Pretty-format: %[+-]x to tweak inter-item newlines

This teaches the "pretty" machinery to expand '%+x' to a LF followed by
the expansion of '%x' if and only if '%x' expands to a non-empty string,
and to remove LFs before '%-x' if '%x' expands to an empty string.  This
works for any supported expansion placeholder 'x'.

This is expected to be immediately useful to reproduce the commit log
message with "%s%+b%n"; "%s%n%b%n" adds one extra LF if the log message is
a one-liner.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 40 insertions, 2 deletions

diff --git a/pretty.c b/pretty.c
index f5983f8ba..081feb660 100644
--- a/pretty.c
+++ b/pretty.c
@@ -595,8 +595,8 @@ static void format_decoration(struct strbuf *sb, const struct commit *commit)
 		strbuf_addch(sb, ')');
 }
 
-static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
-                               void *context)
+static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
+				void *context)
 {
 	struct format_commit_context *c = context;
 	const struct commit *commit = c->commit;
@@ -739,6 +739,44 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 	return 0;	/* unknown placeholder */
 }
 
+static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
+				 void *context)
+{
+	int consumed;
+	size_t orig_len;
+	enum {
+		NO_MAGIC,
+		ADD_LF_BEFORE_NON_EMPTY,
+		DEL_LF_BEFORE_EMPTY,
+	} magic = NO_MAGIC;
+
+	switch (placeholder[0]) {
+	case '-':
+		magic = DEL_LF_BEFORE_EMPTY;
+		break;
+	case '+':
+		magic = ADD_LF_BEFORE_NON_EMPTY;
+		break;
+	default:
+		break;
+	}
+	if (magic != NO_MAGIC)
+		placeholder++;
+
+	orig_len = sb->len;
+	consumed = format_commit_one(sb, placeholder, context);
+	if (magic == NO_MAGIC)
+		return consumed;
+
+	if ((orig_len == sb->len) && magic == DEL_LF_BEFORE_EMPTY) {
+		while (sb->len && sb->buf[sb->len - 1] == '\n')
+			strbuf_setlen(sb, sb->len - 1);
+	} else if ((orig_len != sb->len) && magic == ADD_LF_BEFORE_NON_EMPTY) {
+		strbuf_insert(sb, orig_len, "\n", 1);
+	}
+	return consumed + 1;
+}
+
 void format_commit_message(const struct commit *commit,
 			   const void *format, struct strbuf *sb,
 			   enum date_mode dmode)