diff --git a/corepkgs/nix-pull/download.pl b/corepkgs/nix-pull/download.pl
new file mode 100755
index 0000000000..5c7b33a996
--- /dev/null
+++ b/corepkgs/nix-pull/download.pl
@@ -0,0 +1,218 @@
+#! /usr/bin/perl -w -I/home/eelco/Dev/nix/scripts
+
+use strict;
+use readmanifest;
+
+my $manifestDir = "/home/eelco/Dev/nix/patch/test";
+
+
+# Check the arguments.
+die unless scalar @ARGV == 1;
+my $targetPath = $ARGV[0];
+
+
+# Load all manifests.
+my %narFiles;
+my %patches;
+my %successors;
+
+for my $manifest (glob "$manifestDir/*.nixmanifest") {
+    print STDERR "reading $manifest\n";
+    readManifest $manifest, \%narFiles, \%patches, \%successors;
+}
+
+
+# Build a graph of all store paths that might contribute to the
+# construction of $targetPath, and the special node "start".  The
+# edges are either patch operations, or downloads of full NAR files.
+# The latter edges only occur between "start" and a store path.
+
+my %graph;
+
+$graph{"start"} = {d => 0, pred => undef, edges => []};
+
+my @queue = ();
+my $queueFront = 0;
+my %done;
+
+sub addToQueue {
+    my $v = shift;
+    return if defined $done{$v};
+    $done{$v} = 1;
+    push @queue, $v;
+}
+
+sub addNode {
+    my $u = shift;
+    $graph{$u} = {d => 999999999999, pred => undef, edges => []}
+        unless defined $graph{$u};
+}
+
+sub addEdge {
+    my $u = shift;
+    my $v = shift;
+    my $w = shift;
+    my $type = shift;
+    my $info = shift;
+    addNode $u;
+    push @{$graph{$u}->{edges}},
+        {weight => $w, start => $u, end => $v, type => $type, info => $info};
+    my $n = scalar @{$graph{$u}->{edges}};
+}
+
+addToQueue $targetPath;
+
+while ($queueFront < scalar @queue) {
+    my $u = $queue[$queueFront++];
+    print "$u\n";
+
+    addNode $u;
+
+    # If the path already exists, it has distance 0 from the "start"
+    # node.
+    system "nix-store --isvalid '$u' 2> /dev/null";
+    if ($? == 0) {
+        addEdge "start", $u, 0, "present", undef;
+    }
+
+    else {
+
+        # Add patch edges.
+        my $patchList = $patches{$u};
+        foreach my $patch (@{$patchList}) {
+            # !!! this should be cached
+            my $hash = `nix-hash "$patch->{basePath}"`;
+            chomp $hash;
+            print "  MY HASH is $hash\n";
+            if ($hash ne $patch->{baseHash}) {
+                print "  REJECTING PATCH from $patch->{basePath}\n";
+                next;
+            }
+            print "  PATCH from $patch->{basePath}\n";
+            addToQueue $patch->{basePath};
+            addEdge $patch->{basePath}, $u, $patch->{size}, "patch", $patch;
+        }
+
+        # Add NAR file edges to the start node.
+        my $narFileList = $narFiles{$u};
+        foreach my $narFile (@{$narFileList}) {
+            print "  NAR from $narFile->{url}\n";
+            addEdge "start", $u, $narFile->{size}, "narfile", $narFile;
+        }
+
+    }
+}
+
+
+# Run Dijkstra's shortest path algorithm to determine the shortest
+# sequence of download and/or patch actions that will produce
+# $targetPath.
+
+sub byDistance { # sort by distance, reversed
+    return -($graph{$a}->{d} <=> $graph{$b}->{d});
+}
+
+my @todo = keys %graph;
+
+while (scalar @todo > 0) {
+
+    # Remove the closest element from the todo list.
+    @todo = sort byDistance @todo;
+    my $u = pop @todo;
+
+    my $u_ = $graph{$u};
+
+    print "IN $u $u_->{d}\n";
+
+    foreach my $edge (@{$u_->{edges}}) {
+        my $v_ = $graph{$edge->{end}};
+        if ($v_->{d} > $u_->{d} + $edge->{weight}) {
+            $v_->{d} = $u_->{d} + $edge->{weight};
+            # Store the edge; to edge->start is actually the
+            # predecessor.
+            $v_->{pred} = $edge; 
+            print "  RELAX $edge->{end} $v_->{d}\n";
+        }
+    }
+}
+
+
+# Retrieve the shortest path from "start" to $targetPath.
