#! @perl@ -w

use strict;

my $rootsDir = "@localstatedir@/nix/gcroots/channels";


# Figure out the name of the `.nix-channels' file to use.
my $home = $ENV{"HOME"};
die '$HOME not set' unless defined $home;
my $channelsList = "$home/.nix-channels";
    

my @channels;


# Reads the list of channels from the file $channelsList;
sub readChannels {
    return if (!-f $channelsList);
    open CHANNELS, "<$channelsList" or die "cannot open `$channelsList'";
    while (<CHANNELS>) {
        chomp;
        push @channels, $_;
    }
    close CHANNELS;
}


# Writes the list of channels to the file $channelsList;
sub writeChannels {
    open CHANNELS, ">$channelsList" or die "cannot open `$channelsList'";
    foreach my $url (@channels) {
        print CHANNELS "$url\n";
    }
    close CHANNELS;
}


# Adds a channel to the file $channelsList;
sub addChannel {
    my $url = shift;
    readChannels;
    foreach my $url2 (@channels) {
        return if $url eq $url2;
    }
    push @channels, $url;
    writeChannels;
}


# Fetch Nix expressions and pull cache manifests from the subscribed
# channels.
sub update {
    readChannels;

    # Pull cache manifests.
    foreach my $url (@channels) {
        print "pulling cache manifest from `$url'\n";
        system "@bindir@/nix-pull '$url'/MANIFEST";
        die "cannot pull cache manifest from `$url'" if ($? != 0);
    }

    # Create a Nix expression that fetches and unpacks the channel Nix
    # expressions.

    my $nixExpr = "[";
    
    foreach my $url (@channels) {
        my $fullURL = "$url/nixexprs.tar.bz2";
        my $hash = `@bindir@/nix-prefetch-url '$fullURL' 2> /dev/null`
            or die "cannot fetch `$fullURL'";
        chomp $hash;
        # !!! escaping
        $nixExpr .= "((import @datadir@/nix/corepkgs/fetchurl) " .
            "{url = $fullURL; md5 = \"$hash\"; system = \"@system@\";}) "
    }

    $nixExpr .= "]";

    $nixExpr =
        "(import @datadir@/nix/corepkgs/channels/unpack.nix) " .
        "{inputs = $nixExpr; system = \"@system@\";}";

    # Instantiate the Nix expression.
    my $storeExpr = `echo '$nixExpr' | @bindir@/nix-instantiate -`
        or die "cannot instantiate Nix expression";
    chomp $storeExpr;

    # Register the store expression as a root of the garbage
    # collector.
    my $userName = getpwuid($<);
    die "who ARE you? go away" unless defined $userName;

    my $rootFile = "$rootsDir/$userName.gcroot";
    my $tmpRootFile = "$rootsDir/$userName-tmp.gcroot";
    
    open ROOT, ">$tmpRootFile" or die "cannot create `$tmpRootFile'";
    print ROOT "$storeExpr";
    close ROOT;

    # Realise the store expression.
    my $outPath = `nix-store -qnf '$storeExpr'`
        or die "cannot realise store expression";
    chomp $outPath;

    # Make it the default Nix expression for `nix-env'.
    system "@bindir@/nix-env --import '$outPath'";
    die "cannot pull set default Nix expression to `$outPath'" if ($? != 0);

    rename $tmpRootFile, $rootFile or die "cannot rename `$tmpRootFile' to `$rootFile'";
}


while (scalar @ARGV) {
    my $arg = shift @ARGV;

    if ($arg eq "--add") {
        die "syntax: nix-channel --add URL" if (scalar @ARGV != 1);
        addChannel (shift @ARGV);
        last;
    }

    elsif ($arg eq "--update") {
        die "syntax: nix-channel --update" if (scalar @ARGV != 0);
        update;
        last;
    }

    else {
        die "unknown argument `$arg'";
    }
}