#!/usr/bin/perl

use strict;
use warnings;

use URI::Escape;

my $prgname = $0;


#### find_setup_ini_file ###############################################
# Parse Cygwin's setup.rc file to find the last setup.ini file it used.

sub find_setup_ini_file {
	open my $rc, '<', '/etc/setup/setup.rc'
			or usage("could not read setup.rc file: $!");

	my ($path, $mirror);
	while (<$rc>) {
		chomp;

		if ($_ eq 'last-cache') {
			$path = <$rc>;
			chomp $path;
			$path =~ s/^\s+//;
			open my $cp, '-|', "cygpath -u '$path'";
			$path = <$cp>;
			chomp $path;
			close $cp;
		}
		elsif ($_ eq 'last-mirror') {
			$mirror = <$rc>;
			chomp $mirror;
			$mirror =~ s/^\s+//;
			$mirror = uri_escape($mirror);
		}
	}

	close $rc;

	usage("could not find last Cygwin cache dir") unless $path;
	usage("could not find last Cygwin DL mirror") unless $mirror;

	for my $parent (glob("$path/$mirror/x86*")) {
		my $path = "$parent/setup.ini";
		return $path if -r $path;
	}
	
	usage("could not find setup.ini");
	return;
}


#### get_installed_package_list ########################################
# Return a list of names of installed packages

sub get_installed_package_list {
	open my $db, '<', '/etc/setup/installed.db'
			or usage("failed to read installed package DB file: $!");

	my $header = <$db>;
	my @pkgnames;
	while (<$db>) {
		my ($name) = split;
		push @pkgnames, $name;
	}

	return \@pkgnames;
}


#### parse_cygwin_setup_ini_file #######################################
# Extract dependency info from the Cygwin setup.ini file.

sub parse_cygwin_setup_ini_file {
	my ($inifile, $piref) = @_;

	open my $ini, '<', $inifile
			or die "Cannot read INI file $inifile: $!\n";

	# Skip to first package entry
	while (<$ini>) { last if /^@/; }

	# Parse package entries
	my %deps;
	while (defined $_) {
		chomp;
		my $p = substr $_, 2;
		my $obs = 0;

		while (<$ini>) {
			if (/^@/) {
				# Found next package entry; restart outer loop
				last;
			}
			elsif (/^category: Base$/) {
				# Mark this one as a special sort of root package: one
				# we're going to install regardless of user selection,
				# so we need not list it in our output.
				$piref->{$p} = 2;
			}
			elsif (/^category: _obsolete$/) {
				# Select this package's replacement instead below.
				$piref->{$p} = 0;
				$obs = 1;
			}
			elsif (/^requires:/) {
				# Save this package's requirements as its dependents list.
				my ($junk, @deps) = split;
				$deps{$p} = \@deps;

				# If this package was marked obsolete above, select its
				# replacement as provisionally to-be-installed.  That
				# package still might end up removed from our output list
				# if it in turn is a dependent of one of the packages we 
				# consider a "root" package at the end.
				$piref->{$deps[0]} = 1 if $obs;
			}
		}
	}

	close $ini;
	return \%deps;
}


#### usage #############################################################
# Print usage message plus optional error string, then exit

sub usage {
	my ($error) = @_;
	print "ERROR: $error\n\n" if length($error);

	print <<"USAGE";
usage: $prgname

    Finds the last-used Cygwin setup.ini file, then uses the
    package dependency info found within it to pare the list of
    currently-installed Cygwin packages down to a "root" set,
    being those that will implicitly install all of the others
    as dependencies.
    
    The output is a list suitable for passing to setup.exe -P.
USAGE
	exit ($error ? 1 : 0);
}


#### main ##############################################################

my $inifile = find_setup_ini_file;

# Convert package list to a hash so we can mark them non-root by name
my $pkgnames = get_installed_package_list;
my %packages = map { $_ => 1 } @$pkgnames;

my $deps = parse_cygwin_setup_ini_file($inifile, \%packages);

# For each given package name, mark any of its dependencies also found
# on the command line as as non-root.
for my $p (@$pkgnames) {
	my $pdref = $deps->{$p};
	for my $d (@$pdref) {
		$packages{$d} = 0;
	}
}

# Collect list of root packages and print it out
print join ',', sort(grep { $packages{$_} == 1 } @$pkgnames);