refactor(grid): reimplement 'rightleft' as a post-processing step

problem: checks for wp->w_p_rl are all over the place, making simple
things like "advance column one cell" incredibly complicated.

solution: always fill linebuf_char[] using an incrementing counter,
and then mirror the buffer as a post-processing step

This was "easier" that I first feared, because the stupid but simple
workaround for things like keeping linenumbers still left-right,
e.g. "mirror them and them mirror them once more" is more or less
what vim did already. So let's just keep doing that.

1 files changed, 20 insertions, 14 deletions

diff --git a/src/nvim/grid.c b/src/nvim/grid.c
index d59c3b4803..45bffdcab5 100644
--- a/src/nvim/grid.c
+++ b/src/nvim/grid.c
@@ -477,14 +477,21 @@ void grid_line_mirror(void)
   if (grid_line_first >= grid_line_last) {
     return;
   }
+  linebuf_mirror(&grid_line_first, &grid_line_last, grid_line_maxcol);
+}
+
+void linebuf_mirror(int *firstp, int *lastp, int maxcol)
+{
+  int first = *firstp;
+  int last = *lastp;
 
-  size_t n = (size_t)(grid_line_last - grid_line_first);
-  int mirror = grid_line_maxcol - 1;  // Mirrors are more fun than television.
+  size_t n = (size_t)(last - first);
+  int mirror = 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++) {
+  memcpy(scratch_char + first, linebuf_char + first, n * sizeof(schar_T));
+  for (int col = first; col < last; col++) {
     int rev = mirror - col;
-    if (col + 1 < grid_line_last && scratch_char[col + 1] == 0) {
+    if (col + 1 < last && scratch_char[col + 1] == 0) {
       linebuf_char[rev - 1] = scratch_char[col];
       linebuf_char[rev] = 0;
       col++;
@@ -495,20 +502,19 @@ void grid_line_mirror(void)
 
   // 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++) {
+  memcpy(scratch_attr + first, linebuf_attr + first, n * sizeof(sattr_T));
+  for (int col = first; col < 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++) {
+  memcpy(scratch_vcol + first, linebuf_vcol + first, n * sizeof(colnr_T));
+  for (int col = first; col < 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;
+  *lastp = maxcol - first;
+  *firstp = maxcol - last;
 }
 
 /// End a group of grid_line_puts calls and send the screen buffer to the UI layer.
@@ -698,10 +704,10 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
       }
     }
     col = endcol + 1;
-    endcol = (clear_width > 0 ? clear_width : -clear_width);
+    endcol = clear_width;
   }
 
-  if (p_arshape && !p_tbidi) {
+  if (p_arshape && !p_tbidi && endcol > col) {
     line_do_arabic_shape(linebuf_char + col, endcol - col);
   }