Extract CPU sampling routines. (#8697)

tests/cpp/tree/hist/test_sampler.cc

@@ -0,0 +1,57 @@
+/**
+ * Copyright 2023 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstddef>  // std::size_t
+#include <string>   // std::to_string
+
+#include "../../../../src/tree/hist/sampler.h"  // SampleGradient
+#include "../../../../src/tree/param.h"         // TrainParam
+#include "xgboost/base.h"                       // GradientPair,bst_target_t
+#include "xgboost/context.h"                    // Context
+#include "xgboost/data.h"                       // MetaInfo
+#include "xgboost/linalg.h"                     // Matrix,Constants
+
+namespace xgboost {
+namespace tree {
+TEST(Sampler, Basic) {
+  std::size_t constexpr kRows = 1024;
+  double constexpr kSubsample = .2;
+  TrainParam param;
+  param.UpdateAllowUnknown(Args{{"subsample", std::to_string(kSubsample)}});
+  Context ctx;
+
+  auto run = [&](bst_target_t n_targets) {
+    auto init = GradientPair{1.0f, 1.0f};
+    linalg::Matrix<GradientPair> gpair = linalg::Constant(&ctx, init, kRows, n_targets);
+    auto h_gpair = gpair.HostView();
+    SampleGradient(&ctx, param, h_gpair);
+    std::size_t n_sampled{0};
+    for (std::size_t i = 0; i < kRows; ++i) {
+      bool sampled{false};
+      if (h_gpair(i, 0).GetGrad() - .0f != .0f) {
+        sampled = true;
+        n_sampled++;
+      }
+      for (bst_target_t t = 1; t < n_targets; ++t) {
+        if (sampled) {
+          ASSERT_EQ(h_gpair(i, t).GetGrad() - init.GetGrad(), .0f);
+          ASSERT_EQ(h_gpair(i, t).GetHess() - init.GetHess(), .0f);
+
+        } else {
+          ASSERT_EQ(h_gpair(i, t).GetGrad() - .0f, .0f);
+          ASSERT_EQ(h_gpair(i, t).GetHess() - .0f, .0f);
+        }
+      }
+    }
+    auto ratio = static_cast<double>(n_sampled) / static_cast<double>(kRows);
+    ASSERT_LT(ratio, kSubsample * 1.5);
+    ASSERT_GT(ratio, kSubsample * 0.5);
+  };
+
+  run(1);
+  run(3);
+}
+}  // namespace tree
+}  // namespace xgboost