1 files changed, 365 insertions, 0 deletions
diff --git a/match-trees.c b/match-trees.c
new file mode 100644
index 000000000000..f6c194c1cca6
--- /dev/null
+++ b/match-trees.c
@@ -0,0 +1,365 @@
+#include "cache.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "object-store.h"
+
+static int score_missing(unsigned mode)
+{
+	int score;
+
+	if (S_ISDIR(mode))
+		score = -1000;
+	else if (S_ISLNK(mode))
+		score = -500;
+	else
+		score = -50;
+	return score;
+}
+
+static int score_differs(unsigned mode1, unsigned mode2)
+{
+	int score;
+
+	if (S_ISDIR(mode1) != S_ISDIR(mode2))
+		score = -100;
+	else if (S_ISLNK(mode1) != S_ISLNK(mode2))
+		score = -50;
+	else
+		score = -5;
+	return score;
+}
+
+static int score_matches(unsigned mode1, unsigned mode2)
+{
+	int score;
+
+	/* Heh, we found SHA-1 collisions between different kind of objects */
+	if (S_ISDIR(mode1) != S_ISDIR(mode2))
+		score = -100;
+	else if (S_ISLNK(mode1) != S_ISLNK(mode2))
+		score = -50;
+
+	else if (S_ISDIR(mode1))
+		score = 1000;
+	else if (S_ISLNK(mode1))
+		score = 500;
+	else
+		score = 250;
+	return score;
+}
+
+static void *fill_tree_desc_strict(struct tree_desc *desc,
+				   const struct object_id *hash)
+{
+	void *buffer;
+	enum object_type type;
+	unsigned long size;
+
+	buffer = read_object_file(hash, &type, &size);
+	if (!buffer)
+		die("unable to read tree (%s)", oid_to_hex(hash));
+	if (type != OBJ_TREE)
+		die("%s is not a tree", oid_to_hex(hash));
+	init_tree_desc(desc, buffer, size);
+	return buffer;
+}
+
+static int base_name_entries_compare(const struct name_entry *a,
+				     const struct name_entry *b)
+{
+	return base_name_compare(a->path, tree_entry_len(a), a->mode,
+				 b->path, tree_entry_len(b), b->mode);
+}
+
+/*
+ * Inspect two trees, and give a score that tells how similar they are.
+ */
+static int score_trees(const struct object_id *hash1, const struct object_id *hash2)
+{
+	struct tree_desc one;
+	struct tree_desc two;
+	void *one_buf = fill_tree_desc_strict(&one, hash1);
+	void *two_buf = fill_tree_desc_strict(&two, hash2);
+	int score = 0;
+
+	for (;;) {
+		int cmp;
+
+		if (one.size && two.size)
+			cmp = base_name_entries_compare(&one.entry, &two.entry);
+		else if (one.size)
+			/* two lacks this entry */
+			cmp = -1;
+		else if (two.size)
+			/* two has more entries */
+			cmp = 1;
+		else
+			break;
+
+		if (cmp < 0) {
+			/* path1 does not appear in two */
+			score += score_missing(one.entry.mode);
+			update_tree_entry(&one);
+		} else if (cmp > 0) {
+			/* path2 does not appear in one */
+			score += score_missing(two.entry.mode);
+			update_tree_entry(&two);
+		} else {
+			/* path appears in both */
+			if (!oideq(&one.entry.oid, &two.entry.oid)) {
+				/* they are different */
+				score += score_differs(one.entry.mode,
+						       two.entry.mode);
+			} else {
+				/* same subtree or blob */
+				score += score_matches(one.entry.mode,
+						       two.entry.mode);
+			}
+			update_tree_entry(&one);
+			update_tree_entry(&two);
+		}
+	}
+	free(one_buf);
+	free(two_buf);
+	return score;
+}
+
+/*
+ * Match one itself and its subtrees with two and pick the best match.
+ */
+static void match_trees(const struct object_id *hash1,
+			const struct object_id *hash2,
+			int *best_score,
+			char **best_match,
+			const char *base,
+			int recurse_limit)
+{
+	struct tree_desc one;
+	void *one_buf = fill_tree_desc_strict(&one, hash1);
+
+	while (one.size) {
+		const char *path;
+		const struct object_id *elem;
+		unsigned short mode;
+		int score;
+
+		elem = tree_entry_extract(&one, &path, &mode);
+		if (!S_ISDIR(mode))
+			goto next;
+		score = score_trees(elem, hash2);
+		if (*best_score < score) {
+			free(*best_match);
+			*best_match = xstrfmt("%s%s", base, path);
+			*best_score = score;
+		}
+		if (recurse_limit) {
+			char *newbase = xstrfmt("%s%s/", base, path);
+			match_trees(elem, hash2, best_score, best_match,
+				    newbase, recurse_limit - 1);
+			free(newbase);
+		}
+
+	next:
+		update_tree_entry(&one);
+	}
+	free(one_buf);
+}
+
+/*
+ * A tree "oid1" has a subdirectory at "prefix".  Come up with a tree object by
+ * replacing it with another tree "oid2".
