#!/usr/bin/perl

=head1 NAME

dh_ifexists - Add dependencies iff they are satisfiable at build time

=cut

use strict;
use warnings;

use Debian::Debhelper::Dh_Lib;
use Dpkg::Deps;
use Dpkg::Version;
use AptPkg::Cache;

=head1 SYNOPSIS

B<dh_ifexists> [S<I<debhelper options>>]

=head1 DESCRIPTION

dh_ifexists will examine Depends-If-Exists lines in debian/control,
check each listed dependency for satisfiability (according to apt in the build
environment), and add it to ${misc:Depends} if so.

It also does the same for other -If-Exists lines (such as
Recommends-If-Exists), adding them to ${misc:Recommends}, etc. Note that
these substvars are much less common than ${misc:Depends}, and so your
control file almost certainly doesn't already have them listed.

=cut

init();
my $cache = AptPkg::Cache->new;

my $package = "";

open (CONTROL, 'debian/control') ||
	error("cannot read debian/control: $!\n");
while (<CONTROL>) {
	chomp;
	s/\s+$//;
	if (/^Package:\s*(.*)/) {
		$package = $1;
	}
	if ($package && /^(.*?)-If-Exists: (.*)/) {
		addsubstvar($package, "misc:$1", satisfiable($2));
	}
	if (!$_ or eof) {
		$package = "";
	}
}

sub satisfiable {
	my $fulldeps = deps_parse(shift, use_arch => 1, reduce_arch => 1, union => 1);
	my $deps = Dpkg::Deps::Union->new;
	foreach my $dep ($fulldeps->get_deps()) {
		my $pkg = $cache->get($dep->{package});
		next unless $pkg;
		if ($dep->{relation}) {
			foreach my $candidate (@{$pkg->{VersionList}}) {
				if (version_compare_relation($candidate->{VerStr}, $dep->{relation}, $dep->{version})) {
					$deps->add($dep);
					last;
				}
			}
		} else {
			$deps->add($dep);
		}
	}
	return $deps;
}

=head1 BUGS

We don't handle apt_preferences / policy, although we do handle versioned
dependencies. Hopefully you use clean build chroots, and hopefully you have no
need for apt preferences in those. (This isn't hard to fix, just annoying.)

We should probably have some way to specify that you want to want to add
a group of dependencies if it's all satisfiable. The easy example is
Depends-If-Exists: foo (>= 1.0), foo (<< 2.0), where it would be useless
to keep one but not the other; we'd want both or neither.

=head1 SEE ALSO

L<debhelper(7)>

=head1 AUTHOR

Geoffrey Thomas <geofft@mit.edu>

=cut