thodg/libgit2

diff --git a/include/git2/diff.h b/include/git2/diff.h
index 7f16504..379f469 100644
--- a/include/git2/diff.h
+++ b/include/git2/diff.h
@@ -379,7 +379,7 @@ typedef struct git_diff_patch git_diff_patch;
 typedef enum {
 	/** look for renames? (`--find-renames`) */
 	GIT_DIFF_FIND_RENAMES = (1 << 0),
-	/** consider old size of modified for renames? (`--break-rewrites=N`) */
+	/** consider old side of modified for renames? (`--break-rewrites=N`) */
 	GIT_DIFF_FIND_RENAMES_FROM_REWRITES = (1 << 1),
 
 	/** look for copies? (a la `--find-copies`) */
@@ -897,11 +897,12 @@ GIT_EXTERN(int) git_diff_patch_to_str(
  *
  * NULL is allowed for either `old_blob` or `new_blob` and will be treated
  * as an empty blob, with the `oid` set to NULL in the `git_diff_file` data.
+ * Passing NULL for both blobs is a noop; no callbacks will be made at all.
  *
- * We do run a binary content check on the two blobs and if either of the
- * blobs looks like binary data, the `git_diff_delta` binary attribute will
- * be set to 1 and no call to the hunk_cb nor line_cb will be made (unless
- * you pass `GIT_DIFF_FORCE_TEXT` of course).
+ * We do run a binary content check on the blob content and if either blob
+ * looks like binary data, the `git_diff_delta` binary attribute will be set
+ * to 1 and no call to the hunk_cb nor line_cb will be made (unless you pass
+ * `GIT_DIFF_FORCE_TEXT` of course).
  *
  * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
@@ -921,6 +922,11 @@ GIT_EXTERN(int) git_diff_blobs(
  * so the `git_diff_file` parameters to the callbacks will be faked a la the
  * rules for `git_diff_blobs()`.
  *
+ * Passing NULL for `old_blob` will be treated as an empty blob (i.e. the
+ * `file_cb` will be invoked with GIT_DELTA_ADDED and the diff will be the
+ * entire content of the buffer added).  Passing NULL to the buffer will do
+ * the reverse, with GIT_DELTA_REMOVED and blob content removed.
+ *
  * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_blob_to_buffer(
diff --git a/src/diff_output.c b/src/diff_output.c
index 5e9d5fe..13434be 100644
--- a/src/diff_output.c
+++ b/src/diff_output.c
@@ -456,7 +456,7 @@ static void release_content(git_diff_file *file, git_map *map, git_blob *blob)
 }
 
 
-static void diff_context_init(
+static int diff_context_init(
 	diff_context *ctxt,
 	git_diff_list *diff,
 	git_repository *repo,
@@ -468,6 +468,12 @@ static void diff_context_init(
 {
 	memset(ctxt, 0, sizeof(diff_context));
 
+	if (!repo && diff)
+		repo = diff->repo;
+
+	if (!opts && diff)
+		opts = &diff->opts;
+
 	ctxt->repo = repo;
 	ctxt->diff = diff;
 	ctxt->opts = opts;
@@ -478,6 +484,8 @@ static void diff_context_init(
 	ctxt->error = 0;
 
 	setup_xdiff_options(ctxt->opts, &ctxt->xdiff_config, &ctxt->xdiff_params);
+
+	return 0;
 }
 
 static int diff_delta_file_callback(
@@ -922,6 +930,15 @@ static int diff_patch_line_cb(
 	return 0;
 }
 
+static int diff_required(git_diff_list *diff, const char *action)
+{
+	if (!diff) {
+		giterr_set(GITERR_INVALID, "Must provide valid diff to %s", action);
+		return -1;
+	}
+
+	return 0;
+}
 
 int git_diff_foreach(
 	git_diff_list *diff,
@@ -935,9 +952,12 @@ int git_diff_foreach(
 	size_t idx;
 	git_diff_patch patch;
 
-	diff_context_init(
-		&ctxt, diff, diff->repo, &diff->opts,
-		file_cb, hunk_cb, data_cb, payload);
+	if (diff_required(diff, "git_diff_foreach") < 0)
+		return -1;
+
+	if (diff_context_init(
+			&ctxt, diff, NULL, NULL, file_cb, hunk_cb, data_cb, payload) < 0)
+		return -1;
 
 	diff_patch_init(&ctxt, &patch);
 
