Extract partial sum into an independent function. (#7889)

tests/cpp/common/test_numeric.cc

@@ -0,0 +1,33 @@
+/*!
+ * Copyright 2022, XGBoost contributors.
+ */
+#include <gtest/gtest.h>
+
+#include <numeric>
+
+#include "../../../src/common/numeric.h"
+
+namespace xgboost {
+namespace common {
+TEST(Numeric, PartialSum) {
+  {
+    std::vector<size_t> values{1, 2, 3, 4};
+    std::vector<size_t> result(values.size() + 1);
+    Context ctx;
+    PartialSum(ctx.Threads(), values.begin(), values.end(), static_cast<size_t>(0), result.begin());
+    std::vector<size_t> sol(values.size() + 1, 0);
+    std::partial_sum(values.begin(), values.end(), sol.begin() + 1);
+    ASSERT_EQ(sol, result);
+  }
+  {
+    std::vector<double> values{1.5, 2.5, 3.5, 4.5};
+    std::vector<double> result(values.size() + 1);
+    Context ctx;
+    PartialSum(ctx.Threads(), values.begin(), values.end(), 0.0, result.begin());
+    std::vector<double> sol(values.size() + 1, 0.0);
+    std::partial_sum(values.begin(), values.end(), sol.begin() + 1);
+    ASSERT_EQ(sol, result);
+  }
+}
+}  // namespace common
+}  // namespace xgboost

tests/cpp/tree/test_approx.cc

@@ -3,6 +3,7 @@
  */
 #include <gtest/gtest.h>
 
+#include "../../../src/common/numeric.h"
 #include "../../../src/tree/updater_approx.h"
 #include "../helpers.h"
 #include "test_partitioner.h"