[PATCH 6/6] pretty: support padding placeholders, %< %> and %<>

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Either %<, %> or %<> standing before a placeholder specifies how many
columns (at least as the placeholder can exceed it) it takes. Each
differs on how spaces are padded:

  %< pads on the right (aka left alignment)
  %> pads on the left (aka right alignment)
  %<> pads both ways equally (aka centered)

The (<N>) follows them, e.g. `%<(100)', to specify the number of
columns the next placeholder takes.

However, if '|' stands before (<N>), e.g. `%>|(100)', then the number
of columns is calculated so that it reaches the Nth column on screen.

TODO: %>> allows the next placeholder to eat the trailing spaces in
resulting string if it exceeds its allocated space. Maybe useful in
combination with %<(100)%s%>(40)%d, where spaces are wasted sometimes.

TODO: maybe support modifier '!' in addition to '|' above to cut
exceeding part in the next placeholder.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/pretty-formats.txt |   4 ++
 pretty.c                         | 106 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 109 insertions(+), 1 deletion(-)

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 8304325..7710ea8 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -149,6 +149,10 @@ The placeholders are:
 - '%x00': print a byte from a hex code
 - '%w([<w>[,<i1>[,<i2>]]])': switch line wrapping, like the -w option of
   linkgit:git-shortlog[1].
+- '%<(<N>)': make the next placeholder take at least N columns, padding spaces on the right if necessary
+- '%<|(<N>)': make the next placeholder take at least until Nth columns, padding spaces on the right if necessary
+- '%>(<N>)', '%>|(<N>)': similar to '%<(<N<)', '%<|(<N<)' respectively, but padding spaces on the left
+- '%<>(<N>)', '%<>|(<N>)': similar to '%<(<N<)', '%<|(<N<)' respectively, but padding both sides (i.e. the text is centered)
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index 45fe878..6662f52 100644
--- a/pretty.c
+++ b/pretty.c
@@ -617,12 +617,20 @@ struct chunk {
 	size_t len;
 };
 
+enum flush_type {
+	no_flush,
+	flush_right,
+	flush_left,
+	flush_both
+};
+
 struct format_commit_context {
 	const struct commit *commit;
 	const struct pretty_print_context *pretty_ctx;
 	unsigned commit_header_parsed:1;
 	unsigned commit_message_parsed:1;
 	unsigned commit_signature_parsed:1;
+	enum flush_type flush_type;
 	struct {
 		char *gpg_output;
 		char good_bad;
@@ -631,6 +639,7 @@ struct format_commit_context {
 	char *message;
 	size_t width, indent1, indent2;
 	unsigned use_color;
+	int padding;
 
 	/* These offsets are relative to the start of the commit message. */
 	struct chunk author;
@@ -916,6 +925,52 @@ static size_t parse_color_placeholder(struct strbuf *sb,
 	return 0;
 }
 
+static size_t parse_padding_placeholder(struct strbuf *sb,
+					const char *placeholder,
+					struct format_commit_context *c)
+{
+	const char *ch = placeholder;
+	enum flush_type flush_type;
+	int to_column = 0;
+
+	switch (*ch++) {
+	case '<':
+		if (*ch == '>') {
+			flush_type = flush_both;
+			ch++;
+		} else
+			flush_type = flush_right;
+		break;
+	case '>':
+		flush_type = flush_left;
+		break;
+	default:
+		return 0;
+	}
+
+	/* the next value means "wide enough to that column" */
+	if (*ch == '|') {
+		to_column = 1;
+		ch++;
+	}
+
+	if (*ch == '(') {
+		const char *start = ch + 1;
+		const char *end = strchr(start, ')');
+		char *next;
+		int width;
+		if (!end || end == start)
+			return 0;
+		width = strtoul(start, &next, 10);
+		if (next == start || width == 0)
+			return 0;
+		c->padding = to_column ? -width : width;
+		c->flush_type = flush_type;
+		return end - placeholder + 1;
+	}
+	return 0;
+}
+
 static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 				void *context)
 {
@@ -966,6 +1021,10 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 			return end - placeholder + 1;
 		} else
 			return 0;
+
+	case '<':
+	case '>':
+		return parse_padding_placeholder(sb, placeholder, c);
 	}
 
 	/* these depend on the commit */
@@ -1125,6 +1184,7 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 				 void *context)
 {
 	struct format_commit_context *c = context;
+	struct strbuf local_sb = STRBUF_INIT;
 	int consumed;
 	size_t orig_len;
 	enum {
@@ -1151,7 +1211,51 @@ static size_t format_commit_item(struct strbuf *sb, const char *placeholder,
 		placeholder++;
 
 	orig_len = sb->len;
-	consumed = format_commit_one(sb, placeholder, context);
+	if (c->flush_type != no_flush) {
+		int len, padding = c->padding;
+		if (padding < 0) {
+			const char *start = strrchr(sb->buf, '\n');
+			int occupied;
+			if (!start)
+				start = sb->buf;
+			occupied = utf8_strnwidth(start, -1, 1);
+			padding = (-padding) - occupied;
+		}
+		consumed = format_commit_one(&local_sb, placeholder, context);
+		len = utf8_strnwidth(local_sb.buf, -1, 1);
+		if (len >= padding)
+			/* Maybe we can trim the exceeded part */
+			strbuf_addstr(sb, local_sb.buf);
+		else {
+			int sb_len = sb->len, offset;
+			switch (c->flush_type) {
+			case flush_left:
+				offset = padding - len;
+				break;
+			case flush_right:
+				offset = 0;
+				break;
+			case flush_both:
+				offset = (padding - len) / 2;
+				break;
+			case no_flush: /* to make gcc happy */
+				break;
+			}
+			/*
+			 * we calculate padding in columns, now
+			 * convert it back to chars
+			 */
+			padding = padding - len + local_sb.len;
+			strbuf_grow(sb, padding);
+			strbuf_setlen(sb, sb_len + padding);
+			memset(sb->buf + sb_len, ' ', sb->len - sb_len);
+			memcpy(sb->buf + sb_len + offset, local_sb.buf,
+			       local_sb.len);
+		}
+		strbuf_release(&local_sb);
+		c->flush_type = no_flush;
+	} else
+		consumed = format_commit_one(sb, placeholder, context);
 	if (c->use_color)
 		c->use_color--;
 	if (magic == NO_MAGIC)
-- 
1.7.12.1.384.g7b808e7