lineedit: fix moving backwards across lines with wide chars

function                                             old     new   delta
input_backward                                       212     208      -4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 46 insertions, 21 deletions

diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index bc089ab1c..836fc0089 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -413,12 +413,22 @@ static void beep(void)
         bb_putchar('\007');
 }
 
+static void put_prompt(void)
+{
+        unsigned w;
+
+        out1str(cmdedit_prompt);
+        fflush_all();
+        cursor = 0;
+        w = cmdedit_termw; /* read volatile var once */
+        cmdedit_y = cmdedit_prmt_len / w; /* new quasireal y */
+        cmdedit_x = cmdedit_prmt_len % w;
+}
+
 /* Move back one character */
 /* (optimized for slow terminals) */
 static void input_backward(unsigned num)
 {
-        int count_y;
-
         if (num > cursor)
                 num = cursor;
         if (num == 0)
@@ -456,29 +466,44 @@ static void input_backward(unsigned num)
         }
 
         /* Need to go one or more lines up */
-//FIXME: this does not work correctly if prev line has one "unfilled" screen position
-//caused by wide unicode char not fitting in that one screen position.
-        num -= cmdedit_x;
-        {
-                unsigned w = cmdedit_termw; /* volatile var */
+        if (ENABLE_UNICODE_WIDE_WCHARS) {
+                /* With wide chars, it is hard to "backtrack"
+                 * and reliably figure out where to put cursor.
+                 * Example (<> is a wide char; # is an ordinary char, _ cursor):
+                 * |prompt: <><> |
+                 * |<><><><><><> |
+                 * |_            |
+                 * and user presses left arrow. num = 1, cmdedit_x = 0,
+                 * We need to go up one line, and then - how do we know that
+                 * we need to go *10* positions to the right? Because
+                 * |prompt: <>#<>|
+                 * |<><><>#<><><>|
+                 * |_            |
+                 * in this situation we need to go *11* positions to the right.
+                 *
+                 * A simpler thing to do is to redraw everything from the start
+                 * up to new cursor position (which is already known):
+                 */
+                unsigned sv_cursor;
+                if (cmdedit_y > 0)  /* up to start y */
+                        printf("\033[%uA", cmdedit_y);
+                bb_putchar('\r');
+                cmdedit_y = 0;
+                sv_cursor = cursor;
+                put_prompt(); /* sets cursor to 0 */
+                while (cursor < sv_cursor)
+                        put_cur_glyph_and_inc_cursor();
+        } else {
+                int count_y;
+                unsigned w;
+                num -= cmdedit_x;
+                w = cmdedit_termw; /* read volatile var once */
                 count_y = 1 + (num / w);
                 cmdedit_y -= count_y;
                 cmdedit_x = w * count_y - num;
+                /* go to 1st column; go up; go to correct column */
+                printf("\r" "\033[%uA" "\033[%uC", count_y, cmdedit_x);
         }
-        /* go to 1st column; go up; go to correct column */
-        printf("\r" "\033[%uA" "\033[%uC", count_y, cmdedit_x);
-}
-
-static void put_prompt(void)
-{
-        unsigned w;
-
-        out1str(cmdedit_prompt);
-        fflush_all();
-        cursor = 0;
-        w = cmdedit_termw; /* read volatile var once */
-        cmdedit_y = cmdedit_prmt_len / w; /* new quasireal y */
-        cmdedit_x = cmdedit_prmt_len % w;
 }
 
 /* draw prompt, editor line, and clear tail */