blame: optionally track line fingerprints during fill_blame_origin()

fill_blame_origin() is a convenient place to store data that we will use
throughout the lifetime of a blame_origin.  Some heuristics for
ignoring commits during a blame session can make use of this storage.
In particular, we will calculate a fingerprint for each line of a file
for blame_origins involved in an ignored commit.

In this commit, we only calculate the line_starts, reusing the existing
code from the scoreboard's line_starts.  In an upcoming commit, we will
actually compute the fingerprints.

This feature will be used when we attempt to pass blame entries to
parents when we "ignore" a commit.  Most uses of fill_blame_origin()
will not require this feature, hence the flag parameter.  Multiple calls
to fill_blame_origin() are idempotent, and any of them can request the
creation of the fingerprints structure.

Suggested-by: Michael Platings <michael@platin.gs>
Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

blame.c

@@ -310,12 +310,58 @@ static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b,
 	return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
 }
 
+static const char *get_next_line(const char *start, const char *end)
+{
+	const char *nl = memchr(start, '\n', end - start);
+
+	return nl ? nl + 1 : end;
+}
+
+static int find_line_starts(int **line_starts, const char *buf,
+			    unsigned long len)
+{
+	const char *end = buf + len;
+	const char *p;
+	int *lineno;
+	int num = 0;
+
+	for (p = buf; p < end; p = get_next_line(p, end))
+		num++;
+
+	ALLOC_ARRAY(*line_starts, num + 1);
+	lineno = *line_starts;
+
+	for (p = buf; p < end; p = get_next_line(p, end))
+		*lineno++ = p - buf;
+
+	*lineno = len;
+
+	return num;
+}
+
+static void fill_origin_fingerprints(struct blame_origin *o, mmfile_t *file)
+{
+	int *line_starts;
+
+	if (o->fingerprints)
+		return;
+	o->num_lines = find_line_starts(&line_starts, o->file.ptr,
+					o->file.size);
+	/* TODO: Will fill in fingerprints in a future commit */
+	free(line_starts);
+}
+
+static void drop_origin_fingerprints(struct blame_origin *o)
+{
+}
+
 /*
  * Given an origin, prepare mmfile_t structure to be used by the
  * diff machinery
  */
 static void fill_origin_blob(struct diff_options *opt,
-			     struct blame_origin *o, mmfile_t *file, int *num_read_blob)
+			     struct blame_origin *o, mmfile_t *file,
+			     int *num_read_blob, int fill_fingerprints)
 {
 	if (!o->file.ptr) {
 		enum object_type type;
@@ -339,11 +385,14 @@ static void fill_origin_blob(struct diff_options *opt,
 	}
 	else
 		*file = o->file;
+	if (fill_fingerprints)
+		fill_origin_fingerprints(o, file);
 }
 
 static void drop_origin_blob(struct blame_origin *o)
 {
 	FREE_AND_NULL(o->file.ptr);
+	drop_origin_fingerprints(o);
 }
 
 /*
@@ -1140,8 +1189,10 @@ static void pass_blame_to_parent(struct blame_scoreboard *sb,
 	d.ignore_diffs = ignore_diffs;
 	d.dstq = &newdest; d.srcq = &target->suspects;
 
-	fill_origin_blob(&sb->revs->diffopt, parent, &file_p, &sb->num_read_blob);
-	fill_origin_blob(&sb->revs->diffopt, target, &file_o, &sb->num_read_blob);
+	fill_origin_blob(&sb->revs->diffopt, parent, &file_p,
+			 &sb->num_read_blob, ignore_diffs);
+	fill_origin_blob(&sb->revs->diffopt, target, &file_o,
+			 &sb->num_read_blob, ignore_diffs);
 	sb->num_get_patch++;
 
 	if (diff_hunks(&file_p, &file_o, blame_chunk_cb, &d, sb->xdl_opts))
@@ -1352,7 +1403,8 @@ static void find_move_in_parent(struct blame_scoreboard *sb,
 	if (!unblamed)
 		return; /* nothing remains for this target */
 
-	fill_origin_blob(&sb->revs->diffopt, parent, &file_p, &sb->num_read_blob);
+	fill_origin_blob(&sb->revs->diffopt, parent, &file_p,
+			 &sb->num_read_blob, 0);
 	if (!file_p.ptr)
 		return;
 
@@ -1481,7 +1533,8 @@ static void find_copy_in_parent(struct blame_scoreboard *sb,
 			norigin = get_origin(parent, p->one->path);
 			oidcpy(&norigin->blob_oid, &p->one->oid);
 			norigin->mode = p->one->mode;
-			fill_origin_blob(&sb->revs->diffopt, norigin, &file_p, &sb->num_read_blob);
+			fill_origin_blob(&sb->revs->diffopt, norigin, &file_p,
+					 &sb->num_read_blob, 0);
 			if (!file_p.ptr)
 				continue;
 
@@ -1820,37 +1873,14 @@ void assign_blame(struct blame_scoreboard *sb, int opt)
 	}
 }
 
-static const char *get_next_line(const char *start, const char *end)
-{
-	const char *nl = memchr(start, '\n', end - start);
-	return nl ? nl + 1 : end;
-}
-
 /*
  * To allow quick access to the contents of nth line in the
  * final image, prepare an index in the scoreboard.
  */
 static int prepare_lines(struct blame_scoreboard *sb)
 {
-	const char *buf = sb->final_buf;
-	unsigned long len = sb->final_buf_size;
-	const char *end = buf + len;
-	const char *p;
-	int *lineno;
-	int num = 0;
-
-	for (p = buf; p < end; p = get_next_line(p, end))
-		num++;
-
-	ALLOC_ARRAY(sb->lineno, num + 1);
-	lineno = sb->lineno;
-
-	for (p = buf; p < end; p = get_next_line(p, end))
-		*lineno++ = p - buf;
-
-	*lineno = len;
-
-	sb->num_lines = num;
+	sb->num_lines = find_line_starts(&sb->lineno, sb->final_buf,
+					 sb->final_buf_size);
 	return sb->num_lines;
 }
 

blame.h

@@ -51,6 +51,8 @@ struct blame_origin {
 	 */
 	struct blame_entry *suspects;
 	mmfile_t file;
+	int num_lines;
+	void *fingerprints;
 	struct object_id blob_oid;
 	unsigned mode;
 	/* guilty gets set when shipping any suspects to the final