git-pickaxe: introduce heuristics to avoid "trivial" chunks

This adds scoring logic to blame_entry to prevent blames on very
trivial chunks (e.g. lots of empty lines, indent followed by a
closing brace) from being passed down to unrelated lines in the
parent.

The current heuristics are quite simple and may need to be
tweaked later, but we need to start somewhere.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pickaxe.c

@@ -40,6 +40,15 @@ static int max_score_digits;
 #define PICKAXE_BLAME_COPY		02
 #define PICKAXE_BLAME_COPY_HARDER	04
 
+/*
+ * blame for a blame_entry with score lower than these thresholds
+ * is not passed to the parent using move/copy logic.
+ */
+static unsigned blame_move_score;
+static unsigned blame_copy_score;
+#define BLAME_DEFAULT_MOVE_SCORE	20
+#define BLAME_DEFAULT_COPY_SCORE	40
+
 /* bits #0..7 in revision.h, #8..11 used for merge_bases() in commit.c */
 #define METAINFO_SHOWN		(1u<<12)
 #define MORE_THAN_ONE_PATH	(1u<<13)
@@ -645,7 +654,8 @@ static int find_move_in_parent(struct scoreboard *sb,
 		if (ent->suspect != target || ent->guilty)
 			continue;
 		find_copy_in_blob(sb, ent, parent, split, &file_p);
-		if (split[1].suspect)
+		if (split[1].suspect &&
+		    blame_move_score < ent_score(sb, &split[1]))
 			split_blame(sb, split, ent);
 	}
 	free(blob_p);
@@ -716,7 +726,8 @@ static int find_copy_in_parent(struct scoreboard *sb,
 			find_copy_in_blob(sb, ent, norigin, this, &file_p);
 			copy_split_if_better(sb, split, this);
 		}
-		if (split[1].suspect)
+		if (split[1].suspect &&
+		    blame_copy_score < ent_score(sb, &split[1]))
 			split_blame(sb, split, ent);
 	}
 	diff_flush(&diff_opts);
@@ -1180,6 +1191,15 @@ static int has_path_in_work_tree(const char *path)
 	return !lstat(path, &st);
 }
 
+static unsigned parse_score(const char *arg)
+{
+	char *end;
+	unsigned long score = strtoul(arg, &end, 10);
+	if (*end)
+		return 0;
+	return score;
+}
+
 int cmd_pickaxe(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info revs;
@@ -1209,12 +1229,15 @@ int cmd_pickaxe(int argc, const char **argv, const char *prefix)
 			output_option |= OUTPUT_LONG_OBJECT_NAME;
 		else if (!strcmp("-S", arg) && ++i < argc)
 			revs_file = argv[i];
-		else if (!strcmp("-M", arg))
+		else if (!strncmp("-M", arg, 2)) {
 			opt |= PICKAXE_BLAME_MOVE;
-		else if (!strcmp("-C", arg)) {
+			blame_move_score = parse_score(arg+2);
+		}
+		else if (!strncmp("-C", arg, 2)) {
 			if (opt & PICKAXE_BLAME_COPY)
 				opt |= PICKAXE_BLAME_COPY_HARDER;
 			opt |= PICKAXE_BLAME_COPY | PICKAXE_BLAME_MOVE;
+			blame_copy_score = parse_score(arg+2);
 		}
 		else if (!strcmp("-L", arg) && ++i < argc) {
 			char *term;
@@ -1252,6 +1275,11 @@ int cmd_pickaxe(int argc, const char **argv, const char *prefix)
 			argv[unk++] = arg;
 	}
 
+	if (!blame_move_score)
+		blame_move_score = BLAME_DEFAULT_MOVE_SCORE;
+	if (!blame_copy_score)
+		blame_copy_score = BLAME_DEFAULT_COPY_SCORE;
+
 	/* We have collected options unknown to us in argv[1..unk]
 	 * which are to be passed to revision machinery if we are
 	 * going to do the "bottom" procesing.