perf(ui): build nodes without per-node marshalling tables

build() copied a node's children into a second table so it could nil them
out of the spec, and applyStyle() copied the style keys into a third. Both
existed only to hand C a table it was already holding: tree.create reads
named fields and never touches the array part, so children come straight
off the spec, and create now applies the style itself. That is one C call
per styled node instead of two, and the sparse-style decision is made by
the set mask rather than by a loop over key names in Lua.

Measured in the emulator, 12 sensors / 131 nodes: build 106ms -> 74ms, Lua
transient +18.1kB -> +12.0kB, free heap at build end 18572 -> 27936. Tree
footprint is byte-identical, so the same styles are applied. ui.lua stripped
bytecode 10024 -> 9621 bytes, saved in every app state.

An explicit fill now wins over background, where the Lua version had
background clobber it; no caller sets both.

lua/lib/ui.lua

@@ -163,34 +163,6 @@ tree.setPainter(function(id, x, y, w, h, clipX, clipY, clipW, clipH)
   end
 end)
 
-local STYLE_KEYS = {
-  "color",
-  "fill",
-  "border",
-  "face",
-  "pressedFace",
-  "pressedColor",
-  "focusColor",
-  "radius",
-  "font",
-  "textStyle",
-}
-
-local function applyStyle(id, spec)
-  local style, hasStyle = {}, false
-  for _, key in ipairs(STYLE_KEYS) do
-    if spec[key] ~= nil then
-      style[key], hasStyle = spec[key], true
-    end
-  end
-  if spec.background ~= nil then
-    style.background, style.fill, hasStyle = spec.background, spec.background, true
-  end
-  if hasStyle then
-    tree.setStyle(id, style)
-  end
-end
-
 local function build(spec, kind)
   spec = spec or {}
   -- Nodes outside a build would reset the arena under the screen already on the
@@ -199,20 +171,16 @@ local function build(spec, kind)
     error("build nodes from the function ui.mount() was given, then ui.rebuild()", 3)
   end
 
-  local children = {}
-  for index, child in ipairs(spec) do
-    children[index] = child
-    spec[index] = nil
-  end
-
   spec.type = kind
   spec.interactive = spec.on_enter ~= nil or spec.on_exit ~= nil or spec.on_click ~= nil
+  -- Children come straight off the spec's array part. tree.create reads only named
+  -- fields, so copying them into a second table cost one extra Lua table per node
+  -- -- roughly half again the spec's own cost, on the build path that peaks the heap.
   local id = tree.create(nil, spec)
-  for _, child in ipairs(children) do
+  for _, child in ipairs(spec) do
     tree.attach(id, child)
   end
 
-  applyStyle(id, spec)
   enterHandlers[id] = spec.on_enter
   exitHandlers[id] = spec.on_exit
   clickHandlers[id] = spec.on_click