+ */
+static int splice_tree(const struct object_id *oid1, const char *prefix,
+		       const struct object_id *oid2, struct object_id *result)
+{
+	char *subpath;
+	int toplen;
+	char *buf;
+	unsigned long sz;
+	struct tree_desc desc;
+	unsigned char *rewrite_here;
+	const struct object_id *rewrite_with;
+	struct object_id subtree;
+	enum object_type type;
+	int status;
+
+	subpath = strchrnul(prefix, '/');
+	toplen = subpath - prefix;
+	if (*subpath)
+		subpath++;
+
+	buf = read_object_file(oid1, &type, &sz);
+	if (!buf)
+		die("cannot read tree %s", oid_to_hex(oid1));
+	init_tree_desc(&desc, buf, sz);
+
+	rewrite_here = NULL;
+	while (desc.size) {
+		const char *name;
+		unsigned short mode;
+
+		tree_entry_extract(&desc, &name, &mode);
+		if (strlen(name) == toplen &&
+		    !memcmp(name, prefix, toplen)) {
+			if (!S_ISDIR(mode))
+				die("entry %s in tree %s is not a tree", name,
+				    oid_to_hex(oid1));
+
+			/*
+			 * We cast here for two reasons:
+			 *
+			 *   - to flip the "char *" (for the path) to "unsigned
+			 *     char *" (for the hash stored after it)
+			 *
+			 *   - to discard the "const"; this is OK because we
+			 *     know it points into our non-const "buf"
+			 */
+			rewrite_here = (unsigned char *)(desc.entry.path +
+							 strlen(desc.entry.path) +
+							 1);
+			break;
+		}
+		update_tree_entry(&desc);
+	}
+	if (!rewrite_here)
+		die("entry %.*s not found in tree %s", toplen, prefix,
+		    oid_to_hex(oid1));
+	if (*subpath) {
+		struct object_id tree_oid;
+		hashcpy(tree_oid.hash, rewrite_here);
+		status = splice_tree(&tree_oid, subpath, oid2, &subtree);
+		if (status)
+			return status;
+		rewrite_with = &subtree;
+	} else {
+		rewrite_with = oid2;
+	}
+	hashcpy(rewrite_here, rewrite_with->hash);
+	status = write_object_file(buf, sz, tree_type, result);
+	free(buf);
+	return status;
+}
+
+/*
+ * We are trying to come up with a merge between one and two that
+ * results in a tree shape similar to one.  The tree two might
+ * correspond to a subtree of one, in which case it needs to be
+ * shifted down by prefixing otherwise empty directories.  On the
+ * other hand, it could cover tree one and we might need to pick a
+ * subtree of it.
+ */
+void shift_tree(struct repository *r,
+		const struct object_id *hash1,
+		const struct object_id *hash2,
+		struct object_id *shifted,
+		int depth_limit)
+{
+	char *add_prefix;
+	char *del_prefix;
+	int add_score, del_score;
+
+	/*
+	 * NEEDSWORK: this limits the recursion depth to hardcoded
+	 * value '2' to avoid excessive overhead.
+	 */
+	if (!depth_limit)
+		depth_limit = 2;
+
+	add_score = del_score = score_trees(hash1, hash2);
+	add_prefix = xcalloc(1, 1);
+	del_prefix = xcalloc(1, 1);
+
+	/*
+	 * See if one's subtree resembles two; if so we need to prefix
+	 * two with a few fake trees to match the prefix.
+	 */
+	match_trees(hash1, hash2, &add_score, &add_prefix, "", depth_limit);
+
+	/*
+	 * See if two's subtree resembles one; if so we need to
+	 * pick only subtree of two.
+	 */
+	match_trees(hash2, hash1, &del_score, &del_prefix, "", depth_limit);
+
+	/* Assume we do not have to do any shifting */
+	oidcpy(shifted, hash2);
+
+	if (add_score < del_score) {
+		/* We need to pick a subtree of two */
+		unsigned short mode;
+
+		if (!*del_prefix)
+			return;
+
+		if (get_tree_entry(r, hash2, del_prefix, shifted, &mode))
+			die("cannot find path %s in tree %s",
+			    del_prefix, oid_to_hex(hash2));
+		return;
+	}
+
+	if (!*add_prefix)
+		return;
+
+	splice_tree(hash1, add_prefix, hash2, shifted);
+}
+
+/*
+ * The user says the trees will be shifted by this much.
+ * Unfortunately we cannot fundamentally tell which one to
+ * be prefixed, as recursive merge can work in either direction.
+ */
+void shift_tree_by(struct repository *r,
+		   const struct object_id *hash1,
+		   const struct object_id *hash2,
+		   struct object_id *shifted,
+		   const char *shift_prefix)
+{
+	struct object_id sub1, sub2;
+	unsigned short mode1, mode2;
+	unsigned candidate = 0;
+
+	/* Can hash2 be a tree at shift_prefix in tree hash1? */
+	if (!get_tree_entry(r, hash1, shift_prefix, &sub1, &mode1) &&
+	    S_ISDIR(mode1))
+		candidate |= 1;
+
+	/* Can hash1 be a tree at shift_prefix in tree hash2? */
+	if (!get_tree_entry(r, hash2, shift_prefix, &sub2, &mode2) &&
+	    S_ISDIR(mode2))
+		candidate |= 2;
+
+	if (candidate == 3) {
+		/* Both are plausible -- we need to evaluate the score */
+		int best_score = score_trees(hash1, hash2);
+		int score;
+
+		candidate = 0;
+		score = score_trees(&sub1, hash2);
+		if (score > best_score) {
+			candidate = 1;
+			best_score = score;
+		}
+		score = score_trees(&sub2, hash1);
+		if (score > best_score)
+			candidate = 2;
+	}
+
+	if (!candidate) {
+		/* Neither is plausible -- do not shift */
+		oidcpy(shifted, hash2);
+		return;
+	}
+
+	if (candidate == 1)
+		/*
+		 * shift tree2 down by adding shift_prefix above it
+		 * to match tree1.
+		 */
+		splice_tree(hash1, shift_prefix, hash2, shifted);
+	else
+		/*
+		 * shift tree2 up by removing shift_prefix from it
+		 * to match tree1.
+		 */
+		oidcpy(shifted, &sub2);
+}