Clarify the behavior of invalid categorical value handling. (#7529)

tests/cpp/common/test_categorical.cc

@@ -0,0 +1,43 @@
+/*!
+ * Copyright 2021 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <limits>
+
+#include "../../../src/common/categorical.h"
+
+namespace xgboost {
+namespace common {
+TEST(Categorical, Decision) {
+  // inf
+  float a = std::numeric_limits<float>::infinity();
+
+  ASSERT_TRUE(common::InvalidCat(a));
+  std::vector<uint32_t> cats(256, 0);
+  ASSERT_TRUE(Decision(cats, a, true));
+
+  // larger than size
+  a = 256;
+  ASSERT_TRUE(Decision(cats, a, true));
+
+  // negative
+  a = -1;
+  ASSERT_TRUE(Decision(cats, a, true));
+
+  CatBitField bits{cats};
+  bits.Set(0);
+  a = -0.5;
+  ASSERT_TRUE(Decision(cats, a, true));
+
+  // round toward 0
+  a = 0.5;
+  ASSERT_FALSE(Decision(cats, a, true));
+
+  // valid
+  a = 13;
+  bits.Set(a);
+  ASSERT_FALSE(Decision(bits.Bits(), a, true));
+}
+}  // namespace common
+}  // namespace xgboost