perf(ui): reduce allocation overhead when encoding "redraw" events

Note for external UIs: Nvim can now emit multiple "redraw" event batches
before a final "flush" event is received. To retain existing behavior,
clients should make sure to update visible state at an explicit "flush"
event, not just the end of a "redraw" batch of event.

* Get rid of copy_object() blizzard in the auto-generated ui_event layer
* Special case "grid_line" by encoding screen state directly to
  msgpack events with no intermediate API events.
* Get rid of the arcane notion of referring to the screen as the "shell"
* Array and Dictionary are kvec_t:s, so define them as such.
* Allow kvec_t:s, such as Arrays and Dictionaries, to be allocated with
  a predetermined size within an arena.
* Eliminate redundant capacity checking when filling such kvec_t:s
  with values.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index ff6a4c37e8..693d946088 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -669,6 +669,32 @@ void api_free_string(String value)
   xfree(value.data);
 }
 
+Array arena_array(Arena *arena, size_t max_size)
+{
+  Array arr = ARRAY_DICT_INIT;
+  kv_fixsize_arena(arena, arr, max_size);
+  return arr;
+}
+
+Dictionary arena_dict(Arena *arena, size_t max_size)
+{
+  Dictionary dict = ARRAY_DICT_INIT;
+  kv_fixsize_arena(arena, dict, max_size);
+  return dict;
+}
+
+String arena_string(Arena *arena, String str)
+{
+  if (str.size) {
+    char *mem = arena_alloc(arena, str.size + 1, false);
+    memcpy(mem, str.data, str.size);
+    mem[str.size] = NUL;
+    return cbuf_as_string(mem, str.size);
+  } else {
+    return (String)STRING_INIT;
+  }
+}
+
 void api_free_object(Object value)
 {
   switch (value.type) {