diff: add an internal option to dual-color diffs of diffs

When diffing diffs, it can be quite daunting to figure out what the heck
is going on, as there are nested +/- signs.

Let's make this easier by adding a flag in diff_options that allows
color-coding the outer diff sign with inverted colors, so that the
preimage and postimage is colored like the diff it is.

Of course, this really only makes sense when the preimage and postimage
*are* diffs. So let's not expose this flag via a command-line option for
now.

This is a feature that was invented by git-tbdiff, and it will be used
by `git range-diff` in the next commit, by offering it via a new option:
`--dual-color`.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

diff.c

@@ -562,14 +562,18 @@ static void check_blank_at_eof(mmfile_t *mf1, mmfile_t *mf2,
 	ecbdata->blank_at_eof_in_postimage = (at - l2) + 1;
 }
 
-static void emit_line_0(struct diff_options *o, const char *set, const char *reset,
+static void emit_line_0(struct diff_options *o,
+			const char *set, unsigned reverse, const char *reset,
 			int first, const char *line, int len)
 {
 	int has_trailing_newline, has_trailing_carriage_return;
 	int nofirst;
 	FILE *file = o->file;
 
-	fputs(diff_line_prefix(o), file);
+	if (first)
+		fputs(diff_line_prefix(o), file);
+	else if (!len)
+		return;
 
 	if (len == 0) {
 		has_trailing_newline = (first == '\n');
@@ -587,8 +591,10 @@ static void emit_line_0(struct diff_options *o, const char *set, const char *res
 	}
 
 	if (len || !nofirst) {
+		if (reverse && want_color(o->use_color))
+			fputs(GIT_COLOR_REVERSE, file);
 		fputs(set, file);
-		if (!nofirst)
+		if (first && !nofirst)
 			fputc(first, file);
 		fwrite(line, len, 1, file);
 		fputs(reset, file);
@@ -602,7 +608,7 @@ static void emit_line_0(struct diff_options *o, const char *set, const char *res
 static void emit_line(struct diff_options *o, const char *set, const char *reset,
 		      const char *line, int len)
 {
-	emit_line_0(o, set, reset, line[0], line+1, len-1);
+	emit_line_0(o, set, 0, reset, line[0], line+1, len-1);
 }
 
 enum diff_symbol {
@@ -962,7 +968,8 @@ static void dim_moved_lines(struct diff_options *o)
 
 static void emit_line_ws_markup(struct diff_options *o,
 				const char *set, const char *reset,
-				const char *line, int len, char sign,
+				const char *line, int len,
+				const char *set_sign, char sign,
 				unsigned ws_rule, int blank_at_eof)
 {
 	const char *ws = NULL;
@@ -973,14 +980,20 @@ static void emit_line_ws_markup(struct diff_options *o,
 			ws = NULL;
 	}
 
-	if (!ws)
-		emit_line_0(o, set, reset, sign, line, len);
-	else if (blank_at_eof)
+	if (!ws && !set_sign)
+		emit_line_0(o, set, 0, reset, sign, line, len);
+	else if (!ws) {
+		/* Emit just the prefix, then the rest. */
+		emit_line_0(o, set_sign ? set_sign : set, !!set_sign, reset,
+			    sign, "", 0);
+		emit_line_0(o, set, 0, reset, 0, line, len);
+	} else if (blank_at_eof)
 		/* Blank line at EOF - paint '+' as well */
-		emit_line_0(o, ws, reset, sign, line, len);
+		emit_line_0(o, ws, 0, reset, sign, line, len);
 	else {
 		/* Emit just the prefix, then the rest. */
-		emit_line_0(o, set, reset, sign, "", 0);
+		emit_line_0(o, set_sign ? set_sign : set, !!set_sign, reset,
+			    sign, "", 0);
 		ws_check_emit(line, len, ws_rule,
 			      o->file, set, reset, ws);
 	}
@@ -990,7 +1003,7 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 					 struct emitted_diff_symbol *eds)
 {
 	static const char *nneof = " No newline at end of file\n";
-	const char *context, *reset, *set, *meta, *fraginfo;
+	const char *context, *reset, *set, *set_sign, *meta, *fraginfo;
 	struct strbuf sb = STRBUF_INIT;
 