@@ -1306,8 +1326,10 @@ static int diff_single_init(
 
 	memset(data, 0, sizeof(*data));
 
-	diff_context_init(
-		&data->ctxt, NULL, repo, opts, file_cb, hunk_cb, data_cb, payload);
+	if (diff_context_init(
+			&data->ctxt, NULL, repo, opts,
+			file_cb, hunk_cb, data_cb, payload) < 0)
+		return -1;
 
 	diff_patch_init(&data->ctxt, &data->patch);
 
@@ -1374,6 +1396,9 @@ int git_diff_blobs(
 		new_blob ? git_object_owner((const git_object *)new_blob) :
 		old_blob ? git_object_owner((const git_object *)old_blob) : NULL;
 
+	if (!repo) /* Hmm, given two NULL blobs, silently do no callbacks? */
+		return 0;
+
 	if ((error = diff_single_init(
 			&d, repo, options, file_cb, hunk_cb, data_cb, payload)) < 0)
 		return error;
@@ -1405,6 +1430,9 @@ int git_diff_blob_to_buffer(
 	git_repository *repo =
 		old_blob ? git_object_owner((const git_object *)old_blob) : NULL;
 
+	if (!repo && !buf) /* Hmm, given NULLs, silently do no callbacks? */
+		return 0;
+
 	if ((error = diff_single_init(
 			&d, repo, options, file_cb, hunk_cb, data_cb, payload)) < 0)
 		return error;
@@ -1453,11 +1481,19 @@ int git_diff_get_patch(
 
 	if (patch_ptr)
 		*patch_ptr = NULL;
+	if (delta_ptr)
+		*delta_ptr = NULL;
+
+	if (diff_required(diff, "git_diff_get_patch") < 0)
+		return -1;
+
+	if (diff_context_init(
+			&ctxt, diff, NULL, NULL,
+			NULL, diff_patch_hunk_cb, diff_patch_line_cb, NULL) < 0)
+		return -1;
 
 	delta = git_vector_get(&diff->deltas, idx);
 	if (!delta) {
-		if (delta_ptr)
-			*delta_ptr = NULL;
 		giterr_set(GITERR_INVALID, "Index out of range for delta in diff");
 		return GIT_ENOTFOUND;
 	}
@@ -1470,10 +1506,6 @@ int git_diff_get_patch(
 		 (diff->opts.flags & GIT_DIFF_SKIP_BINARY_CHECK) != 0))
 		return 0;
 
-	diff_context_init(
-		&ctxt, diff, diff->repo, &diff->opts,
-		NULL, diff_patch_hunk_cb, diff_patch_line_cb, NULL);
-
 	if (git_diff_delta__should_skip(ctxt.opts, delta))
 		return 0;
 
diff --git a/src/diff_tform.c b/src/diff_tform.c
index e051732..48332d3 100644
--- a/src/diff_tform.c
+++ b/src/diff_tform.c
@@ -7,6 +7,7 @@
 #include "common.h"
 #include "diff.h"
 #include "git2/config.h"
+#include "git2/blob.h"
 #include "hashsig.h"
 
 static git_diff_delta *diff_delta__dup(
@@ -362,24 +363,86 @@ on_error:
 	return -1;
 }
 
-typedef struct {
-	/* array of delta index * 2 + (old_file/new_file) -> file hashes */
-	git_hashsig *sigs;
-} diff_similarity_cache;
+GIT_INLINE(git_diff_file *) similarity_get_file(git_diff_list *diff, size_t idx)
+{
+	git_diff_delta *delta = git_vector_get(&diff->deltas, idx / 2);
+	return (idx & 1) ? &delta->new_file : &delta->old_file;
+}
 
