Implement iterative DMatrix for CPU. (#8116)

tests/cpp/data/test_iterative_dmatrix.cc

@@ -0,0 +1,36 @@
+/*!
+ * Copyright 2022 XGBoost contributors
+ */
+#include "test_iterative_dmatrix.h"
+
+#include <gtest/gtest.h>
+
+#include "../../../src/data/gradient_index.h"
+#include "../../../src/data/iterative_dmatrix.h"
+#include "../helpers.h"
+
+namespace xgboost {
+namespace data {
+TEST(IterativeDMatrix, Ref) {
+  TestRefDMatrix<GHistIndexMatrix, NumpyArrayIterForTest>(
+      [&](GHistIndexMatrix const& page) { return page.cut; });
+}
+
+TEST(IterativeDMatrix, IsDense) {
+  int n_bins = 16;
+  auto test = [n_bins](float sparsity) {
+    NumpyArrayIterForTest iter(sparsity);
+    IterativeDMatrix m(&iter, iter.Proxy(), nullptr, Reset, Next,
+                       std::numeric_limits<float>::quiet_NaN(), 0, n_bins);
+    if (sparsity == 0.0) {
+      ASSERT_TRUE(m.IsDense());
+    } else {
+      ASSERT_FALSE(m.IsDense());
+    }
+  };
+  test(0.0);
+  test(0.1);
+  test(1.0);
+}
+}  // namespace data
+}  // namespace xgboost