--- a/write-tree.c
+++ b/write-tree.c
@@ -4,6 +4,8 @@
  * Copyright (C) Linus Torvalds, 2005
  */
 #include "cache.h"
+static char **dirs;
+static int num_dirs = 0;
 
 static int check_valid_sha1(unsigned char *sha1)
 {
@@ -27,15 +29,47 @@ static int prepend_integer(char *buffer,
 	return i;
 }
 
+static int find_sha(char *buffer, int size, const char *base, int baselen, char *returnsha1)
+{
+	while(size) {
+		int len = strlen(buffer)+1;
+		unsigned char *sha1 = buffer + len;
+		char *path = strchr(buffer, ' ')+1;
+		unsigned int mode;
+
+		if (size < len + 20 || sscanf(buffer, "%o", &mode) != 1)
+			die("corrupt 'tree' file");
+		buffer = sha1 + 20;
+		size -= len + 20;
+		if (strncmp(path, base, baselen) == 0 &&
+		    strlen(path) == baselen) {
+			memcpy(returnsha1, sha1, 20);
+			return 0;
+		}
+	}
+	return -1;
+}
+
 #define ORIG_OFFSET (40)	/* Enough space to add the header of "tree <size>\0" */
 
-static int write_tree(struct cache_entry **cachep, int maxentries, const char *base, int baselen, unsigned char *returnsha1)
+static int write_tree(struct cache_entry **cachep, int maxentries, const char *base, int baselen, unsigned char *returnsha1, char *treesha)
 {
 	unsigned char subdir_sha1[20];
 	unsigned long size, offset;
 	char *buffer;
 	int i, nr;
-
+	char *tree = NULL;
+	unsigned long tree_size;
+	char type[20];
+	if (treesha) {
+		tree = read_sha1_file(treesha, type, &tree_size);
+		if (!tree) {
+			die("unable to read sha1 file");
+		} else {
+		}
+		if (strcmp(type, "tree"))
+			die("expected a tree node");
+	}
 	/* Guess at some random initial size */
 	size = 8192;
 	buffer = malloc(size);
@@ -55,15 +89,60 @@ static int write_tree(struct cache_entry
 
 		sha1 = ce->sha1;
 		mode = ntohl(ce->ce_mode);
-
 		/* Do we have _further_ subdirectories? */
 		filename = pathname + baselen;
 		dirname = strchr(filename, '/');
 		if (dirname) {
 			int subdir_written;
-
-			subdir_written = write_tree(cachep + nr, maxentries - nr, pathname, dirname-pathname+1, subdir_sha1);
-			nr += subdir_written;
+			int dirlen = dirname - pathname;
+			int dirmatch = 1;
+			if (tree && num_dirs > 0) {
+				dirmatch = 0;
+				for(i = 0 ; i < num_dirs; i++) {
+					int len = strlen(dirs[i]);
+					if (len <= baselen)
+						continue;
+					if (memcmp(dirs[i], pathname, dirlen)==0 &&
+					    pathname[dirlen] == '/') {
+						dirmatch = 1;
+						break;
+					}
+				}
+				if (!dirmatch && find_sha(tree, tree_size, 
+							 filename, 
+							 dirname-filename, 
+							 subdir_sha1)) {
+					dirmatch = 1;
+				}
+			}
+			if (!dirmatch) {
+				/* eat all the entries in this dir */
+				while(++nr < maxentries) {
+					char *p;
+					ce = cachep[nr];
+					p = strchr(ce->name + baselen, '/');
+					if (!p)
+						break;
+					if (p - ce->name != dirname-pathname)
+						break;
+					if (memcmp(pathname, ce->name, p-ce->name))
+						break;
+				}
+			} else {
+				unsigned char thisdir_sha1[20];
+				char *p = thisdir_sha1;
+				if (num_dirs && tree) {
+				    if (find_sha(tree, tree_size, filename, 
+				                 dirname-filename, p)) {
+				    	num_dirs = 0;
+					p = NULL;
+				    }
+				} else {
+					p = NULL;
+				}
+				subdir_written = write_tree(cachep + nr, maxentries - nr, pathname, dirname-pathname+1, subdir_sha1, p);
+				nr += subdir_written;
+			}
 
 			/* Now we need to write out the directory entry into this tree.. */
 			mode = S_IFDIR;
@@ -92,9 +172,10 @@ static int write_tree(struct cache_entry
 	i = prepend_integer(buffer, offset - ORIG_OFFSET, ORIG_OFFSET);
 	i -= 5;
 	memcpy(buffer+i, "tree ", 5);
-
 	write_sha1_file(buffer + i, offset - i, returnsha1);
 	free(buffer);
+	if (tree)
+		free(tree);
 	return nr;
 }
 
@@ -103,7 +184,19 @@ int main(int argc, char **argv)
 	int i, unmerged;
 	int entries = read_cache();
 	unsigned char sha1[20];
+	unsigned char treesha1[20];
+	char *p = NULL;
 
+	if (argc > 1) {
+		if (argc < 3)
+			die("usage: write-tree [sha1 dir1 dir2 ...]");
+		num_dirs = argc - 2;
+		dirs = argv + 2;
+		if (get_sha1_hex(argv[1], treesha1) < 0)
+			die("bad sha1 given");
+		p = treesha1;
+
+	}
 	if (entries <= 0)
 		die("write-tree: no cache contents to write");
 
@@ -123,7 +216,7 @@ int main(int argc, char **argv)
 		die("write-tree: not able to write tree");
 
 	/* Ok, write it out */
-	if (write_tree(active_cache, entries, "", 0, sha1) != entries)
+	if (write_tree(active_cache, entries, "", 0, sha1, p) != entries)
 		die("write-tree: internal error");
 	printf("%s\n", sha1_to_hex(sha1));
 	return 0;