[GPU-Plugin] Add load balancing search to gpu_hist. Add compressed iterator. (#2504)

tests/cpp/common/test_compressed_iterator.cc

@@ -0,0 +1,54 @@
+#include "../../../src/common/compressed_iterator.h"
+#include "gtest/gtest.h"
+
+namespace xgboost {
+namespace common {
+TEST(CompressedIterator, Test) {
+  ASSERT_TRUE(detail::SymbolBits(256) == 8);
+  ASSERT_TRUE(detail::SymbolBits(150) == 8);
+  std::vector<int> test_cases = {3, 426, 21, 64, 256, 100000, INT32_MAX};
+  int num_elements = 1000;
+  int repetitions = 1000;
+  srand(9);
+
+  for (auto alphabet_size : test_cases) {
+    for (int i = 0; i < repetitions; i++) {
+      std::vector<int> input(num_elements);
+      std::generate(input.begin(), input.end(),
+        [=]() { return rand() % alphabet_size; });
+      CompressedBufferWriter cbw(alphabet_size);
+
+      // Test write entire array
+      std::vector<unsigned char> buffer(
+        CompressedBufferWriter::CalculateBufferSize(input.size(),
+          alphabet_size));
+
+      cbw.Write(buffer.data(), input.begin(), input.end());
+
+      CompressedIterator<int> ci(buffer.data(), alphabet_size);
+      std::vector<int> output(input.size());
+      for (int i = 0; i < input.size(); i++) {
+        output[i] = ci[i];
+      }
+
+      ASSERT_TRUE(input == output);
+
+      // Test write Symbol
+      std::vector<unsigned char> buffer2(
+        CompressedBufferWriter::CalculateBufferSize(input.size(),
+          alphabet_size));
+      for (int i = 0; i < input.size(); i++) {
+        cbw.WriteSymbol(buffer2.data(), input[i], i);
+      }
+      CompressedIterator<int> ci2(buffer.data(), alphabet_size);
+      std::vector<int> output2(input.size());
+      for (int i = 0; i < input.size(); i++) {
+        output2[i] = ci2[i];
+      }
+      ASSERT_TRUE(input == output2);
+    }
+  }
+}
+
+}  // namespace common
+}  // namespace xgboost