native/src/bindings/features/screen/tree.cpp

@@ -144,6 +144,50 @@ int reset(lua_State* state) {
   return 0;
 }
 
+void readStyleColor(lua_State* state, int index, const char* key,
+                    int32_t& field, uint16_t flag, uint16_t& set) {
+  lua_getfield(state, index, key);
+  if (!lua_isnoneornil(state, -1)) {
+    field = static_cast<int32_t>(luaL_checkinteger(state, -1));
+    set |= flag;
+  }
+  lua_pop(state, 1);
+}
+
+// Reads whatever style fields a spec carries, returning whether it named any. The
+// Lua wrapper used to do this by copying the style keys into a second table and
+// mapping background itself, which cost a table per styled node and a rehash of the
+// caller's spec to express the mapping.
+bool readStyleInto(lua_State* state, int index, ui::Style& style) {
+  const uint16_t before = style.set;
+  readStyleColor(state, index, "color", style.color, ui::S_COLOR, style.set);
+  readStyleColor(state, index, "background", style.bg, ui::S_BG, style.set);
+  readStyleColor(state, index, "fill", style.fill, ui::S_FILL, style.set);
+  readStyleColor(state, index, "border", style.border, ui::S_BORDER, style.set);
+  readStyleColor(state, index, "face", style.face, ui::S_FACE, style.set);
+  readStyleColor(state, index, "pressedFace", style.pressedFace,
+                 ui::S_PRESSED_FACE, style.set);
+  readStyleColor(state, index, "pressedColor", style.pressedColor,
+                 ui::S_PRESSED_COLOR, style.set);
+  readStyleColor(state, index, "focusColor", style.focusColor,
+                 ui::S_FOCUS_COLOR, style.set);
+  readStyleColor(state, index, "font", style.font, ui::S_FONT, style.set);
+  readStyleColor(state, index, "textStyle", style.textStyle, ui::S_TEXT_STYLE,
+                 style.set);
+
+  int32_t radius = style.radius;
+  readStyleColor(state, index, "radius", radius, ui::S_RADIUS, style.set);
+  style.radius = static_cast<uint8_t>(radius);
+
+  // A background is the surface its own corners blend into, so it implies fill.
+  // An explicit fill wins, which is the way round the Lua version had it backwards.
+  if ((style.set & ui::S_BG) && !(style.set & ui::S_FILL)) {
+    style.fill = style.bg;
+    style.set |= ui::S_FILL;
+  }
+  return style.set != before;
+}
+
 int create(lua_State* state) {
   const bool hasParent = !lua_isnoneornil(state, 1);
   const uint16_t parent = hasParent ? checkNode(state, 1) : ui::NONE;
@@ -199,6 +243,14 @@ int create(lua_State* state) {
   if (label)
     tree(state).setLabel(id, label);
   lua_pop(state, 1);
+
+  // Styled here rather than by a second call from Lua: the spec is already on the
+  // stack, and a node naming no colours must cost no Style entry, which is a
+  // decision the set mask makes better than a loop over key names in Lua.
+  ui::Style style;
+  if (readStyleInto(state, 2, style))
+    tree(state).styleFor(id) = style;
+
   lua_pushinteger(state, id);
   return 1;
 }
@@ -316,39 +368,11 @@ int getParent(lua_State* state) {
   return 1;
 }
 
-void readStyleColor(lua_State* state, int index, const char* key,
-                    int32_t& field, uint16_t flag, uint16_t& set) {
-  lua_getfield(state, index, key);
-  if (!lua_isnoneornil(state, -1)) {
-    field = static_cast<int32_t>(luaL_checkinteger(state, -1));
-    set |= flag;
-  }
-  lua_pop(state, 1);
-}
-
 int setStyle(lua_State* state) {
   const uint16_t id = checkNode(state, 1);
   luaL_checktype(state, 2, LUA_TTABLE);
 
-  ui::Style& style = tree(state).styleFor(id);
-  readStyleColor(state, 2, "color", style.color, ui::S_COLOR, style.set);
-  readStyleColor(state, 2, "background", style.bg, ui::S_BG, style.set);
-  readStyleColor(state, 2, "fill", style.fill, ui::S_FILL, style.set);
-  readStyleColor(state, 2, "border", style.border, ui::S_BORDER, style.set);
-  readStyleColor(state, 2, "face", style.face, ui::S_FACE, style.set);
-  readStyleColor(state, 2, "pressedFace", style.pressedFace, ui::S_PRESSED_FACE,
-                 style.set);
-  readStyleColor(state, 2, "pressedColor", style.pressedColor,
-                 ui::S_PRESSED_COLOR, style.set);
-  readStyleColor(state, 2, "focusColor", style.focusColor, ui::S_FOCUS_COLOR,
-                 style.set);
-  readStyleColor(state, 2, "font", style.font, ui::S_FONT, style.set);
-  readStyleColor(state, 2, "textStyle", style.textStyle, ui::S_TEXT_STYLE,
-                 style.set);
-
-  int32_t radius = style.radius;
-  readStyleColor(state, 2, "radius", radius, ui::S_RADIUS, style.set);
-  style.radius = static_cast<uint8_t>(radius);
+  readStyleInto(state, 2, tree(state).styleFor(id));
 
   tree(state).nodes[id].flags |= ui::DIRTY;
   return 0;