Make --color-words work well with --graph

'--color-words' algorithm can be described as:

  1. collect a the minus/plus lines of a diff hunk, divided into
     minus-lines and plus-lines;

  2. break both minus-lines and plus-lines into words and
     place them into two mmfile_t with one word for each line;

  3. use xdiff to run diff on the two mmfile_t to get the words level diff;

And for the common parts of the both file, we output the plus side text.
diff_words->current_plus is used to trace the current position of the plus file
which printed. diff_words->last_minus is used to trace the last minus word
printed.

For '--graph' to work with '--color-words', we need to output the graph prefix
on each line of color words output. Generally, there are two conditions on
which we should output the prefix.

  1. diff_words->last_minus == 0 &&
     diff_words->current_plus == diff_words->plus.text.ptr

     that is: the plus text must start as a new line, and if there is no minus
     word printed, a graph prefix must be printed.

  2. diff_words->current_plus > diff_words->plus.text.ptr &&
     *(diff_words->current_plus - 1) == '\n'

     that is: a graph prefix must be printed following a '\n'

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -622,7 +622,8 @@ struct diff_words_style diff_words_styles[] = {
 struct diff_words_data {
 	struct diff_words_buffer minus, plus;
 	const char *current_plus;
-	FILE *file;
+	int last_minus;
+	struct diff_options *opt;
 	regex_t *word_regex;
 	enum diff_words_type type;
 	struct diff_words_style *style;
@@ -631,10 +632,15 @@ struct diff_words_data {
 static int fn_out_diff_words_write_helper(FILE *fp,
 					  struct diff_words_style_elem *st_el,
 					  const char *newline,
-					  size_t count, const char *buf)
+					  size_t count, const char *buf,
+					  const char *line_prefix)
 {
+	int print = 0;
+
 	while (count) {
 		char *p = memchr(buf, '\n', count);
+		if (print)
+			fputs(line_prefix, fp);
 		if (p != buf) {
 			if (st_el->color && fputs(st_el->color, fp) < 0)
 				return -1;
@@ -652,21 +658,74 @@ static int fn_out_diff_words_write_helper(FILE *fp,
 			return -1;
 		count -= p + 1 - buf;
 		buf = p + 1;
+		print = 1;
 	}
 	return 0;
 }
 
+/*
+ * '--color-words' algorithm can be described as:
+ *
+ *   1. collect a the minus/plus lines of a diff hunk, divided into
+ *      minus-lines and plus-lines;
+ *
+ *   2. break both minus-lines and plus-lines into words and
+ *      place them into two mmfile_t with one word for each line;
+ *
+ *   3. use xdiff to run diff on the two mmfile_t to get the words level diff;
+ *
+ * And for the common parts of the both file, we output the plus side text.
+ * diff_words->current_plus is used to trace the current position of the plus file
+ * which printed. diff_words->last_minus is used to trace the last minus word
+ * printed.
+ *
+ * For '--graph' to work with '--color-words', we need to output the graph prefix
+ * on each line of color words output. Generally, there are two conditions on
+ * which we should output the prefix.
+ *
+ *   1. diff_words->last_minus == 0 &&
+ *      diff_words->current_plus == diff_words->plus.text.ptr
+ *
+ *      that is: the plus text must start as a new line, and if there is no minus
+ *      word printed, a graph prefix must be printed.
+ *
+ *   2. diff_words->current_plus > diff_words->plus.text.ptr &&
+ *      *(diff_words->current_plus - 1) == '\n'
+ *
+ *      that is: a graph prefix must be printed following a '\n'
+ */
+static int color_words_output_graph_prefix(struct diff_words_data *diff_words)
+{
+	if ((diff_words->last_minus == 0 &&
+		diff_words->current_plus == diff_words->plus.text.ptr) ||
+		(diff_words->current_plus > diff_words->plus.text.ptr &&
+		*(diff_words->current_plus - 1) == '\n')) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
 static void fn_out_diff_words_aux(void *priv, char *line, unsigned long len)
 {
 	struct diff_words_data *diff_words = priv;
 	struct diff_words_style *style = diff_words->style;
 	int minus_first, minus_len, plus_first, plus_len;
 	const char *minus_begin, *minus_end, *plus_begin, *plus_end;
+	struct diff_options *opt = diff_words->opt;
+	struct strbuf *msgbuf;
+	char *line_prefix = "";
 
 	if (line[0] != '@' || parse_hunk_header(line, len,
 			&minus_first, &minus_len, &plus_first, &plus_len))
 		return;
 
