package Aspect::Advice::Before;

use strict;
use warnings;

# Added by eilara as hack around caller() core dump
# NOTE: Now we've switched to Sub::Uplevel can this be removed? --ADAMK
use Carp::Heavy    (); 
use Carp           ();
use Aspect::Hook   ();
use Aspect::Advice ();
use Aspect::Point  ();

our $VERSION = '1.02';
our @ISA     = 'Aspect::Advice';

sub _install {
	my $self     = shift;
	my $pointcut = $self->pointcut;
	my $code     = $self->code;
	my $lexical  = $self->lexical;

	# Get the curried version of the pointcut we will use for the
	# runtime checks instead of the original.
	# Because $MATCH_RUN is used in boolean conditionals, if there
	# is nothing to do the compiler will optimise away the code entirely.
	my $curried   = $pointcut->curry_runtime;
	my $compiled  = $curried ? $curried->compiled_runtime : undef;
	my $MATCH_RUN = $compiled ? '$compiled->()' : 1;

	# When an aspect falls out of scope, we don't attempt to remove
	# the generated hook code, because it might (for reasons potentially
	# outside our control) have been recursively hooked several times
	# by both Aspect and other modules.
	# Instead, we store an "out of scope" flag that is used to shortcut
	# past the hook as quickely as possible.
	# This flag is shared between all the generated hooks for each
	# installed Aspect.
	# If the advice is going to last lexical then we don't need to
	# check or use the $out_of_scope variable.
	my $out_of_scope = undef;
	my $MATCH_DISABLED = $lexical ? '$out_of_scope' : '0';

	# Find all pointcuts that are statically matched
	# wrap the method with advice code and install the wrapper
	foreach my $name ( $pointcut->match_all ) {
		my $NAME = $name; # For completeness

		no strict 'refs';
		my $original = *$name{CODE};
		unless ( $original ) {
			Carp::croak("Can't wrap non-existent subroutine ", $name);
		}

		# Any way to set prototypes other than eval?
		my $PROTOTYPE = prototype($original);
		   $PROTOTYPE = defined($PROTOTYPE) ? "($PROTOTYPE)" : '';

		# Generate the new function
		no warnings 'redefine';
		eval <<"END_PERL"; die $@ if $@;
		package Aspect::Hook;

		*$NAME = sub $PROTOTYPE {
			# Is this a lexically scoped hook that has finished
			goto &\$original if $MATCH_DISABLED;

			# Apply any runtime-specific context checks
			my \$wantarray = wantarray;
			local \$Aspect::POINT = bless {
				type      => 'before',
				pointcut  => \$pointcut,
				original  => \$original,
				sub_name  => \$name,
				wantarray => \$wantarray,
				args      => \\\@_,
				exception => \$\@, ### Not used (yet)
			}, 'Aspect::Point';

			local \$_ = \$Aspect::POINT;
			goto &\$original unless $MATCH_RUN;

			# Run the advice code
			&\$code(\$_);

			# Shortcut if they set a return value
			if ( exists \$_->{return_value} ) {
				return \@{\$_->{return_value}} if \$wantarray;
				return \$_->{return_value};
			}

			# Proceed to the original function
			\@_ = \$_->args; ### Superfluous?
			goto &\$original;
		};
END_PERL
		$self->{installed}++;
	}

	# If this will run lexical we don't need a descoping hook
	return unless $lexical;

	# Return the lexical descoping hook.
	# This MUST be stored and run at DESTROY-time by the
	# parent object calling _install. This is less bullet-proof
	# than the DESTROY-time self-executing blessed coderef
	return sub { $out_of_scope = 1 };
}

# Check for pointcut usage not supported by the advice type
sub _validate {
	my $self     = shift;
	my $pointcut = $self->pointcut;

	# The method used by the Highest pointcut is incompatible
	# with the goto optimisation used by the before() advice.
	if ( $pointcut->match_contains('Aspect::Pointcut::Highest') ) {
		return 'The pointcut highest is not currently supported by before advice';
	}

	# Pointcuts using "throwing" are irrelevant in before advice
	if ( $pointcut->match_contains('Aspect::Pointcut::Throwing') ) {
		return 'The pointcut throwing is illegal when used by before advice';
	}

	# Pointcuts using "throwing" are irrelevant in before advice
	if ( $pointcut->match_contains('Aspect::Pointcut::Returning') ) {
		return 'The pointcut returning is illegal when used by before advice';
	}

	$self->SUPER::_validate(@_);
}

1;

__END__

=pod

=head1 NAME

Aspect::Advice::Before - Execute code before a function is called

=head1 SYNOPSIS

  use Aspect;
  
  before {
  
      # Trace all calls to your module
      print STDERR "Called my function " . $_->sub_name . "\n";
  
      # Shortcut calls to foo() to always be true
      if ( $_->short_name eq 'foo' ) {
          return $_->return_value(1);
      }
  
      # Add an extra flag to bar() but call as normal
      if ( $_->short_name eq 'bar' ) {
          $_->args( $_->args, 'flag' );
      }

  } call qr/^ MyModule::\w+ $/

=head1 DESCRIPTION

The C<before> advice type is used to execute advice code prior to entry
into a target function. It is implemented by B<Aspect::Advice::Before>.

As well as creating side effects that run before the main code, the
C<before> advice type is particularly useful for changing parameters or
shortcutting calls to functions entirely and replacing the value they
would normally return with a different value.

Please note that the C<highest> pointcut (L<Aspect::Pointcut::Highest>) is
incompatible with C<before>. Creating a C<before> advice with a pointcut
tree that contains a C<highest> pointcut will result in an exception.

If speed is important to your program then C<before> is particular
interesting as the C<before> implementation is the only one that can take
advantage of tail calls via Perl's C<goto> function, where the rest of the
advice types need the more costly L<Sub::Uplevel> to keep caller() returning
correctly.

=head1 AUTHORS

Adam Kennedy E<lt>adamk@cpan.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2010 Adam Kennedy.

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut