#! /usr/bin/perl -w

use strict;
use IPC::Open2;
use POSIX qw(tmpnam);

my $tmpdir;
do { $tmpdir = tmpnam(); }
until mkdir $tmpdir, 0777;

my $manifest = "$tmpdir/manifest";
my $conffile = "@sysconfdir@/nix/prebuilts.conf";

#END { unlink $manifest; rmdir $tmpdir; }

my @srcpaths;
my @subs;
my @sucs;

my $fullexpr = "[";


sub processURL {
    my $url = shift;
    $url =~ s/\/$//;
    print "obtaining list of Nix archives at $url...\n";

    system "wget --cache=off '$url'/MANIFEST -O '$manifest' 2> /dev/null"; # !!! escape
    if ($?) { die "`wget' failed"; }
        
    open MANIFEST, "<$manifest";

    my $inside = 0;

    my $storepath;
    my $narname;
    my $hash;
    my @preds;

    while (<MANIFEST>) {
        chomp;
        s/\#.*$//g;
        next if (/^$/);

        if (!$inside) {
            if (/^\{$/) { 
                $inside = 1;
                undef $storepath;
                undef $narname;
                undef $hash;
                @preds = ();
                }
            else { die "bad line: $_"; }
        } else {
            if (/^\}$/) {
                $inside = 0;
                my $fullurl = "$url/$narname";
#                print "$storepath\n";

                # Construct a Nix expression that fetches and unpacks a
                # Nix archive from the network.
                my $fetch =
                    "(import @datadir@/nix/corepkgs/fetchurl) " .
                    "{url = $fullurl; md5 = \"$hash\"; system = \"@system@\"}";
                my $nixexpr =
                    "((import @datadir@/nix/corepkgs/nar/unnar.nix) " .
                    "{narFile = ($fetch); outPath = \"$storepath\"; system = \"@system@\"}) ";
                $fullexpr .= $nixexpr; # !!! O(n^2)?

                push @srcpaths, $storepath;

                foreach my $p (@preds) {
                    push @sucs, $p;
                    push @sucs, $storepath;
                }

            }
            elsif (/^\s*StorePath:\s*(\/\S+)\s*$/) {
                $storepath = $1;
            }
            elsif (/^\s*NarName:\s*(\S+)\s*$/) {
                $narname = $1;
                }
            elsif (/^\s*MD5:\s*(\S+)\s*$/) {
                $hash = $1;
            }
            elsif (/^\s*SuccOf:\s*(\/\S+)\s*$/) {
                push @preds, $1;
            }
            else { die "bad line: $_"; }
        }
    }

    close MANIFEST;
}


# Obtain URLs either from the command line or from a configuration file.
if (scalar @ARGV > 0) {
    while (@ARGV) {
        my $url = shift @ARGV;
            processURL $url;
    }
} else {
    open CONFFILE, "<$conffile";
    while (<CONFFILE>) {
        chomp;
        if (/^\s*(\S+)\s*(\#.*)?$/) {
            my $url = $1;
            processURL $url;
        }
    }
    close CONFFILE;
}

$fullexpr .= "]";


# Instantiate store expressions from the Nix expressions we created above.
print STDERR "instantiating Nix expression...\n";
my $pid = open2(\*READ, \*WRITE, "nix-instantiate -") or die "cannot run nix-instantiate";

print WRITE $fullexpr;
close WRITE;
my $i = 0;
while (<READ>) {
    chomp;
    die unless /^\//;
    my $subpath = $_;
    die unless ($i < scalar @srcpaths);
    my $srcpath = $srcpaths[$i++];
    push @subs, $srcpath;
    push @subs, $subpath;
}

waitpid $pid, 0;
$? == 0 or die "nix-instantiate failed";


# Register all substitutes.
print STDERR "registering substitutes...\n";
system "nix-store --substitute @subs";
if ($?) { die "`nix-store --substitute' failed"; }


# Register all successors.
print STDERR "registering successors...\n";
system "nix-store --successor @sucs";
if ($?) { die "`nix-store --successor' failed"; }