 	enum diff_symbol s = eds->s;
@@ -1003,7 +1016,7 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 		context = diff_get_color_opt(o, DIFF_CONTEXT);
 		reset = diff_get_color_opt(o, DIFF_RESET);
 		putc('\n', o->file);
-		emit_line_0(o, context, reset, '\\',
+		emit_line_0(o, context, 0, reset, '\\',
 			    nneof, strlen(nneof));
 		break;
 	case DIFF_SYMBOL_SUBMODULE_HEADER:
@@ -1030,7 +1043,18 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 	case DIFF_SYMBOL_CONTEXT:
 		set = diff_get_color_opt(o, DIFF_CONTEXT);
 		reset = diff_get_color_opt(o, DIFF_RESET);
-		emit_line_ws_markup(o, set, reset, line, len, ' ',
+		set_sign = NULL;
+		if (o->flags.dual_color_diffed_diffs) {
+			char c = !len ? 0 : line[0];
+
+			if (c == '+')
+				set = diff_get_color_opt(o, DIFF_FILE_NEW);
+			else if (c == '@')
+				set = diff_get_color_opt(o, DIFF_FRAGINFO);
+			else if (c == '-')
+				set = diff_get_color_opt(o, DIFF_FILE_OLD);
+		}
+		emit_line_ws_markup(o, set, reset, line, len, set_sign, ' ',
 				    flags & (DIFF_SYMBOL_CONTENT_WS_MASK), 0);
 		break;
 	case DIFF_SYMBOL_PLUS:
@@ -1057,7 +1081,20 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 			set = diff_get_color_opt(o, DIFF_FILE_NEW);
 		}
 		reset = diff_get_color_opt(o, DIFF_RESET);
-		emit_line_ws_markup(o, set, reset, line, len, '+',
+		if (!o->flags.dual_color_diffed_diffs)
+			set_sign = NULL;
+		else {
+			char c = !len ? 0 : line[0];
+
+			set_sign = set;
+			if (c == '-')
+				set = diff_get_color_opt(o, DIFF_FILE_OLD);
+			else if (c == '@')
+				set = diff_get_color_opt(o, DIFF_FRAGINFO);
+			else if (c != '+')
+				set = diff_get_color_opt(o, DIFF_CONTEXT);
+		}
+		emit_line_ws_markup(o, set, reset, line, len, set_sign, '+',
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK,
 				    flags & DIFF_SYMBOL_CONTENT_BLANK_LINE_EOF);
 		break;
@@ -1085,7 +1122,20 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 			set = diff_get_color_opt(o, DIFF_FILE_OLD);
 		}
 		reset = diff_get_color_opt(o, DIFF_RESET);
-		emit_line_ws_markup(o, set, reset, line, len, '-',
+		if (!o->flags.dual_color_diffed_diffs)
+			set_sign = NULL;
+		else {
+			char c = !len ? 0 : line[0];
+
+			set_sign = set;
+			if (c == '+')
+				set = diff_get_color_opt(o, DIFF_FILE_NEW);
+			else if (c == '@')
+				set = diff_get_color_opt(o, DIFF_FRAGINFO);
+			else if (c != '-')
+				set = diff_get_color_opt(o, DIFF_CONTEXT);
+		}
+		emit_line_ws_markup(o, set, reset, line, len, set_sign, '-',
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK, 0);
 		break;
 	case DIFF_SYMBOL_WORDS_PORCELAIN:
@@ -1276,6 +1326,7 @@ static void emit_hunk_header(struct emit_callback *ecbdata,
 	const char *frag = diff_get_color(ecbdata->color_diff, DIFF_FRAGINFO);
 	const char *func = diff_get_color(ecbdata->color_diff, DIFF_FUNCINFO);
 	const char *reset = diff_get_color(ecbdata->color_diff, DIFF_RESET);
+	const char *reverse = ecbdata->color_diff ? GIT_COLOR_REVERSE : "";
 	static const char atat[2] = { '@', '@' };
 	const char *cp, *ep;
 	struct strbuf msgbuf = STRBUF_INIT;
@@ -1296,6 +1347,8 @@ static void emit_hunk_header(struct emit_callback *ecbdata,
 	ep += 2; /* skip over @@ */
 
 	/* The hunk header in fraginfo color */
+	if (ecbdata->opt->flags.dual_color_diffed_diffs)
+		strbuf_addstr(&msgbuf, reverse);
 	strbuf_addstr(&msgbuf, frag);
 	strbuf_add(&msgbuf, line, ep - line);
 	strbuf_addstr(&msgbuf, reset);

diff.h

@@ -95,6 +95,7 @@ struct diff_flags {
 	unsigned default_follow_renames:1;
 	unsigned stat_with_summary:1;
 	unsigned suppress_diff_headers:1;
+	unsigned dual_color_diffed_diffs:1;
 };
 
 static inline void diff_flags_or(struct diff_flags *a,