Unify CPU hist sketching (#5880)

tests/cpp/common/test_quantile.h

@@ -0,0 +1,54 @@
+#include <rabit/rabit.h>
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "../helpers.h"
+
+namespace xgboost {
+namespace common {
+inline void InitRabitContext(std::string msg, size_t n_workers) {
+  auto port = std::getenv("DMLC_TRACKER_PORT");
+  std::string port_str;
+  if (port) {
+    port_str = port;
+  } else {
+    LOG(WARNING) << msg << " as `DMLC_TRACKER_PORT` is not set up.";
+    return;
+  }
+
+  std::vector<std::string> envs{
+      "DMLC_TRACKER_PORT=" + port_str,
+      "DMLC_TRACKER_URI=127.0.0.1",
+      "DMLC_NUM_WORKER=" + std::to_string(n_workers)};
+  char* c_envs[] {&(envs[0][0]), &(envs[1][0]), &(envs[2][0])};
+  rabit::Init(3, c_envs);
+}
+
+template <typename Fn> void RunWithSeedsAndBins(size_t rows, Fn fn) {
+  std::vector<int32_t> seeds(4);
+  SimpleLCG lcg;
+  SimpleRealUniformDistribution<float> dist(3, 1000);
+  std::generate(seeds.begin(), seeds.end(), [&](){ return dist(&lcg); });
+
+  std::vector<size_t> bins(8);
+  for (size_t i = 0; i < bins.size() - 1; ++i) {
+    bins[i] = i * 35 + 2;
+  }
+  bins.back() = rows + 80;  // provide a bin number greater than rows.
+
+  std::vector<MetaInfo> infos(2);
+  auto& h_weights = infos.front().weights_.HostVector();
+  h_weights.resize(rows);
+  std::generate(h_weights.begin(), h_weights.end(), [&]() { return dist(&lcg); });
+
+  for (auto seed : seeds) {
+    for (auto n_bin : bins) {
+      for (auto const& info : infos) {
+        fn(seed, n_bin, info);
+      }
+    }
+  }
+}
+}  // namespace common
+}  // namespace xgboost