checkpoint: ground work for calculating binary light trees

computeLightTree() takes a list of light as
a bitfield and produces a depth-first 
binary tree array that can be used to efficiently
check which lights volumes contain a given z
coordinate in screen space.

Currently computeLightTree() produces the array
locally on the stack, which isn’t useful, but
before we can make use of it, a lot of other
things have to happen.

libs/utils/test/test_BinaryTreeArray.cpp

@@ -0,0 +1,146 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+
+#include <utils/BinaryTreeArray.h>
+
+using namespace utils;
+
+TEST(BinaryTreeArray, basic) {
+
+
+    struct Node {
+        size_t index;
+        size_t next;
+    };
+    Node array[256];
+
+    BinaryTreeArray::traverse(1,
+            [&array](size_t index, size_t parent, size_t col, size_t next) {
+                array[index] = Node{ index, next };
+            },
+            [&array](size_t index, size_t parent, size_t l, size_t r, size_t next) {
+                array[index] = Node{ index, next };
+            });
+
+    EXPECT_EQ(0, array[0].index);
+    EXPECT_EQ(1, array[0].next);
+
+
+    BinaryTreeArray::traverse(2,
+            [&array](size_t index, size_t parent, size_t col, size_t next) {
+                array[index] = Node{ index, next };
+            },
+            [&array](size_t index, size_t parent, size_t l, size_t r, size_t next) {
+                array[index] = Node{ index, next };
+            });
+
+    EXPECT_EQ(0, array[0].index);
+    EXPECT_EQ(3, array[0].next);
+
+        EXPECT_EQ(1, array[1].index);
+        EXPECT_EQ(2, array[1].next);
+
+        EXPECT_EQ(2, array[2].index);
+        EXPECT_EQ(3, array[2].next);
+
+
+    BinaryTreeArray::traverse(3,
+            [&array](size_t index, size_t parent, size_t col, size_t next) {
+                array[index] = Node{ index, next };
+            },
+            [&array](size_t index, size_t parent, size_t l, size_t r, size_t next) {
+                array[index] = Node{ index, next };
+            });
+
+    EXPECT_EQ(0, array[0].index);
+    EXPECT_EQ(7, array[0].next);
+
+        EXPECT_EQ(1, array[1].index);
+        EXPECT_EQ(4, array[1].next);
+
+            EXPECT_EQ(2, array[2].index);
+            EXPECT_EQ(3, array[2].next);
+
+            EXPECT_EQ(3, array[3].index);
+            EXPECT_EQ(4, array[3].next);
+
+        EXPECT_EQ(4, array[4].index);
+        EXPECT_EQ(7, array[4].next);
+
+            EXPECT_EQ(5, array[5].index);
+            EXPECT_EQ(6, array[5].next);
+
+            EXPECT_EQ(6, array[6].index);
+            EXPECT_EQ(7, array[6].next);
+
+
+    BinaryTreeArray::traverse(4,
+            [&array](size_t index, size_t parent, size_t col, size_t next) {
+                array[index] = Node{ index, next };
+            },
+            [&array](size_t index, size_t parent, size_t l, size_t r, size_t next) {
+                array[index] = Node{ index, next };
+            });
+
+    EXPECT_EQ(0, array[0].index);
+    EXPECT_EQ(15, array[0].next);
+
+        EXPECT_EQ(1, array[1].index);
+        EXPECT_EQ(8, array[1].next);
+
+            EXPECT_EQ(2, array[2].index);
+            EXPECT_EQ(5, array[2].next);
+
+                EXPECT_EQ(3, array[3].index);
+                EXPECT_EQ(4, array[3].next);
+
+                EXPECT_EQ(4, array[4].index);
+                EXPECT_EQ(5, array[4].next);
+
+            EXPECT_EQ(5, array[5].index);
+            EXPECT_EQ(8, array[5].next);
+
+                EXPECT_EQ(6, array[6].index);
+                EXPECT_EQ(7, array[6].next);
+
+                EXPECT_EQ(7, array[7].index);
+                EXPECT_EQ(8, array[7].next);
+
+        EXPECT_EQ(8, array[8].index);
+        EXPECT_EQ(15, array[8].next);
+
+            EXPECT_EQ(9, array[9].index);
+            EXPECT_EQ(12, array[9].next);
+
+                EXPECT_EQ(10, array[10].index);
+                EXPECT_EQ(11, array[10].next);
+
+                EXPECT_EQ(11, array[11].index);
+                EXPECT_EQ(12, array[11].next);
+
+            EXPECT_EQ(12, array[12].index);
+            EXPECT_EQ(15, array[12].next);
+
+                EXPECT_EQ(13, array[13].index);
+                EXPECT_EQ(14, array[13].next);
+
+                EXPECT_EQ(14, array[14].index);
+                EXPECT_EQ(15, array[14].next);
+
+}
+