Make "subtree" part more orthogonal to the rest of merge-recursive.

This makes "subtree" more orthogonal to the rest of recursive merge, so
that you can use subtree and ours/theirs features at the same time.  For
example, you can now say:

	git merge -s subtree -Xtheirs other

to merge with "other" branch while shifting it up or down to match the
shape of the tree of the current branch, and resolving conflicts favoring
the changes "other" branch made over changes made in the current branch.

It also allows the prefix used to shift the trees to be specified using
the "-Xsubtree=$prefix" option.  Giving an empty prefix tells the command
to figure out how much to shift trees automatically as we have always
done.  "merge -s subtree" is the same as "merge -s recursive -Xsubtree="
(or "merge -s recursive -Xsubtree").

Based on an old patch done back in the days when git-merge was a script;
Avery ported the script part to builtin-merge.c.  Bugs in shift_tree()
is mine.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 68 insertions, 1 deletions

diff --git a/match-trees.c b/match-trees.c
index 0fd6df7..26f7ed1 100644
--- a/match-trees.c
+++ b/match-trees.c
@@ -185,7 +185,7 @@ static void match_trees(const unsigned char *hash1,
  * tree object by replacing it with another tree "hash2".
  */
 static int splice_tree(const unsigned char *hash1,
-		       char *prefix,
+		       const char *prefix,
 		       const unsigned char *hash2,
 		       unsigned char *result)
 {
@@ -264,6 +264,13 @@ void shift_tree(const unsigned char *hash1,
 	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);
@@ -301,3 +308,63 @@ void shift_tree(const unsigned char *hash1,
 
 	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(const unsigned char *hash1,
+		   const unsigned char *hash2,
+		   unsigned char *shifted,
+		   const char *shift_prefix)
+{
+	unsigned char sub1[20], sub2[20];
+	unsigned mode1, mode2;
+	unsigned candidate = 0;
+
+	/* Can hash2 be a tree at shift_prefix in tree hash1? */
+	if (!get_tree_entry(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(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 */
+		hashcpy(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.
+		 */
+		hashcpy(shifted, sub2);
+}