+	assert(opt);
+	if (opt->output_prefix) {
+		msgbuf = opt->output_prefix(opt, opt->output_prefix_data);
+		line_prefix = msgbuf->buf;
+	}
+
 	/* POSIX requires that first be decremented by one if len == 0... */
 	if (minus_len) {
 		minus_begin = diff_words->minus.orig[minus_first].begin;
@@ -682,21 +741,32 @@ static void fn_out_diff_words_aux(void *priv, char *line, unsigned long len)
 	} else
 		plus_begin = plus_end = diff_words->plus.orig[plus_first].end;
 
-	if (diff_words->current_plus != plus_begin)
+	if (color_words_output_graph_prefix(diff_words)) {
-		fn_out_diff_words_write_helper(diff_words->file,
+		fputs(line_prefix, diff_words->opt->file);
+	}
+	if (diff_words->current_plus != plus_begin) {
+		fn_out_diff_words_write_helper(diff_words->opt->file,
 				&style->ctx, style->newline,
 				plus_begin - diff_words->current_plus,
-				diff_words->current_plus);
+				diff_words->current_plus, line_prefix);
-	if (minus_begin != minus_end)
+		if (*(plus_begin - 1) == '\n')
-		fn_out_diff_words_write_helper(diff_words->file,
+			fputs(line_prefix, diff_words->opt->file);
+	}
+	if (minus_begin != minus_end) {
+		fn_out_diff_words_write_helper(diff_words->opt->file,
 				&style->old, style->newline,
-				minus_end - minus_begin, minus_begin);
+				minus_end - minus_begin, minus_begin,
-	if (plus_begin != plus_end)
+				line_prefix);
-		fn_out_diff_words_write_helper(diff_words->file,
+	}
+	if (plus_begin != plus_end) {
+		fn_out_diff_words_write_helper(diff_words->opt->file,
 				&style->new, style->newline,
-				plus_end - plus_begin, plus_begin);
+				plus_end - plus_begin, plus_begin,
+				line_prefix);
+	}
 
 	diff_words->current_plus = plus_end;
+	diff_words->last_minus = minus_first;
 }
 
 /* This function starts looking at *begin, and returns 0 iff a word was found. */
@@ -777,16 +847,29 @@ static void diff_words_show(struct diff_words_data *diff_words)
 	mmfile_t minus, plus;
 	struct diff_words_style *style = diff_words->style;
 
+	struct diff_options *opt = diff_words->opt;
+	struct strbuf *msgbuf;
+	char *line_prefix = "";
+
+	assert(opt);
+	if (opt->output_prefix) {
+		msgbuf = opt->output_prefix(opt, opt->output_prefix_data);
+		line_prefix = msgbuf->buf;
+	}
+
 	/* special case: only removal */
 	if (!diff_words->plus.text.size) {
-		fn_out_diff_words_write_helper(diff_words->file,
+		fputs(line_prefix, diff_words->opt->file);
+		fn_out_diff_words_write_helper(diff_words->opt->file,
 			&style->old, style->newline,
-			diff_words->minus.text.size, diff_words->minus.text.ptr);
+			diff_words->minus.text.size,
+			diff_words->minus.text.ptr, line_prefix);
 		diff_words->minus.text.size = 0;
 		return;
 	}
 
 	diff_words->current_plus = diff_words->plus.text.ptr;
+	diff_words->last_minus = 0;
 
 	memset(&xpp, 0, sizeof(xpp));
 	memset(&xecfg, 0, sizeof(xecfg));
@@ -800,11 +883,15 @@ static void diff_words_show(struct diff_words_data *diff_words)
 	free(minus.ptr);
 	free(plus.ptr);
 	if (diff_words->current_plus != diff_words->plus.text.ptr +
-			diff_words->plus.text.size)
+			diff_words->plus.text.size) {
-		fn_out_diff_words_write_helper(diff_words->file,
+		if (color_words_output_graph_prefix(diff_words))
+			fputs(line_prefix, diff_words->opt->file);
+		fn_out_diff_words_write_helper(diff_words->opt->file,
 			&style->ctx, style->newline,
 			diff_words->plus.text.ptr + diff_words->plus.text.size
-			- diff_words->current_plus, diff_words->current_plus);
+			- diff_words->current_plus, diff_words->current_plus,
+			line_prefix);
+	}
 	diff_words->minus.text.size = diff_words->plus.text.size = 0;
 }
 
@@ -1902,8 +1989,8 @@ static void builtin_diff(const char *name_a,
 
 			ecbdata.diff_words =
 				xcalloc(1, sizeof(struct diff_words_data));
-			ecbdata.diff_words->file = o->file;
 			ecbdata.diff_words->type = o->word_diff;
+			ecbdata.diff_words->opt = o;
 			if (!o->word_regex)
 				o->word_regex = userdiff_word_regex(one);
 			if (!o->word_regex)