+my @path = ();
+my $cur = $targetPath;
+die "don't know how to produce $targetPath\n"
+    unless defined $graph{$targetPath}->{pred};
+while ($cur ne "start") {
+    push @path, $graph{$cur}->{pred};
+    $cur = $graph{$cur}->{pred}->{start};
+}
+
+
+# Traverse the shortest path, perform the actions described by the
+# edges.
+my $curStep = 1;
+my $maxStep = scalar @path;
+
+sub downloadFile {
+    my $url = shift;
+    my $hash = shift;
+    $ENV{"PRINT_PATH"} = 1;
+    $ENV{"QUIET"} = 1;
+    my ($hash2, $path) = `nix-prefetch-url '$url' '$hash'`;
+    chomp $hash2;
+    chomp $path;
+    die "hash mismatch" if $hash ne $hash2;
+    return $path;
+}
+
+while (scalar @path > 0) {
+    my $edge = pop @path;
+    my $u = $edge->{start};
+    my $v = $edge->{end};
+
+    print "\n*** Step $curStep/$maxStep: ";
+    $curStep++;
+
+    if ($edge->{type} eq "present") {
+        print "using already present path `$v'\n";
+    }
+
+    elsif ($edge->{type} eq "patch") {
+        my $patch = $edge->{info};
+        print "applying patch `$patch->{url}' to `$u' to create `$v'\n";
+
+        # Download the patch.
+        print "  downloading patch...\n";
+        my $patchPath = downloadFile "$patch->{url}", "$patch->{hash}";
+
+        # Turn the base path into a NAR archive, to which we can
+        # actually apply the patch.
+        print "  packing base path...\n";
+        system "nix-store --dump $patch->{basePath} > /tmp/nar";
+        die "cannot dump `$patch->{basePath}'" if ($? != 0);
+
+        # Apply the patch.
+        print "  applying patch...\n";
+        system "bspatch /tmp/nar /tmp/nar2 $patchPath";
+        die "cannot apply patch `$patchPath' to /tmp/nar" if ($? != 0);
+
+        # Unpack the resulting NAR archive into the target path.
+        print "  unpacking patched archive...\n";
+        system "nix-store --restore $targetPath < /tmp/nar2";
+        die "cannot unpack /tmp/nar2 into `$targetPath'" if ($? != 0);
+    }
+
+    elsif ($edge->{type} eq "narfile") {
+        my $narFile = $edge->{info};
+        print "downloading `$narFile->{url}' into `$v'\n";
+
+        # Download the archive.
+        print "  downloading archive...\n";
+        my $narFilePath = downloadFile "$narFile->{url}", "$narFile->{hash}";
+
+        # Unpack the archive into the target path.
+        print "  unpacking archive...\n";
+        system "bunzip2 < '$narFilePath' | nix-store --restore '$targetPath'";
+        die "cannot unpack `$narFilePath' into `$targetPath'" if ($? != 0);
+    }
+}
diff --git a/scripts/nix-pull.in b/scripts/nix-pull.in
index a802760a52..66d99ff7c9 100644
--- a/scripts/nix-pull.in
+++ b/scripts/nix-pull.in
@@ -19,14 +19,24 @@ my $confFile = "@sysconfdir@/nix/prebuilts.conf";
 my %storePaths2urls;
 my %urls2hashes;
 my %successors;
-sub doURL {
+
+sub processURL {
     my $url = shift;
-    processURL $manifest, $url, \%storePaths2urls, \%urls2hashes, \%successors;
+
+    $url =~ s/\/$//;
+    print "obtaining list of Nix archives at $url...\n";
+
+    system("@curl@ --fail --silent --show-error --location --max-redirs 20 " .
+           "'$url' > '$manifest'") == 0
+           or die "curl failed: $?";
+    
+    readManifest $manifest, \%storePaths2urls, \%urls2hashes, \%successors;
 }
+
 if (scalar @ARGV > 0) {
     while (@ARGV) {
         my $url = shift @ARGV;
-	doURL $url;
+	processURL $url;
     }
 } else {
     open CONFFILE, "<$confFile";
@@ -34,7 +44,7 @@ if (scalar @ARGV > 0) {
         chomp;
         if (/^\s*(\S+)\s*(\#.*)?$/) {
             my $url = $1;
-	    doURL $url;
+	    processURL $url;
         }
     }
     close CONFFILE;
diff --git a/scripts/readmanifest.pm.in b/scripts/readmanifest.pm.in
index d5527bf3b3..8d6694ff2e 100644
--- a/scripts/readmanifest.pm.in
+++ b/scripts/readmanifest.pm.in
@@ -1,27 +1,24 @@
 use strict;
 
