layer optimizaton

styles/layers.h

@@ -15,10 +15,8 @@
 // opaque. If the base color is transparent, the final result may also be transparent,
 // depending on what the layers paint on top of the base color.
 
-template<class BASE, class... LAYERS> class Layers : public BASE {};
-
 template<class BASE, class L1>
-class Layers<BASE, L1> {
+class Compose {
 public:
   LayerRunResult run(BladeBase* blade) {
     LayerRunResult base_run_result = RunLayer(&base_, blade);
@@ -44,7 +42,15 @@ class Layers<BASE, L1> {
   }
 };
 
-template<class BASE, class L1, class L2, class... LAYERS>
-  class Layers<BASE, L1, L2, LAYERS...> : public Layers<Layers<BASE, L1>, L2, LAYERS...> {};
+
+template<class BASE, class ... LAYERS> struct LayerSelector {};
+template<class BASE> struct LayerSelector<BASE> { typedef BASE type; };
+template<class BASE, class L1> struct LayerSelector<AlphaL<BASE, Int<0>>, L1> { typedef L1 type; };
+template<class BASE, class L1> struct LayerSelector<BASE, L1> { typedef Compose<BASE, L1> type; };
+template<class BASE, class L1, class ... REST> struct LayerSelector<BASE, L1, REST...> {
+  typedef typename LayerSelector<Compose<BASE, L1>, REST...>::type type;
+};
+
+template<class BASE, class... LAYERS> using Layers = typename LayerSelector<BASE, LAYERS...>::type;
 
 #endif

styles/tests.cpp

@@ -909,7 +909,17 @@ void test_mix() {
   CHECK_COLOR(mock_blade.colors[0], 0, 65535, 0, 1);
 }
 
+#define CHECK(X) do {                                           \
+    if (!(X)) { fprintf(stderr, "%s failed, line %d\n", #X, __LINE__); exit(1); } \
+} while(0)
+
+void test_layers() {
+  CHECK( (is_same_type<Layers<AlphaL<Red, Int<0>>, Blue>, Blue>::value) );
+  CHECK( (!is_same_type<Layers<AlphaL<Red, Int<1>>, Blue>, Blue>::value) );
+}
+
 int main() {
+  test_layers();
   test_mix();
   test_gradient();
   test_style6();