#! @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;
}


# Remove a channel from the file $channelsList;
sub removeChannel {
    my $url = shift;
    my @left = ();
    readChannels;
    foreach my $url2 (@channels) {
        push @left, $url2 if $url ne $url2;
    }
    @channels = @left;
    writeChannels;
}


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

    # Get rid of all the old substitutes.
    system "@bindir@/nix-store --clear-substitutes";
    die "cannot clear substitutes" if ($? != 0);

    # 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@\";}";

    # Figure out a name for the GC root.
    my $userName = getpwuid($<);
    die "who ARE you? go away" unless defined $userName;

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

    # Build the resulting derivation.
    my $outPath = `nix-store --add-root '$rootFile' -r '$storeExpr'`
        or die "cannot realise store expression";
    chomp $outPath;

    unlink "$rootFile.tmp";

    # 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);
}


sub usageError {
    print STDERR <<EOF;
Usage:
  nix-channel --add URL
  nix-channel --remove URL
  nix-channel --list
  nix-channel --update
EOF
    exit 1;
}


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

    if ($arg eq "--add") {
        usageError if scalar @ARGV != 1;
        addChannel (shift @ARGV);
        last;
    }

    if ($arg eq "--remove") {
        usageError if scalar @ARGV != 1;
        removeChannel (shift @ARGV);
        last;
    }

    if ($arg eq "--list") {
        usageError if scalar @ARGV != 0;
        readChannels;
        foreach my $url (@channels) {
            print "$url\n";
        }
        last;
    }

    elsif ($arg eq "--update") {
        usageError if scalar @ARGV != 0;
        update;
        last;
    }
    
    elsif ($arg eq "--help") {
        usageError;
    }

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