Add "diff-tree" program to show which files have changed between two trees.

Very useful for creating diffs efficiently, and in general to see what has
changed in the namespace.

Makefile

@@ -2,7 +2,7 @@ CFLAGS=-g -O3 -Wall
 CC=gcc
 
 PROG=	update-cache show-diff init-db write-tree read-tree commit-tree \
-	cat-file fsck-cache checkout-cache
+	cat-file fsck-cache checkout-cache diff-tree
 
 all: $(PROG)
 
@@ -37,6 +37,9 @@ fsck-cache: fsck-cache.o read-cache.o
 checkout-cache: checkout-cache.o read-cache.o
 	$(CC) $(CFLAGS) -o checkout-cache checkout-cache.o read-cache.o $(LIBS)
 
+diff-tree: diff-tree.o read-cache.o
+	$(CC) $(CFLAGS) -o diff-tree diff-tree.o read-cache.o $(LIBS)
+
 read-cache.o: cache.h
 show-diff.o: cache.h
 

diff-tree.c

@@ -0,0 +1,109 @@
+#include "cache.h"
+
+struct tree_entry;
+
+static struct tree_entry *update_tree_entry(void **bufp, unsigned long *sizep)
+{
+	void *buf = *bufp;
+	unsigned long size = *sizep;
+	int len = strlen(buf) + 1 + 20;
+
+	if (size < len)
+		usage("corrupt tree file");
+	*bufp = buf + len;
+	*sizep = size - len;
+	return buf;
+}
+
+static const unsigned char *extract(void *tree, unsigned long size, const char **pathp, unsigned int *modep)
+{
+	int len = strlen(tree)+1;
+	const unsigned char *sha1 = tree + len;
+	const char *path = strchr(tree, ' ');
+
+	if (!path || size < len + 20 || sscanf(tree, "%o", modep) != 1)
+		usage("corrupt tree file");
+	*pathp = path+1;
+	return sha1;
+}
+
+static void show_file(const char *prefix, void *tree, unsigned long size)
+{
+	unsigned mode;
+	const char *path;
+	const unsigned char *sha1 = extract(tree, size, &path, &mode);
+	printf("%s%o %s %s%c", prefix, mode, sha1_to_hex(sha1), path, 0);
+}
+
+static int compare_tree_entry(void *tree1, unsigned long size1, void *tree2, unsigned long size2)
+{
+	unsigned mode1, mode2;
+	const char *path1, *path2;
+	const unsigned char *sha1, *sha2;
+	int cmp;
+
+	sha1 = extract(tree1, size1, &path1, &mode1);
+	sha2 = extract(tree2, size2, &path2, &mode2);
+
+	cmp = cache_name_compare(path1, strlen(path1), path2, strlen(path2));
+	if (cmp < 0) {
+		show_file("-", tree1, size1);
+		return -1;
+	}
+	if (cmp > 0) {
+		show_file("+", tree2, size2);
+		return 1;
+	}
+	if (!memcmp(sha1, sha2, 20) && mode1 == mode2)
+		return 0;
+	show_file("<", tree1, size1);
+	show_file(">", tree2, size2);
+	return 0;
+}
+
+static int diff_tree(void *tree1, unsigned long size1, void *tree2, unsigned long size2)
+{
+	while (size1 | size2) {
+		if (!size1) {
+			show_file("+", tree2, size2);
+			update_tree_entry(&tree2, &size2);
+			continue;
+		}
+		if (!size2) {
+			show_file("-", tree1, size1);
+			update_tree_entry(&tree1, &size1);
+			continue;
+		}
+		switch (compare_tree_entry(tree1, size1, tree2, size2)) {
+		case -1:
+			update_tree_entry(&tree1, &size1);
+			continue;
+		case 0:
+			update_tree_entry(&tree1, &size1);
+			/* Fallthrough */
+		case 1:
+			update_tree_entry(&tree2, &size2);
+			continue;
+		}
+		usage("diff-tree: internal error");
+	}
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	unsigned char old[20], new[20];
+	void *tree1, *tree2;
+	unsigned long size1, size2;
+	char type[20];
+
+	if (argc != 3 || get_sha1_hex(argv[1], old) || get_sha1_hex(argv[2], new))
+		usage("diff-tree <tree sha1> <tree sha1>");
+	tree1 = read_sha1_file(old, type, &size1);
+	if (!tree1 || strcmp(type, "tree"))
+		usage("unable to read source tree");
+	tree2 = read_sha1_file(new, type, &size2);
+	if (!tree2 || strcmp(type, "tree"))
+		usage("unable to read destination tree");
+	return diff_tree(tree1, size1, tree2, size2);
+}