vi: improved handling of backspace in replace mode

In replace mode ('R' command) the backspace character should get
special treatment:

- backspace only goes back to the start of the replacement;
- backspacing over replaced characters restores the original text.

Prior to this commit BusyBox vi deleted the characters both before
and after the cursor in replace mode.

function                                             old     new   delta
undo_pop                                               -     235    +235
char_insert                                          858     884     +26
indicate_error                                        81      84      +3
find_range                                           654     657      +3
static.text_yank                                      77      79      +2
do_cmd                                              4486    4243    -243
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 4/1 up/down: 269/-243)           Total: 26 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 18 insertions, 6 deletions

diff --git a/editors/vi.c b/editors/vi.c
index d37cd48a3..4257c0fdc 100644
--- a/editors/vi.c
+++ b/editors/vi.c
@@ -224,6 +224,7 @@
 
 #endif
 
+#define isbackspace(c) ((c) == term_orig.c_cc[VERASE] || (c) == 8 || (c) == 127)
 
 enum {
 	MAX_TABSTOP = 32, // sanity limit
@@ -342,6 +343,7 @@ struct globals {
 	int last_modified_count; // = -1;
 	int cmdline_filecnt;     // how many file names on cmd line
 	int cmdcnt;              // repetition count
+	char *rstart;            // start of text in Replace mode
 	unsigned rows, columns;	 // the terminal screen is this size
 #if ENABLE_FEATURE_VI_ASK_TERMINAL
 	int get_rowcol_error;
@@ -474,6 +476,7 @@ struct globals {
 #define last_modified_count     (G.last_modified_count)
 #define cmdline_filecnt         (G.cmdline_filecnt    )
 #define cmdcnt                  (G.cmdcnt             )
+#define rstart                  (G.rstart             )
 #define rows                    (G.rows               )
 #define columns                 (G.columns            )
 #define crow                    (G.crow               )
@@ -1212,7 +1215,7 @@ static char *get_input_line(const char *prompt)
 		c = get_one_char();
 		if (c == '\n' || c == '\r' || c == 27)
 			break;		// this is end of input
-		if (c == term_orig.c_cc[VERASE] || c == 8 || c == 127) {
+		if (isbackspace(c)) {
 			// user wants to erase prev char
 			write1("\b \b"); // erase char on screen
 			buf[--i] = '\0';
@@ -2174,8 +2177,16 @@ static char *char_insert(char *p, char c, int undo) // insert the char c at 'p'
 			p += 1 + stupid_insert(p, ' ');
 		}
 #endif
-	} else if (c == term_orig.c_cc[VERASE] || c == 8 || c == 127) { // Is this a BS
-		if (p > text) {
+	} else if (isbackspace(c)) {
+		if (cmd_mode == 2) {
+			// special treatment for backspace in Replace mode
+			if (p > rstart) {
+				p--;
+#if ENABLE_FEATURE_VI_UNDO
+				undo_pop();
+#endif
+			}
+		} else if (p > text) {
 			p--;
 			p = text_hole_delete(p, p, ALLOW_UNDO_QUEUED);	// shrink buffer 1 char
 		}
@@ -3703,9 +3714,9 @@ static void do_cmd(int c)
 			undo_queue_commit();
 		} else {
 			if (1 <= c || Isprint(c)) {
-				if (c != 27)
-					dot = yank_delete(dot, dot, PARTIAL, YANKDEL, ALLOW_UNDO);	// delete char
-				dot = char_insert(dot, c, ALLOW_UNDO_CHAIN);	// insert new char
+				if (c != 27 && !isbackspace(c))
+					dot = yank_delete(dot, dot, PARTIAL, YANKDEL, ALLOW_UNDO);
+				dot = char_insert(dot, c, ALLOW_UNDO_CHAIN);
 			}
 			goto dc1;
 		}
@@ -4264,6 +4275,7 @@ static void do_cmd(int c)
  dc5:
 		cmd_mode = 2;
 		undo_queue_commit();
+		rstart = dot;
 		break;
 	case KEYCODE_DELETE:
 		if (dot < end - 1)