-static unsigned int calc_similarity(
-	void *ref, git_diff_file *old_file, git_diff_file *new_file)
+static int similarity_calc(
+	git_diff_list *diff,
+	git_diff_find_options *opts,
+	size_t file_idx,
+	void **cache)
 {
-	diff_similarity_cache *cache = ref;
+	int error = 0;
+	git_diff_file *file = similarity_get_file(diff, file_idx);
+	git_iterator_type_t src = (file_idx & 1) ? diff->old_src : diff->new_src;
+
+	if (src == GIT_ITERATOR_TYPE_WORKDIR) { /* compute hashsig from file */
+		git_buf path = GIT_BUF_INIT;
+
+		/* TODO: apply wd-to-odb filters to file data if necessary */
+
+		if (!(error = git_buf_joinpath(
+				&path, git_repository_workdir(diff->repo), file->path)))
+			error = opts->metric->file_signature(
+				&cache[file_idx], file, path.ptr, opts->metric->payload);
+
+		git_buf_free(&path);
+	} else { /* compute hashsig from blob buffer */
+		git_blob *blob = NULL;
 
-	GIT_UNUSED(cache);
+		/* TODO: add max size threshold a la diff? */
 
-	if (git_oid_cmp(&old_file->oid, &new_file->oid) == 0)
+		if ((error = git_blob_lookup(&blob, diff->repo, &file->oid)) < 0)
+			return error;
+
+		error = opts->metric->buffer_signature(
+			&cache[file_idx], file, git_blob_rawcontent(blob),
+			git_blob_rawsize(blob), opts->metric->payload);
+
+		git_blob_free(blob);
+	}
+
+	return error;
+}
+
+static int similarity_measure(
+	git_diff_list *diff,
+	git_diff_find_options *opts,
+	void **cache,
+	size_t a_idx,
+	size_t b_idx)
+{
+	int score = 0;
+	git_diff_file *a_file = similarity_get_file(diff, a_idx);
+	git_diff_file *b_file = similarity_get_file(diff, b_idx);
+
+	if (GIT_MODE_TYPE(a_file->mode) != GIT_MODE_TYPE(b_file->mode))
+		return 0;
+
+	if (git_oid_cmp(&a_file->oid, &b_file->oid) == 0)
 		return 100;
 
-	/* TODO: insert actual similarity algo here */
+	/* update signature cache if needed */
+	if (!cache[a_idx] && similarity_calc(diff, opts, a_idx, cache) < 0)
+		return -1;
+	if (!cache[b_idx] && similarity_calc(diff, opts, b_idx, cache) < 0)
+		return -1;
 
-	return 0;
+	/* compare signatures */
+	if (opts->metric->similarity(
+			&score, cache[a_idx], cache[b_idx], opts->metric->payload) < 0)
+		return -1;
+
+	/* clip score */
+	if (score < 0)
+		score = 0;
+	else if (score > 100)
+		score = 100;
+
+	return score;
 }
 
 #define FLAG_SET(opts,flag_name) ((opts.flags & flag_name) != 0)
@@ -388,67 +451,85 @@ int git_diff_find_similar(
 	git_diff_list *diff,
 	git_diff_find_options *given_opts)
 {
-	unsigned int i, j, similarity;
+	size_t i, j, cache_size, *matches;
+	int error = 0, similarity;
 	git_diff_delta *from, *to;
 	git_diff_find_options opts;
-	unsigned int tried_targets, num_changes = 0;
-	git_vector matches = GIT_VECTOR_INIT;
+	size_t tried_targets, num_rewrites = 0;
+	void **cache;
 
-	if (normalize_find_opts(diff, &opts, given_opts) < 0)
-		return -1;
+	if ((error = normalize_find_opts(diff, &opts, given_opts)) < 0)
+		return error;
 
-	/* first do splits if requested */
+	/* TODO: maybe abort if deltas.length > target_limit ??? */
+
+	cache_size = diff->deltas.length * 2; /* must store b/c length may change */
+	cache = git__calloc(cache_size, sizeof(void *));
+	GITERR_CHECK_ALLOC(cache);
+
+	matches = git__calloc(diff->deltas.length, sizeof(size_t));
+	GITERR_CHECK_ALLOC(matches);
+
+	/* first break MODIFIED records that are too different (if requested) */
 
 	if (FLAG_SET(opts, GIT_DIFF_FIND_AND_BREAK_REWRITES)) {
 		git_vector_foreach(&diff->deltas, i, from) {
 			if (from->status != GIT_DELTA_MODIFIED)
 				continue;
 
-			/* Right now, this doesn't work right because the similarity
-			 * algorithm isn't actually implemented...
-			 */
-			similarity = 100;
-			/* calc_similarity(NULL, &from->old_file, from->new_file); */
+			similarity = similarity_measure(
+				diff, &opts, cache, 2 * i, 2 * i + 1);
 
