refactor(grid): make screen rendering more multibyte than ever before

Problem: buffer text with composing chars are converted from UTF-8
to an array of up to seven UTF-32 values and then converted back
to UTF-8 strings.

Solution: Convert buffer text directly to UTF-8 based schar_T values.

The limit of the text size is now in schar_T bytes, which is currently
31+1 but easily could be raised as it no longer multiplies the size
of the entire screen grid when not used, the full size is only required
for temporary scratch buffers.

Also does some general cleanup to win_line text handling, which was
unnecessarily complicated due to multibyte rendering being an "opt-in"
feature long ago. Nowadays, a char is just a char, regardless if it consists
of one ASCII byte or multiple bytes.

1 files changed, 15 insertions, 18 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index c6909245af..8ef208f291 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -5134,7 +5134,7 @@ static void f_nr2char(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     return;
   }
 
-  char buf[MB_MAXBYTES];
+  char buf[MB_MAXCHAR];
   const int len = utf_char2bytes((int)num, buf);
 
   rettv->v_type = VAR_STRING;
@@ -6891,7 +6891,7 @@ static void f_screenchar(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   if (row < 0 || row >= grid->rows || col < 0 || col >= grid->cols) {
     c = -1;
   } else {
-    char buf[MB_MAXBYTES + 1];
+    char buf[MAX_SCHAR_SIZE + 1];
     schar_get(buf, grid_getchar(grid, row, col, NULL));
     c = utf_ptr2char(buf);
   }
@@ -6907,24 +6907,22 @@ static void f_screenchars(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   ScreenGrid *grid;
   screenchar_adjust(&grid, &row, &col);
 
+  tv_list_alloc_ret(rettv, kListLenMayKnow);
   if (row < 0 || row >= grid->rows || col < 0 || col >= grid->cols) {
-    tv_list_alloc_ret(rettv, 0);
     return;
   }
 
-  char buf[MB_MAXBYTES + 1];
+  char buf[MAX_SCHAR_SIZE + 1];
   schar_get(buf, grid_getchar(grid, row, col, NULL));
-  int pcc[MAX_MCO];
-  int c = utfc_ptr2char(buf, pcc);
-  int composing_len = 0;
-  while (composing_len < MAX_MCO && pcc[composing_len] != 0) {
-    composing_len++;
-  }
-  tv_list_alloc_ret(rettv, composing_len + 1);
-  tv_list_append_number(rettv->vval.v_list, c);
-  for (int i = 0; i < composing_len; i++) {
-    tv_list_append_number(rettv->vval.v_list, pcc[i]);
-  }
+
+  // schar values are already processed chars which are always NUL-terminated.
+  // A single [0] is expected when char is NUL.
+  size_t i = 0;
+  do {
+    int c = utf_ptr2char(buf + i);
+    tv_list_append_number(rettv->vval.v_list, c);
+    i += (size_t)utf_ptr2len(buf + i);
+  } while (buf[i] != NUL);
 }
 
 /// "screencol()" function
@@ -6957,7 +6955,7 @@ static void f_screenstring(typval_T *argvars, typval_T *rettv, EvalFuncData fptr
     return;
   }
 
-  char buf[MB_MAXBYTES + 1];
+  char buf[MAX_SCHAR_SIZE + 1];
   schar_get(buf, grid_getchar(grid, row, col, NULL));
   rettv->vval.v_string = xstrdup(buf);
 }
@@ -7413,8 +7411,7 @@ static void f_setcharsearch(typval_T *argvars, typval_T *rettv, EvalFuncData fpt
 
   char *const csearch = tv_dict_get_string(d, "char", false);
   if (csearch != NULL) {
-    int pcc[MAX_MCO];
-    const int c = utfc_ptr2char(csearch, pcc);
+    int c = utf_ptr2char(csearch);
     set_last_csearch(c, csearch, utfc_ptr2len(csearch));
   }