refactor(grid): implement rightleftcmd as a post-processing step

Previously, 'rightleftcmd' was implemented by having all code which
would affect msg_col or output screen cells be conditional on `cmdmsg_rl`.
This change removes all that and instead implements rightleft as a
mirroring post-processing step.

7 files changed, 168 insertions, 83 deletions

diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 93fd62f107..80a1410dbc 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -715,12 +715,8 @@ static uint8_t *command_line_enter(int firstc, int count, int indent, bool clear
   ExpandInit(&s->xpc);
   ccline.xpc = &s->xpc;
 
-  if (curwin->w_p_rl && *curwin->w_p_rlc == 's'
-      && (s->firstc == '/' || s->firstc == '?')) {
-    cmdmsg_rl = true;
-  } else {
-    cmdmsg_rl = false;
-  }
+  cmdmsg_rl = (curwin->w_p_rl && *curwin->w_p_rlc == 's'
+               && (s->firstc == '/' || s->firstc == '?'));
 
   msg_grid_validate();
 
@@ -1564,11 +1560,7 @@ static int command_line_erase_chars(CommandLineState *s)
 
     XFREE_CLEAR(ccline.cmdbuff);        // no commandline to return
     if (!cmd_silent && !ui_has(kUICmdline)) {
-      if (cmdmsg_rl) {
-        msg_col = Columns;
-      } else {
-        msg_col = 0;
-      }
+      msg_col = 0;
       msg_putchar(' ');                             // delete ':'
     }
     s->is_state.search_start = s->is_state.save_cursor;
@@ -2664,7 +2656,7 @@ static int command_line_changed(CommandLineState *s)
     }
   }
 
-  if (cmdmsg_rl || (p_arshape && !p_tbidi)) {
+  if (p_arshape && !p_tbidi) {
     // Always redraw the whole command line to fix shaping and
     // right-left typing.  Not efficient, but it works.
     // Do it only when there are no characters left to read
@@ -3863,18 +3855,10 @@ void cursorcmd(void)
     return;
   }
 
-  if (cmdmsg_rl) {
-    msg_row = cmdline_row + (ccline.cmdspos / (Columns - 1));
-    msg_col = Columns - (ccline.cmdspos % (Columns - 1)) - 1;
-    if (msg_row <= 0) {
-      msg_row = Rows - 1;
-    }
-  } else {
-    msg_row = cmdline_row + (ccline.cmdspos / Columns);
-    msg_col = ccline.cmdspos % Columns;
-    if (msg_row >= Rows) {
-      msg_row = Rows - 1;
-    }
+  msg_row = cmdline_row + (ccline.cmdspos / Columns);
+  msg_col = ccline.cmdspos % Columns;
+  if (msg_row >= Rows) {
+    msg_row = Rows - 1;
   }
 
   msg_cursor_goto(msg_row, msg_col);
@@ -3886,11 +3870,7 @@ void gotocmdline(bool clr)
     return;
   }
   msg_start();
-  if (cmdmsg_rl) {
-    msg_col = Columns - 1;
-  } else {
-    msg_col = 0;  // always start in column 0
-  }
+  msg_col = 0;  // always start in column 0
   if (clr) {  // clear the bottom line(s)
     msg_clr_eos();  // will reset clear_cmdline
   }
diff --git a/src/nvim/grid.c b/src/nvim/grid.c
index efc819c26f..d59c3b4803 100644
--- a/src/nvim/grid.c
+++ b/src/nvim/grid.c
@@ -472,6 +472,45 @@ void grid_line_cursor_goto(int col)
   ui_grid_cursor_goto(grid_line_grid->handle, grid_line_row, col);
 }
 
