package Debug::Filter::PrintExpr;

use strict;
use warnings;
 
use Filter::Simple;

our (@h, $p);

sub gen_print {
	my ($self, $type, $label, $expr, $fh) = @_;
	# resulting code, with EXPR literally substituted by
	# the value of $expr and all in one single line:
	#
	# $:
	# {
	#	local ($\, $,);
	#	print 'line ', __LINE__, ':', ' EXPR = ', "'",
	# 		scalar(EXPR), "'", "\n";
	# }
	#
	# @:
	# {
	#	local ($\, $,);
	#	print 'line ', __LINE__, ':', ' EXPR = ', "('",
	#	join("', '", (EXPR)), "')", "\n";
	# }
	#
	# %:
	# {
	#	local ($\, $,);
	#	print 'line ', __LINE__, ':', ' EXPR = ';
	#	local (@Debug::Filter::PrintExpr::h, $Debug::Filter::PrintExpr::p);
	#	push @Debug::Filter::PrintExpr::h, $Debug::Filter::PrintExpr::p
	#		while $Debug::Filter::PrintExpr::p = [each (EXPR)],
	#			defined $Debug::Filter::PrintExpr::p->[0];
	#	print '(', (join ', ',
	#		map {"'$_->[0]' => '$_->[1]'"} @Debug::Filter::PrintExpr::h),
	#	')', "\n";
	# }
	#
	# quotes q and qq are for code generation
	# quotes " and ' are for generated code
	my $stmt = q[{local ($\, $,); print ];
	$stmt .= qq{$fh } if $fh;
	$stmt .= $label ? qq{'$label'} : q{'line ', __LINE__, ':'};
	if ($expr) {
		$stmt .= qq{, ' $expr = '};
		if ($type eq '$') {
			$stmt .= qq{, "'", scalar($expr), "'"};
		} elsif ($type eq '@') {
			$stmt .= qq{, "('", join("', '", ($expr)), "')"};
		} elsif ($type eq '%') {
			my $h = q{@} . $self . q{::h};
			my $p = q{$} . $self . q{::p};
			$stmt .= qq{; local ($h, $p); };
			$stmt .= qq{push $h, $p };
			$stmt .= qq{while $p = [each ($expr)], };
			$stmt .= qq{defined $p} . q{->[0]; };
			$stmt .= q{print '(', (join ', ', };
			$stmt .= q{map {"'$_->[0]' => '$_->[1]'"} };
			$stmt .= qq{$h), ')'};
		}
	}
	$stmt .= q[, "\n";}];
	return $stmt;
}

FILTER {
	my ($self, %opt) = @_;
	s/
		^\h*\#
		(?<type>[%@\$])
		\{\h*
		(?<label>\w+:)?
		\h*
		(?<expr>[^'\v]+)?
		\}\h*$
	/ gen_print($self, $+{type}, $+{label}, $+{expr}, $opt{-out}) /gmex;
	print if $opt{-debug};
};

1;

__END__

=head1 NAME

Debug::Filter::PrintExpr - Convert comment lines to debug print statements

=head1 SYNOPSIS

	use Debug::Filter::PrintExpr;

	my $s = 'a scalar';
	my @a = qw(this is an array);
	my %h = (key1 => 'value1', key2 => 'value2');

	#${$s}
	#@{@a}
	#%{%h}
	#${ calc: @a * 2 }

When the program is run, the comment lines will produce an output like this:

	line 12: $s = 'a scalar'
	line 13: @a = ('this', 'is', 'an', 'array')
	line 14: %h = ('key1' => 'value1', 'key2' => 'value2')
	calc: @a * 2  = '8'

=head1 DESCRIPTION

=head2 The Problem

Providing debug output often results in a couple of print statements that
output the value of some expression and some kind of description.
When the program is finished, these statements must be made conditional
on some variable or turned into comments.

Often the contents of arrays or hashes need to be presented in a
readable way, leading to repeated lines of similar code.

C programmers use the preprocessor to solve this problem.
As Perl has it's own filter mechanism for preprocessing,
this leads to a similar solution in Perl.

=head2 A Solution

The Filter::Simple module by Damian Conway provides a convenient way
of implementing Perl filters.
Debug::Filter::PrintExpr makes use of this module to transform specially
formed comment lines into print statements for various debugging
purposes.
Just by removing the "use" of Debug::Filter::PrintExpr completely
or disabling it partially by

	no Debug::Filter::PrintExpr;

all these lines (or a part of them) lose their magic and remain
simple comments.

The comment lines to be transformed must follow this format:

# I<sigil> { [I<label>:] [I<expression>] }

or more formally must be matched by the following regexp:
	
	^\s*#([$@%]){\s*(\w+:)?\s*([^']+)?}\s*$

where $0 represents the sigil, $1 an optional label and $2 an optional
expression.

If the label is omitted, it defaults to C<line nnn:>, where nnn is the
line number in the program.

The sigil determines the output format of the given expression:

=over 4

=item $

The expression is evaluated in scalar context and printed inside
single quotes;

=item @

The expression is evaluated in list context and the elements of the
list are printed inside single quoted, separated by commas and gathered
in parentheses.

=item %

The expression is used as argument in a while-each loop and the output
consists of pairs of the form 'key' => 'value' inside parentheses.

=back

The forms #${} and #@{} may be used for any type of expression
and inside the #%{} form, arrays are permitted too.
With the varibles $s, @a and %h defined as listed above, it is possible
to use:
	
	#@{scalar_as_array: $s}
	#${array_as_scalar :@a}
	#@{hash_as_array: %h}
	#%{array_as_hash: @a}

and receive these results:

	scalar_as_array: $s = ('this is a scalar')
	array_as_scalar: @a = '4'
	hash_as_array: %h = ('k1', 'v1', 'k2', 'v2')
	array_as_hash: @a = ('0' => 'this', '1' => 'is', '2' => 'an', '3' => 'array')
	
If the expression is omitted, only the label will be printed.
Any valid sigil may be used in this case.

Requirements for the expression are:

=over 4

=item 1.

It must be a valid Perl expression.

=item 2.

It must not contain single quotes.
(You may use q{} instead.)

=item 3.

In case of the #%{}-form, it must be a valid argument to the
each() builtin function, i.e. it should resolve to an array or hash.

=back

A PrintExpr will be resolved to a block and therefore may be located
anywhere in the program where a block is valid. 
Do not put it in a place, where a block is required (e.g. after a
conditional) as this would break the code when running without the
filter.

=head2 Arguments to Debug::Filter::PrintExpr

The use-statement for C<Debug::Filter::PrintExpr> may contain
a hash of options:

	use Debug::Filter::PrintExpr (-out => 'STDERR', -debug => 1);

=over 4

=item -out

This option is used as the name of the file handle in the
generated print statements.
Note that the argument to C<-out> is not a file handle, but a string
containing the name of the file handle.

=item -debug

When this option is set to true, the resulting source code after
comment transformation is written to the default output file handle.
Only the parts of source where Debug::Filter::PrintExpr is in effect
are printed out.

=back

=head1 AUTHOR

Joerg Sommrey

=head1 LICENCE AND COPYRIGHT

Copyright (c) 2018, Joerg Sommrey. All rights reserved.

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

=head1 DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
DAMAGES.

=cut