Extract CPUExpandEntry and HistParam. (#7321)

* Remove kRootNid.
* Check for empty hessian.

src/tree/hist/expand_entry.h

@@ -0,0 +1,64 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#ifndef XGBOOST_TREE_HIST_EXPAND_ENTRY_H_
+#define XGBOOST_TREE_HIST_EXPAND_ENTRY_H_
+
+#include <utility>
+#include "../param.h"
+
+namespace xgboost {
+namespace tree {
+
+struct CPUExpandEntry {
+  int nid;
+  int depth;
+  SplitEntry split;
+  CPUExpandEntry() = default;
+  XGBOOST_DEVICE
+  CPUExpandEntry(int nid, int depth, SplitEntry split)
+      : nid(nid), depth(depth), split(std::move(split)) {}
+  CPUExpandEntry(int nid, int depth, float loss_chg)
+      : nid(nid), depth(depth)  {
+    split.loss_chg = loss_chg;
+  }
+
+  bool IsValid(const TrainParam& param, int num_leaves) const {
+    if (split.loss_chg <= kRtEps) return false;
+    if (split.left_sum.GetHess() == 0 || split.right_sum.GetHess() == 0) {
+      return false;
+    }
+    if (split.loss_chg < param.min_split_loss) {
+      return false;
+    }
+    if (param.max_depth > 0 && depth == param.max_depth) {
+      return false;
+    }
+    if (param.max_leaves > 0 && num_leaves == param.max_leaves) {
+      return false;
+    }
+    return true;
+  }
+
+  float GetLossChange() const { return split.loss_chg; }
+  bst_node_t GetNodeId() const { return nid; }
+
+  static bool ChildIsValid(const TrainParam& param, int depth, int num_leaves) {
+    if (param.max_depth > 0 && depth >= param.max_depth) return false;
+    if (param.max_leaves > 0 && num_leaves >= param.max_leaves) return false;
+    return true;
+  }
+
+  friend std::ostream& operator<<(std::ostream& os, const CPUExpandEntry& e) {
+    os << "ExpandEntry: \n";
+    os << "nidx: " << e.nid << "\n";
+    os << "depth: " << e.depth << "\n";
+    os << "loss: " << e.split.loss_chg << "\n";
+    os << "left_sum: " << e.split.left_sum << "\n";
+    os << "right_sum: " << e.split.right_sum << "\n";
+    return os;
+  }
+};
+}  // namespace tree
+}  // namespace xgboost
+#endif  // XGBOOST_TREE_HIST_EXPAND_ENTRY_H_

src/tree/hist/param.h

@@ -0,0 +1,23 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#ifndef XGBOOST_TREE_HIST_PARAM_H_
+#define XGBOOST_TREE_HIST_PARAM_H_
+#include "xgboost/parameter.h"
+
+namespace xgboost {
+namespace tree {
+// training parameters specific to this algorithm
+struct CPUHistMakerTrainParam
+    : public XGBoostParameter<CPUHistMakerTrainParam> {
+  bool single_precision_histogram;
+  // declare parameters
+  DMLC_DECLARE_PARAMETER(CPUHistMakerTrainParam) {
+    DMLC_DECLARE_FIELD(single_precision_histogram).set_default(false).describe(
+        "Use single precision to build histograms.");
+  }
+};
+}  // namespace tree
+}  // namespace xgboost
+
+#endif  // XGBOOST_TREE_HIST_PARAM_H_