-			if (similarity < opts.break_rewrite_threshold) {
+			if (similarity < 0) {
+				error = similarity;
+				goto cleanup;
+			}
+
+			if ((unsigned int)similarity < opts.break_rewrite_threshold) {
 				from->flags |= GIT_DIFF_FLAG__TO_SPLIT;
-				num_changes++;
+				num_rewrites++;
 			}
 		}
-
-		/* apply splits as needed */
-		if (num_changes > 0 &&
-			apply_splits_and_deletes(
-				diff, diff->deltas.length + num_changes) < 0)
-			return -1;
 	}
 
 	/* next find the most similar delta for each rename / copy candidate */
 
-	if (git_vector_init(&matches, diff->deltas.length, git_diff_delta__cmp) < 0)
-		return -1;
-
 	git_vector_foreach(&diff->deltas, i, from) {
 		tried_targets = 0;
 
+		/* skip things that aren't blobs */
+		if (GIT_MODE_TYPE(from->old_file.mode) !=
+			GIT_MODE_TYPE(GIT_FILEMODE_BLOB))
+			continue;
+
 		git_vector_foreach(&diff->deltas, j, to) {
 			if (i == j)
 				continue;
 
+			/* skip things that aren't blobs */
+			if (GIT_MODE_TYPE(to->new_file.mode) !=
+				GIT_MODE_TYPE(GIT_FILEMODE_BLOB))
+				continue;
+
 			switch (to->status) {
 			case GIT_DELTA_ADDED:
 			case GIT_DELTA_UNTRACKED:
 			case GIT_DELTA_RENAMED:
 			case GIT_DELTA_COPIED:
 				break;
+			case GIT_DELTA_MODIFIED:
+				if ((to->flags & GIT_DIFF_FLAG__TO_SPLIT) == 0)
+					continue;
+				break;
 			default:
 				/* only the above status values should be checked */
 				continue;
 			}
 
 			/* skip all but DELETED files unless copy detection is on */
-			if (from->status != GIT_DELTA_DELETED &&
-				!FLAG_SET(opts, GIT_DIFF_FIND_COPIES))
+			if (!FLAG_SET(opts, GIT_DIFF_FIND_COPIES) &&
+				from->status != GIT_DELTA_DELETED &&
+				(from->flags & GIT_DIFF_FLAG__TO_SPLIT) == 0)
 				continue;
 
 			/* don't check UNMODIFIED files as source unless given option */
@@ -463,34 +544,44 @@ int git_diff_find_similar(
 			/* calculate similarity and see if this pair beats the
 			 * similarity score of the current best pair.
 			 */
-			similarity = calc_similarity(NULL, &from->old_file, &to->new_file);
+			similarity = similarity_measure(
+				diff, &opts, cache, 2 * i, 2 * j + 1);
+
+			if (similarity < 0) {
+				error = similarity;
+				goto cleanup;
+			}
 
-			if (to->similarity < similarity) {
-				to->similarity = similarity;
-				if (git_vector_set(NULL, &matches, j, from) < 0)
-					return -1;
+			if (to->similarity < (unsigned int)similarity) {
+				to->similarity = (unsigned int)similarity;
+				matches[j] = i + 1;
 			}
 		}
 	}
 
 	/* next rewrite the diffs with renames / copies */
 
-	num_changes = 0;
+	num_rewrites = 0;
 
 	git_vector_foreach(&diff->deltas, j, to) {
-		from = GIT_VECTOR_GET(&matches, j);
-		if (!from) {
+		if (!matches[j]) {
 			assert(to->similarity == 0);
 			continue;
 		}
 
-		/* three possible outcomes here:
+		i = matches[j] - 1;
+		from = GIT_VECTOR_GET(&diff->deltas, i);
+		assert(from);
+
+		/* four possible outcomes here:
 		 * 1. old DELETED and if over rename threshold,
 		 *    new becomes RENAMED and old goes away
-		 * 2. old was MODIFIED but FIND_RENAMES_FROM_REWRITES is on and
+		 * 2. old SPLIT and if over rename threshold,
+		 *    new becomes RENAMED and old becomes ADDED (clear SPLIT)
+		 * 3. old was MODIFIED but FIND_RENAMES_FROM_REWRITES is on and
 		 *    old is more similar to new than it is to itself, in which
 		 *    case, new becomes RENAMED and old becomed ADDED
-		 * 3. otherwise if over copy threshold, new becomes COPIED
+		 * 4. otherwise if over copy threshold, new becomes COPIED
 		 */
 
