Fixed issue #4272 and added tests for partition (#4280)

include/LightGBM/utils/array_args.h

@@ -103,7 +103,9 @@ class ArrayArgs {
     int j = end - 1;
     int p = i;
     int q = j;
-    if (start >= end) {
+    if (start >= end - 1) {
+      *l = start - 1;
+      *r = end;
       return;
     }
     std::vector<VAL_T>& ref = *arr;

tests/cpp_tests/test_array_args.cpp

@@ -0,0 +1,52 @@
+/*!
+ * Copyright (c) 2021 Microsoft Corporation. All rights reserved.
+ * Licensed under the MIT License. See LICENSE file in the project root for license information.
+ */
+
+#include <gtest/gtest.h>
+#include <LightGBM/meta.h>
+#include <LightGBM/utils/array_args.h>
+
+#include <random>
+
+using LightGBM::data_size_t;
+using LightGBM::score_t;
+using LightGBM::ArrayArgs;
+
+
+TEST(Partition, JustWorks) {
+  std::vector<score_t> gradients({0.5f, 5.0f, 1.0f, 2.0f, 2.0f});
+  data_size_t middle_begin, middle_end;
+
+  ArrayArgs<score_t>::Partition(&gradients, 0, gradients.size(), &middle_begin, &middle_end);
+
+  EXPECT_EQ(gradients[middle_begin + 1], gradients[middle_end - 1]);
+  EXPECT_GT(gradients[0], gradients[middle_begin + 1]);
+  EXPECT_GT(gradients[middle_begin + 1], gradients.back());
+}
+
+TEST(Partition, PartitionOneElement) {
+  std::vector<score_t> gradients({0.5f});
+  data_size_t middle_begin, middle_end;
+  ArrayArgs<score_t>::Partition(&gradients, 0, gradients.size(), &middle_begin, &middle_end);
+  EXPECT_EQ(gradients[middle_begin + 1], gradients[middle_end - 1]);
+}
+
+TEST(Partition, Empty) {
+  std::vector<score_t> gradients;
+  data_size_t middle_begin, middle_end;
+  ArrayArgs<score_t>::Partition(&gradients, 0, gradients.size(), &middle_begin, &middle_end);
+
+  EXPECT_EQ(middle_begin, -1);
+  EXPECT_EQ(middle_end, 0);
+}
+
+TEST(Partition, AllEqual) {
+  std::vector<score_t> gradients({0.5f, 0.5f, 0.5f});
+  data_size_t middle_begin, middle_end;
+  ArrayArgs<score_t>::Partition(&gradients, 0, gradients.size(), &middle_begin, &middle_end);
+
+  EXPECT_EQ(gradients[middle_begin + 1], gradients[middle_end - 1]);
+  EXPECT_EQ(middle_begin, -1);
+  EXPECT_EQ(middle_end, 3);
+}