Add "merge-tree" helper program. Maybe it's retarded, maybe it's helpful.

It only works one directory level at a time, so lookout..

Makefile

@@ -14,7 +14,7 @@ CC=gcc
 
 PROG=   update-cache show-diff init-db write-tree read-tree commit-tree \
 	cat-file fsck-cache checkout-cache diff-tree rev-tree show-files \
-	check-files ls-tree
+	check-files ls-tree merge-tree
 
 all: $(PROG)
 
@@ -64,6 +64,9 @@ check-files: check-files.o read-cache.o
 ls-tree: ls-tree.o read-cache.o
 	$(CC) $(CFLAGS) -o ls-tree ls-tree.o read-cache.o $(LIBS)
 
+merge-tree: merge-tree.o read-cache.o
+	$(CC) $(CFLAGS) -o merge-tree merge-tree.o read-cache.o $(LIBS)
+
 read-cache.o: cache.h
 show-diff.o: cache.h
 

merge-tree.c

@@ -0,0 +1,128 @@
+#include "cache.h"
+
+struct tree_entry {
+	unsigned mode;
+	unsigned char *sha1;
+	char *path;
+	struct tree_entry *next;
+};
+
+static struct tree_entry *read_tree(unsigned char *sha1)
+{
+	char type[20];
+	unsigned long size;
+	void *buf = read_sha1_file(sha1, type, &size);
+	struct tree_entry *ret = NULL, **tp = &ret;
+
+	if (!buf || strcmp(type, "tree"))
+		die("unable to read 'tree' object %s", sha1_to_hex(sha1));
+	while (size) {
+		int len = strlen(buf)+1;
+		struct tree_entry * entry = malloc(sizeof(struct tree_entry));
+		if (size < len + 20 || sscanf(buf, "%o", &entry->mode) != 1)
+			die("corrupt 'tree' object %s", sha1_to_hex(sha1));
+		entry->path = strchr(buf, ' ')+1;
+		entry->sha1 = buf + len;
+		entry->next = NULL;
+		*tp = entry;
+		tp = &entry->next;
+		len += 20;
+		buf += len;
+		size -= len;
+	}
+	return ret;
+}
+
+static void show(const struct tree_entry *a, const char *path)
+{
+	printf("select %o %s %s%c", a->mode, sha1_to_hex(a->sha1), path, 0);
+}
+
+static void merge(const struct tree_entry *a, const struct tree_entry *b, const struct tree_entry *c, const char *path)
+{
+	char hex_a[60], hex_b[60], hex_c[60];
+	strcpy(hex_a, sha1_to_hex(a->sha1));
+	strcpy(hex_b, sha1_to_hex(b->sha1));
+	strcpy(hex_c, sha1_to_hex(c->sha1));
+	printf("merge %o->%o,%o %s->%s,%s %s%c",
+		a->mode, b->mode, c->mode,
+		hex_a, hex_b, hex_c, path, 0);
+}
+
+static int same(const struct tree_entry *a, const struct tree_entry *b)
+{
+	return a->mode == b->mode && !memcmp(a->sha1, b->sha1, 20);
+}
+
+static void merge_entry(const struct tree_entry *src, const struct tree_entry *dst1, const struct tree_entry *dst2)
+{
+	static unsigned char nullsha1[20];
+	static const struct tree_entry none = { 0, nullsha1, "", NULL };
+	const char *path = NULL;
+	const struct tree_entry *a, *b, *c;
+
+	a = &none;
+	b = &none;
+	c = &none;
+	if (src) { a = src; path = src->path; }
+	if (dst1) { b = dst1; path = dst1->path; }
+	if (dst2) { c = dst2; path = dst2->path; }
+	if (same(b, c)) {
+		show(b, path);
+		return;
+	}
+	if (same(a, b)) {
+		show(c, path);
+		return;
+	}
+	if (same(a, c)) {
+		show(b, path);
+		return;
+	}
+	merge(a, b, c, path);
+}
+
+/* For two entries, select the smaller one, clear the bigger one */
+static void smaller(struct tree_entry **ap, struct tree_entry **bp)
+{
+	struct tree_entry *a = *ap, *b = *bp;
+	if (a && b) {
+		int cmp = cache_name_compare(a->path, strlen(a->path), b->path, strlen(b->path));
+		if (cmp) {
+			if (cmp < 0)
+				*bp = NULL;
+			else
+				*ap = NULL;
+		}
+	}
+}
+
+static void merge_tree(struct tree_entry *src, struct tree_entry *dst1, struct tree_entry *dst2)
+{
+	while (src || dst1 || dst2) {
+		struct tree_entry *a, *b, *c;
+		a = src;
+		b = dst1;
+		c = dst2;
+		smaller(&a,&b);
+		smaller(&a,&c);
+		smaller(&b,&c);
+		if (a) src = a->next;
+		if (b) dst1 = b->next;
+		if (c) dst2 = c->next;
+		merge_entry(a,b,c);
+	}
+}
+
+int main(int argc, char **argv)
+{
+	unsigned char src[20], dst1[20], dst2[20];
+
+	if (argc != 4 ||
+	    get_sha1_hex(argv[1], src) ||
+	    get_sha1_hex(argv[2], dst1) ||
+	    get_sha1_hex(argv[3], dst2))
+		usage("merge-tree <src> <dst1> <dst2>");
+	merge_tree(read_tree(src), read_tree(dst1), read_tree(dst2));
+	return 0;
+}