#!/usr/bin/env perl
## Copyright © 2008-2009 by Daniel Friesel <derf@finalrewind.org>
## License: WTFPL <http://sam.zoy.org/wtfpl>
use strict;
use warnings;
use 5.010;
use AptPkg::Cache;
use Getopt::Long;

my $cache     = AptPkg::Cache->new;
my $max_depth = 5;
my $all       = 0;
my @deptypes  = qw/Depends Recommends Suggests/;
my @known;

sub revdeps {
	my $name    = shift;
	my $package = $cache->{$name};
	my @return;
	my $rdeps = $package->{RevDependsList};

	foreach my $dependency ( @{$rdeps} ) {
		if (
			not $dependency->{ParentPkg}{Name} ~~ @known
			and (  $all
				or $cache->{ $dependency->{ParentPkg}{Name} }->{CurrentState} eq
				'Installed' )
			and "$dependency->{DepType}" ~~ @deptypes
		)
		{
			push( @return, $dependency->{ParentPkg}{Name} );
		}
	}

	push( @known, @return );
	return (@return);
}

sub recurse {
	my $name  = shift;
	my $depth = (shift) + 1;

	printf( "%s%s\n", "\t" x ( $depth - 1 ), $name );

	if ( $depth >= $max_depth ) {
		return;
	}

	foreach ( revdeps($name) ) {
		recurse( $_, $depth );
	}

	return;
}

GetOptions(
	'all!'       => \$all,
	'deptype=s'  => sub { @deptypes = split( /,/, $_[1] ) },
	'depth=i'    => \$max_depth,
	'h|?|help'   =>
		sub { exec( 'perldoc', '-F', $0 ) or die("See perldoc -F $0\n") },
);

my $packagename = shift
	or die("Usage: $0 [options] <package>; see $0 --help\n");

if ( not defined $cache->{$packagename} ) {
	die("No such package: $packagename\n");
}

if ( $cache->{$packagename}->{Flags} =~ /Essential/ ) {
	print "Essential: Yes\n";
}

recurse( $packagename, 0 );

__END__

=head1 NAME

apt-why - filtered reverse dependency displayer using AptPkg::Cache

=head1 SYNOPSIS

B<apt-why> [ I<options> ] I<package>

=head1 DESCRIPTION

B<apt-why> recursively displays I<package>'s reverse dependencies, thus
showing which installed packages require I<package> on the system.

=head1 OPTIONS

=over

=item B<--all>

Consider all packages, not just installed ones

=item B<--depth>=I<integer> (default: 5)

Specify the maximum recursion depth

=item B<--deptype>=I<types>

Only show reverse dependencies which mach the specified I<types> (comma separated list).
Right now, possible types are Depends, Suggests, Recommends.

=back

=head1 EXIT STATUS

Zero on success, non-zero on error (i.e. no such package).

=head1 CONFIGURATION

None.

=head1 DEPENDENCIES

This program requires the perl module B<AptPkg::Cache>. On Debian, it is
available via the package B<libapt-pkg-perl>.

=head1 BUGS AND LIMITATIONS

There may be problems with virtual packages and "Provides: " stuff.

=head1 AUTHOR

Daniel Friesel E<lt>derf@finalrewind.orgE<gt>

=head1 LICENSE

  0. You just DO WHAT THE FUCK YOU WANT TO.