-sub processURL {
+sub readManifest {
     my $manifest = shift;
-    my $url = shift;
-    my $storePaths2urls = shift;
-    my $urls2hashes = shift;
+    my $narFiles = shift;
+    my $patches = shift;
     my $successors = shift;
 
-    $url =~ s/\/$//;
-    print "obtaining list of Nix archives at $url...\n";
-
-    system("@curl@ --fail --silent --show-error --location --max-redirs 20 " .
-           "'$url' > '$manifest'") == 0
-           or die "curl failed: $?";
-        
     open MANIFEST, "<$manifest";
 
     my $inside = 0;
+    my $type;
 
     my $storePath;
-    my $narurl;
+    my $url;
     my $hash;
+    my $size;
     my @preds;
+    my $basePath;
+    my $baseHash;
+    my $patchType;
 
     while (<MANIFEST>) {
         chomp;
@@ -29,38 +26,102 @@ sub processURL {
         next if (/^$/);
 
         if (!$inside) {
-            if (/^\{$/) { 
+            if (/^\{$/) {
+                $type = "narfile";
                 $inside = 1;
                 undef $storePath;
-                undef $narurl;
+                undef $url;
                 undef $hash;
+                $size = 999999999;
                 @preds = ();
 	    }
+            elsif (/^patch \{$/) {
+                $type = "patch";
+                $inside = 1;
+                undef $url;
+                undef $hash;
+                undef $size;
+                undef $basePath;
+                undef $baseHash;
+                undef $patchType;
+            }
             else { die "bad line: $_"; }
         } else {
+            
             if (/^\}$/) {
                 $inside = 0;
 
-		$$storePaths2urls{$storePath} = $narurl;
-		$$urls2hashes{$narurl} = $hash;
+                if ($type eq "narfile") {
 
-                foreach my $p (@preds) {
-		    $$successors{$p} = $storePath;
+                    $$narFiles{$storePath} = []
+                        unless defined $$narFiles{$storePath};
+
+                    my $narFileList = $$narFiles{$storePath};
+
+                    my $found = 0;
+                    foreach my $narFile (@{$narFileList}) {
+                        if ($narFile->{url} eq $url) {
+                            if ($narFile->{hash} eq $hash) {
+                                $found = 1;
+                            } else {
+                                die "conflicting hashes for URL $url, " .
+                                    "namely $narFile->{hash} and $hash";
+                            }
+                        }
+                    }
+                    if (!$found) {
+                        push @{$narFileList},
+                            {url => $url, hash => $hash, size => $size};
+                    }
+                
+                    foreach my $p (@preds) {
+                        $$successors{$p} = $storePath;
+                    }
+
+                }
+
+                elsif ($type eq "patch") {
+
+                    $$patches{$storePath} = []
+                        unless defined $$patches{$storePath};
+
+                    my $patchList = $$patches{$storePath};
+
+                    my $found = 0;
+                    foreach my $patch (@{$patchList}) {
+                        if ($patch->{url} eq $url) {
+                            if ($patch->{hash} eq $hash) {
+                                $found = 1 if ($patch->{basePath} eq $basePath);
+                            } else {
+                                die "conflicting hashes for URL $url, " .
+                                    "namely $patch->{hash} and $hash";
+                            }
+                        }
+                    }
+                    if (!$found) {
+                        push @{$patchList},
+                            { url => $url, hash => $hash, size => $size
+                            , basePath => $basePath, baseHash => $baseHash
+                            };
+                    }
+                    
                 }
 
             }
-            elsif (/^\s*StorePath:\s*(\/\S+)\s*$/) {
-                $storePath = $1;
-            }
-            elsif (/^\s*NarURL:\s*(\S+)\s*$/) {
-                $narurl = $1;
-	    }
-            elsif (/^\s*MD5:\s*(\S+)\s*$/) {
-                $hash = $1;
-            }
-            elsif (/^\s*SuccOf:\s*(\/\S+)\s*$/) {
-                push @preds, $1;
-            }
+            
+            elsif (/^\s*StorePath:\s*(\/\S+)\s*$/) { $storePath = $1; }
+            elsif (/^\s*Hash:\s*(\S+)\s*$/) { $hash = $1; }
+            elsif (/^\s*URL:\s*(\S+)\s*$/) { $url = $1; }
+            elsif (/^\s*Size:\s*(\d+)\s*$/) { $size = $1; }
+            elsif (/^\s*SuccOf:\s*(\/\S+)\s*$/) { push @preds, $1; }
+            elsif (/^\s*BasePath:\s*(\/\S+)\s*$/) { $basePath = $1; }
+            elsif (/^\s*BaseHash:\s*(\S+)\s*$/) { $baseHash = $1; }
+            elsif (/^\s*Type:\s*(\S+)\s*$/) { $patchType = $1; }
+
+            # Compatibility;
+            elsif (/^\s*NarURL:\s*(\S+)\s*$/) { $url = $1; }
+            elsif (/^\s*MD5:\s*(\S+)\s*$/) { $hash = $1; }
+            
             else { die "bad line: $_"; }
         }
     }