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
in `branch-diff` in the next commit.

Signed-off-by: Johannes Schindelin <johannes.schinde...@gmx.de>
---
 diff.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 diff.h |  5 ++++-
 2 files changed, 59 insertions(+), 11 deletions(-)

diff --git a/diff.c b/diff.c
index f1bda0db3f5..98a41e88620 100644
--- a/diff.c
+++ b/diff.c
@@ -67,6 +67,8 @@ static char diff_colors[][COLOR_MAXLEN] = {
        GIT_COLOR_BOLD_YELLOW,  /* NEW_MOVED ALTERNATIVE */
        GIT_COLOR_FAINT,        /* NEW_MOVED_DIM */
        GIT_COLOR_FAINT_ITALIC, /* NEW_MOVED_ALTERNATIVE_DIM */
+       GIT_COLOR_INV_RED,      /* OLD_INV */
+       GIT_COLOR_INV_GREEN,    /* NEW_INV */
 };
 
 static NORETURN void die_want_option(const char *option_name)
@@ -108,6 +110,10 @@ static int parse_diff_color_slot(const char *var)
                return DIFF_FILE_NEW_MOVED_DIM;
        if (!strcasecmp(var, "newmovedalternativedimmed"))
                return DIFF_FILE_NEW_MOVED_ALT_DIM;
+       if (!strcasecmp(var, "oldinv"))
+               return DIFF_FILE_OLD_INV;
+       if (!strcasecmp(var, "newinv"))
+               return DIFF_FILE_NEW_INV;
        return -1;
 }
 
@@ -577,7 +583,10 @@ static void emit_line_0(struct diff_options *o, const char 
*set, const char *res
        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');
@@ -596,7 +605,7 @@ static void emit_line_0(struct diff_options *o, const char 
*set, const char *res
 
        if (len || !nofirst) {
                fputs(set, file);
-               if (!nofirst)
+               if (first && !nofirst)
                        fputc(first, file);
                fwrite(line, len, 1, file);
                fputs(reset, file);
@@ -970,7 +979,8 @@ 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;
@@ -981,14 +991,18 @@ static void emit_line_ws_markup(struct diff_options *o,
                        ws = NULL;
        }
 
-       if (!ws)
+       if (!ws && set_sign == set)
                emit_line_0(o, set, reset, sign, line, len);
-       else if (blank_at_eof)
+       else if (!ws) {
+               /* Emit just the prefix, then the rest. */
+               emit_line_0(o, set_sign, reset, sign, "", 0);
+               emit_line_0(o, set, 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);
        else {
                /* Emit just the prefix, then the rest. */
-               emit_line_0(o, set, reset, sign, "", 0);
+               emit_line_0(o, set_sign, reset, sign, "", 0);
                ws_check_emit(line, len, ws_rule,
                              o->file, set, reset, ws);
        }
@@ -998,7 +1012,7 @@ 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;
@@ -1038,7 +1052,16 @@ 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 = set;
+               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_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:
@@ -1065,7 +1088,18 @@ 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 = set;
+               else {
+                       char c = !len ? 0 : line[0];
+
+                       set_sign = diff_get_color_opt(o, DIFF_FILE_NEW_INV);
+                       if (c == '-')
+                               set = diff_get_color_opt(o, DIFF_FILE_OLD);
+                       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;
@@ -1093,7 +1127,18 @@ 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 = set;
+               else {
+                       char c = !len ? 0 : line[0];
+
+                       set_sign = diff_get_color_opt(o, DIFF_FILE_OLD_INV);
+                       if (c == '+')
+                               set = diff_get_color_opt(o, DIFF_FILE_NEW);
+                       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:
diff --git a/diff.h b/diff.h
index 0dd6a71af60..c3e5d27967c 100644
--- a/diff.h
+++ b/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,
@@ -242,7 +243,9 @@ enum color_diff {
        DIFF_FILE_NEW_MOVED = 13,
        DIFF_FILE_NEW_MOVED_ALT = 14,
        DIFF_FILE_NEW_MOVED_DIM = 15,
-       DIFF_FILE_NEW_MOVED_ALT_DIM = 16
+       DIFF_FILE_NEW_MOVED_ALT_DIM = 16,
+       DIFF_FILE_OLD_INV = 17,
+       DIFF_FILE_NEW_INV = 18
 };
 const char *diff_get_color(int diff_use_color, enum color_diff ix);
 #define diff_get_color_opt(o, ix) \
-- 
2.17.0.409.g71698f11835


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