+void grid_line_mirror(void)
+{
+  if (grid_line_first >= grid_line_last) {
+    return;
+  }
+
+  size_t n = (size_t)(grid_line_last - grid_line_first);
+  int mirror = grid_line_maxcol - 1;  // Mirrors are more fun than television.
+  schar_T *scratch_char = (schar_T *)linebuf_scratch;
+  memcpy(scratch_char + grid_line_first, linebuf_char + grid_line_first, n * sizeof(schar_T));
+  for (int col = grid_line_first; col < grid_line_last; col++) {
+    int rev = mirror - col;
+    if (col + 1 < grid_line_last && scratch_char[col + 1] == 0) {
+      linebuf_char[rev - 1] = scratch_char[col];
+      linebuf_char[rev] = 0;
+      col++;
+    } else {
+      linebuf_char[rev] = scratch_char[col];
+    }
+  }
+
+  // for attr and vcol: assumes doublewidth chars are self-consistent
+  sattr_T *scratch_attr = (sattr_T *)linebuf_scratch;
+  memcpy(scratch_attr + grid_line_first, linebuf_attr + grid_line_first, n * sizeof(sattr_T));
+  for (int col = grid_line_first; col < grid_line_last; col++) {
+    linebuf_attr[mirror - col] = scratch_attr[col];
+  }
+
+  colnr_T *scratch_vcol = (colnr_T *)linebuf_scratch;
+  memcpy(scratch_vcol + grid_line_first, linebuf_vcol + grid_line_first, n * sizeof(colnr_T));
+  for (int col = grid_line_first; col < grid_line_last; col++) {
+    linebuf_vcol[mirror - col] = scratch_vcol[col];
+  }
+
+  int grid_line_last_copy = grid_line_last;
+  grid_line_last = grid_line_maxcol - grid_line_first;
+  grid_line_first = grid_line_maxcol - grid_line_last_copy;
+}
+
 /// End a group of grid_line_puts calls and send the screen buffer to the UI layer.
 void grid_line_flush(void)
 {
@@ -828,9 +867,11 @@ void grid_alloc(ScreenGrid *grid, int rows, int columns, bool copy, bool valid)
     xfree(linebuf_char);
     xfree(linebuf_attr);
     xfree(linebuf_vcol);
+    xfree(linebuf_scratch);
     linebuf_char = xmalloc((size_t)columns * sizeof(schar_T));
     linebuf_attr = xmalloc((size_t)columns * sizeof(sattr_T));
     linebuf_vcol = xmalloc((size_t)columns * sizeof(colnr_T));
+    linebuf_scratch = xmalloc((size_t)columns * sizeof(sscratch_T));
     linebuf_size = (size_t)columns;
   }
 }
@@ -855,6 +896,7 @@ void grid_free_all_mem(void)
   xfree(linebuf_char);
   xfree(linebuf_attr);
   xfree(linebuf_vcol);
+  xfree(linebuf_scratch);
 }
 
 /// (Re)allocates a window grid if size changed while in ext_multigrid mode.
diff --git a/src/nvim/grid.h b/src/nvim/grid.h
index 1b2c529a93..6e3c4fd770 100644
--- a/src/nvim/grid.h
+++ b/src/nvim/grid.h
@@ -29,6 +29,7 @@ EXTERN bool resizing_screen INIT( = 0);
 EXTERN schar_T *linebuf_char INIT( = NULL);
 EXTERN sattr_T *linebuf_attr INIT( = NULL);
 EXTERN colnr_T *linebuf_vcol INIT( = NULL);
+EXTERN char *linebuf_scratch INIT( = NULL);
 
 // Low-level functions to manipulate individual character cells on the
 // screen grid.
diff --git a/src/nvim/grid_defs.h b/src/nvim/grid_defs.h
index c65c273d57..390b2a848a 100644
--- a/src/nvim/grid_defs.h
+++ b/src/nvim/grid_defs.h
@@ -16,7 +16,9 @@
 // otherwise it must be a UTF-8 string of length maximum 4 (no NUL when n=4)
 
 typedef uint32_t schar_T;
-typedef int sattr_T;
+typedef int32_t sattr_T;
+// must be at least as big as the biggest of schar_T, sattr_T, col_T
+typedef int32_t sscratch_T;
 
 enum {
   kZIndexDefaultGrid = 0,
diff --git a/src/nvim/message.c b/src/nvim/message.c
index c1acef8b0d..3b33559906 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -1231,9 +1231,7 @@ void wait_return(int redraw)
           } else {
             msg_didout = false;
             c = K_IGNORE;
-            msg_col =
-              cmdmsg_rl ? Columns - 1 :
-              0;
+            msg_col = 0;
           }
           if (quit_more) {
             c = CAR;                            // just pretend CR was hit
@@ -1435,7 +1433,7 @@ void msg_start(void)
 
   if (!msg_scroll && full_screen) {     // overwrite last message
     msg_row = cmdline_row;
-    msg_col = cmdmsg_rl ? Columns - 1 : 0;
+    msg_col = 0;
   } else if (msg_didout || (p_ch == 0 && !ui_has(kUIMessages))) {  // start message on next line
     msg_putchar('\n');
     did_return = true;
@@ -2132,7 +2130,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
   int msg_row_pending = -1;
 
   while (true) {
-    if (cmdmsg_rl ? msg_col <= 0 : msg_col >= Columns) {
+    if (msg_col >= Columns) {
       if (p_more && !recurse) {
         // Store text for scrolling back.
         store_sb_text(&sb_str, s, attr, &sb_col, true);
@@ -2141,7 +2139,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
         break;
       }
 
-      msg_col = cmdmsg_rl ? Columns - 1 : 0;
+      msg_col = 0;
       msg_row++;
       msg_didout = false;
     }
@@ -2156,7 +2154,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
 
       if (!recurse) {
         if (msg_row_pending >= 0) {
-          grid_line_flush_if_valid_row();
+          msg_line_flush();
           msg_row_pending = -1;
         }
 
