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>

3 files changed, 70 insertions, 2 deletions

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 2a845b1e57..ca9c6d1f80 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -132,6 +132,14 @@ The placeholders are:
 - '%n': newline
 - '%x00': print a byte from a hex code
 
+If you add a `{plus}` (plus sign) after '%' of a placeholder, a line-feed
+is inserted immediately before the expansion if and only if the
+placeholder expands to a non-empty string.
+
+If you add a `-` (minus sign) after '%' of a placeholder, line-feeds that
+immediately precede the expansion are deleted if and only if the
+placeholder expands to an empty string.
+
 * 'tformat:'
 +
 The 'tformat:' format works exactly like 'format:', except that it
diff --git a/pretty.c b/pretty.c
index f5983f8baa..081feb6602 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)
diff --git a/t/t6006-rev-list-format.sh b/t/t6006-rev-list-format.sh
index 59d1f6283b..18a77a739e 100755
--- a/t/t6006-rev-list-format.sh
+++ b/t/t6006-rev-list-format.sh
@@ -162,4 +162,26 @@ test_expect_success 'empty email' '
 	}
 '
 
+test_expect_success 'del LF before empty (1)' '
+	git show -s --pretty=format:"%s%n%-b%nThanks%n" HEAD^^ >actual &&
+	test $(wc -l <actual) = 2
+'
+
+test_expect_success 'del LF before empty (2)' '
+	git show -s --pretty=format:"%s%n%-b%nThanks%n" HEAD >actual &&
+	test $(wc -l <actual) = 6 &&
+	grep "^$" actual
+'
+
+test_expect_success 'add LF before non-empty (1)' '
+	git show -s --pretty=format:"%s%+b%nThanks%n" HEAD^^ >actual &&
+	test $(wc -l <actual) = 2
+'
+
+test_expect_success 'add LF before non-empty (2)' '
+	git show -s --pretty=format:"%s%+b%nThanks%n" HEAD >actual &&
+	test $(wc -l <actual) = 6 &&
+	grep "^$" actual
+'
+
 test_done