 		if (from->status == GIT_DELTA_DELETED) {
@@ -503,7 +594,26 @@ int git_diff_find_similar(
 			memcpy(&to->old_file, &from->old_file, sizeof(to->old_file));
 
 			from->flags |= GIT_DIFF_FLAG__TO_DELETE;
-			num_changes++;
+			num_rewrites++;
+
+			continue;
+		}
+
+		if (from->status == GIT_DELTA_MODIFIED &&
+			(from->flags & GIT_DIFF_FLAG__TO_SPLIT) != 0)
+		{
+			if (to->similarity < opts.rename_threshold) {
+				to->similarity = 0;
+				continue;
+			}
+
+			to->status = GIT_DELTA_RENAMED;
+			memcpy(&to->old_file, &from->old_file, sizeof(to->old_file));
+
+			from->status = GIT_DELTA_ADDED;
+			from->flags &= ~GIT_DIFF_FLAG__TO_SPLIT;
+			memset(&from->old_file, 0, sizeof(from->old_file));
+			num_rewrites--;
 
 			continue;
 		}
@@ -512,10 +622,15 @@ int git_diff_find_similar(
 			FLAG_SET(opts, GIT_DIFF_FIND_RENAMES_FROM_REWRITES) &&
 			to->similarity > opts.rename_threshold)
 		{
-			similarity = 100;
-			/* calc_similarity(NULL, &from->old_file, from->new_file); */
+			similarity = similarity_measure(
+				diff, &opts, cache, 2 * i, 2 * i + 1);
+
+			if (similarity < 0) {
+				error = similarity;
+				goto cleanup;
+			}
 
-			if (similarity < opts.rename_from_rewrite_threshold) {
+			if ((unsigned int)similarity < opts.rename_from_rewrite_threshold) {
 				to->status = GIT_DELTA_RENAMED;
 				memcpy(&to->old_file, &from->old_file, sizeof(to->old_file));
 
@@ -538,17 +653,23 @@ int git_diff_find_similar(
 		memcpy(&to->old_file, &from->old_file, sizeof(to->old_file));
 	}
 
-	git_vector_free(&matches);
+	if (num_rewrites > 0) {
+		assert(num_rewrites < diff->deltas.length);
 
-	if (num_changes > 0) {
-		assert(num_changes < diff->deltas.length);
+		error = apply_splits_and_deletes(
+			diff, diff->deltas.length - num_rewrites);
+	}
+
+cleanup:
+	git__free(matches);
 
-		if (apply_splits_and_deletes(
-				diff, diff->deltas.length - num_changes) < 0)
-			return -1;
+	for (i = 0; i < cache_size; ++i) {
+		if (cache[i] != NULL)
+			opts.metric->free_signature(cache[i], opts.metric->payload);
 	}
+	git__free(cache);
 
-	return 0;
+	return error;
 }
 
 #undef FLAG_SET
diff --git a/tests-clar/diff/blob.c b/tests-clar/diff/blob.c
index a6950b8..2ac8dbc 100644
--- a/tests-clar/diff/blob.c
+++ b/tests-clar/diff/blob.c
@@ -196,7 +196,7 @@ void test_diff_blob__can_compare_identical_blobs(void)
 		NULL, NULL, &opts, diff_file_cb, diff_hunk_cb, diff_line_cb, &expected));
 
 	cl_assert_equal_i(0, expected.files_binary);
-	assert_identical_blobs_comparison(&expected);
+	cl_assert_equal_i(0, expected.files); /* NULLs mean no callbacks, period */
 
 	memset(&expected, 0, sizeof(expected));
 	cl_git_pass(git_diff_blobs(
@@ -399,7 +399,7 @@ void test_diff_blob__can_compare_blob_to_buffer(void)
 
 	assert_identical_blobs_comparison(&expected);
 
-	/* diff from NULL blob to content of b */
+	/* diff from NULL blob to content of a */
 	memset(&expected, 0, sizeof(expected));
 	cl_git_pass(git_diff_blob_to_buffer(
 		NULL, a_content, strlen(a_content),