@@ -2196,7 +2194,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
       // TODO(bfredl): this logic is messier that it has to be. What
       // messages really want is its own private linebuf_char buffer.
       if (msg_row_pending >= 0) {
-        grid_line_flush_if_valid_row();
+        msg_line_flush();
       }
       grid_line_start(&msg_grid_adj, msg_row);
       msg_row_pending = msg_row;
@@ -2207,7 +2205,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
       // avoid including composing chars after the end
       int l = (maxlen >= 0) ? utfc_ptr2len_len(s, (int)((str + maxlen) - s)) : utfc_ptr2len(s);
 
-      if (cw > 1 && (cmdmsg_rl ? msg_col <= 1 : msg_col == Columns - 1)) {
+      if (cw > 1 && (msg_col == Columns - 1)) {
         // Doesn't fit, print a highlighted '>' to fill it up.
         grid_line_puts(msg_col, ">", 1, HL_ATTR(HLF_AT));
         cw = 1;
@@ -2216,20 +2214,12 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
         s += l;
       }
       msg_didout = true;  // remember that line is not empty
-      if (cmdmsg_rl) {
-        msg_col -= cw;
-      } else {
-        msg_col += cw;
-      }
+      msg_col += cw;
     } else {
       char c = *s++;
       if (c == '\n') {  // go to next line
         msg_didout = false;  // remember that line is empty
-        if (cmdmsg_rl) {
-          msg_col = Columns - 1;
-        } else {
-          msg_col = 0;
-        }
+        msg_col = 0;
         msg_row++;
         if (p_more && !recurse) {
           // Store text for scrolling back.
@@ -2244,9 +2234,9 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
       } else if (c == TAB) {  // translate Tab into spaces
         do {
           grid_line_puts(msg_col, " ", 1, print_attr);
-          msg_col += cmdmsg_rl ? -1 : 1;
+          msg_col += 1;
 
-          if (msg_col == (cmdmsg_rl ? 0 : Columns)) {
+          if (msg_col == Columns) {
             break;
           }
         } while (msg_col & 7);
@@ -2257,7 +2247,7 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
   }
 
   if (msg_row_pending >= 0) {
-    grid_line_flush_if_valid_row();
+    msg_line_flush();
   }
   msg_cursor_goto(msg_row, msg_col);
 
@@ -2268,9 +2258,20 @@ static void msg_puts_display(const char *str, int maxlen, int attr, int recurse)
   msg_check();
 }
 
+void msg_line_flush(void)
+{
+  if (cmdmsg_rl) {
+    grid_line_mirror();
+  }
+  grid_line_flush_if_valid_row();
+}
+
 void msg_cursor_goto(int row, int col)
 {
   ScreenGrid *grid = &msg_grid_adj;
+  if (cmdmsg_rl) {
+    col = Columns - 1 - col;
+  }
   grid_adjust(&grid, &row, &col);
   ui_grid_cursor_goto(grid->handle, row, col);
 }
@@ -2656,18 +2657,10 @@ static void msg_puts_printf(const char *str, const ptrdiff_t maxlen)
 
     int cw = utf_char2cells(utf_ptr2char(s));
     // primitive way to compute the current column
-    if (cmdmsg_rl) {
-      if (*s == '\r' || *s == '\n') {
-        msg_col = Columns - 1;
-      } else {
-        msg_col -= cw;
-      }
+    if (*s == '\r' || *s == '\n') {
+      msg_col = 0;
     } else {
-      if (*s == '\r' || *s == '\n') {
-        msg_col = 0;
-      } else {
-        msg_col += cw;
-      }
+      msg_col += cw;
     }
     s += len;
   }
@@ -2915,8 +2908,6 @@ static int do_more_prompt(int typed_char)
   if (quit_more) {
     msg_row = Rows - 1;
     msg_col = 0;
-  } else if (cmdmsg_rl) {
-    msg_col = Columns - 1;
   }
 
   entered = false;
@@ -3014,7 +3005,7 @@ void msg_clr_eos_force(void)
     return;
   }
   int msg_startcol = (cmdmsg_rl) ? 0 : msg_col;
-  int msg_endcol = (cmdmsg_rl) ? msg_col + 1 : Columns;
+  int msg_endcol = (cmdmsg_rl) ? Columns - msg_col : Columns;
 
   if (msg_grid.chars && msg_row < msg_grid_pos) {
     // TODO(bfredl): ugly, this state should already been validated at this
@@ -3028,7 +3019,7 @@ void msg_clr_eos_force(void)
             ' ', ' ', HL_ATTR(HLF_MSG));
 
