsequent calls to get().

Can be called from within an iterator, to insert values that will be returned 
after the current call, e.g. calling from the iterator:

  $stream->unget(2..3); return 1;

will result in the stream 1,2,3 being returned from the stream.

=cut

#------------------------------------------------------------------------------

sub unget {
    my($self, @input) = @_;
    unshift @$self, @input;					# save current head
    $self->_check_head;
}

#------------------------------------------------------------------------------

=head2 iterator

Return an iterator function that returns the next stream element on each call.

=cut

#------------------------------------------------------------------------------

sub iterator {
    my($self) = @_;
	return sub { $self->get };
}

#------------------------------------------------------------------------------

=head1 ACKNOWLEDGEMENTS

Inspired in L<HOP::Stream|HOP::Stream>.

=head1 BUGS, FEEDBACK, AUTHOR, LICENCE and COPYRIGHT

See L<Asm::Preproc|Asm::Preproc>.

=cut

#------------------------------------------------------------------------------

1;