   redraw_cmdline = true;  // overwritten the command line
-  if (msg_row < Rows - 1 || msg_col == (cmdmsg_rl ? Columns : 0)) {
+  if (msg_row < Rows - 1 || msg_col == 0) {
     clear_cmdline = false;  // command line has been cleared
     mode_displayed = false;  // mode cleared or overwritten
   }
@@ -3383,14 +3374,8 @@ void msg_advance(int col)
   if (col >= Columns) {         // not enough room
     col = Columns - 1;
   }
-  if (cmdmsg_rl) {
-    while (msg_col > Columns - col) {
-      msg_putchar(' ');
-    }
-  } else {
-    while (msg_col < col) {
-      msg_putchar(' ');
-    }
+  while (msg_col < col) {
+    msg_putchar(' ');
   }
 }
 
diff --git a/src/nvim/spellsuggest.c b/src/nvim/spellsuggest.c
index 564c4ca12d..39265718c6 100644
--- a/src/nvim/spellsuggest.c
+++ b/src/nvim/spellsuggest.c
@@ -537,22 +537,18 @@ void spell_suggest(int count)
   } else {
     // When 'rightleft' is set the list is drawn right-left.
     cmdmsg_rl = curwin->w_p_rl;
-    if (cmdmsg_rl) {
-      msg_col = Columns - 1;
-    }
 
     // List the suggestions.
     msg_start();
     msg_row = Rows - 1;         // for when 'cmdheight' > 1
     lines_left = Rows;          // avoid more prompt
-    vim_snprintf(IObuff, IOSIZE, _("Change \"%.*s\" to:"),
-                 sug.su_badlen, sug.su_badptr);
-    if (cmdmsg_rl && strncmp(IObuff, "Change", 6) == 0) {
+    char *fmt = _("Change \"%.*s\" to:");
+    if (cmdmsg_rl && strncmp(fmt, "Change", 6) == 0) {
       // And now the rabbit from the high hat: Avoid showing the
       // untranslated message rightleft.
-      vim_snprintf(IObuff, IOSIZE, ":ot \"%.*s\" egnahC",
-                   sug.su_badlen, sug.su_badptr);
+      fmt = ":ot \"%.*s\" egnahC";
     }
+    vim_snprintf(IObuff, IOSIZE, fmt, sug.su_badlen, sug.su_badptr);
     msg_puts(IObuff);
     msg_clr_eos();
     msg_putchar('\n');
diff --git a/test/functional/ui/cmdline_spec.lua b/test/functional/ui/cmdline_spec.lua
index 497b2e7f4c..188b9ee87b 100644
--- a/test/functional/ui/cmdline_spec.lua
+++ b/test/functional/ui/cmdline_spec.lua
@@ -24,6 +24,7 @@ local function new_screen(opt)
     [7] = {bold = true, foreground = Screen.colors.Brown},
     [8] = {background = Screen.colors.LightGrey},
     [9] = {bold = true},
+    [10] = {background = Screen.colors.Yellow1};
   })
   return screen
 end
@@ -771,6 +772,84 @@ describe('cmdline redraw', function()
       :^abc                                    |
     ]])
   end)
+
+  it('with rightleftcmd', function()
+    command('set rightleft rightleftcmd=search shortmess+=s')
+    meths.buf_set_lines(0, 0, -1, true, {"let's rock!"})
+    screen:expect{grid=[[
+                    !kcor s'te^l|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                               |
+    ]]}
+
+    feed '/'
+    screen:expect{grid=[[
+                    !kcor s'tel|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                             ^ /|
+    ]]}
+
+    feed "let's"
+    -- note: cursor looks off but looks alright in real use
+    -- when rendered as a block so it touches the end of the text
+    screen:expect{grid=[[
+                    !kcor {2:s'tel}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                        ^ s'tel/|
+    ]]}
+
+    -- cursor movement
+    feed "<space>"
+    screen:expect{grid=[[
+                    !kcor{2: s'tel}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                       ^  s'tel/|
+    ]]}
+
+    feed "rock"
+    screen:expect{grid=[[
+                    !{2:kcor s'tel}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                   ^ kcor s'tel/|
+    ]]}
+
+    feed "<right>"
+    screen:expect{grid=[[
+                    !{2:kcor s'tel}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                    ^kcor s'tel/|
+    ]]}
+
+    feed "<left>"
+    screen:expect{grid=[[
+                    !{2:kcor s'tel}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+                   ^ kcor s'tel/|
+    ]]}
+
+    feed "<cr>"
+    screen:expect{grid=[[
+                    !{10:kcor s'te^l}|
+      {1:                        ~}|
+      {1:                        ~}|
+      {1:                        ~}|
+      kcor s'tel/              |
+    ]]}
+  end)
 end)
 
 describe('statusline is redrawn on entering cmdline', function()