Extract make metric name from ranking metric. (#8768)

- Extract the metric parsing routine from ranking.
- Add a test.
- Accept null for string view.

tests/cpp/common/test_ranking_utils.cc

@@ -0,0 +1,38 @@
+/**
+ * Copyright 2023 by XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <cstdint>  // std::uint32_t
+
+#include "../../../src/common/ranking_utils.h"
+
+namespace xgboost {
+namespace ltr {
+TEST(RankingUtils, MakeMetricName) {
+  std::uint32_t topn{32};
+  bool minus{false};
+  auto name = MakeMetricName("ndcg", "3-", &topn, &minus);
+  ASSERT_EQ(name, "ndcg@3-");
+  ASSERT_EQ(topn, 3);
+  ASSERT_TRUE(minus);
+
+  name = MakeMetricName("ndcg", "6", &topn, &minus);
+  ASSERT_EQ(topn, 6);
+  ASSERT_TRUE(minus);  // unchanged
+
+  minus = false;
+  name = MakeMetricName("ndcg", "-", &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);
+
+  name = MakeMetricName("ndcg", nullptr, &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);  // unchanged
+
+  name = MakeMetricName("ndcg", StringView{}, &topn, &minus);
+  ASSERT_EQ(topn, 6);  // unchanged
+  ASSERT_TRUE(minus);  // unchanged
+}
+}  // namespace ltr
+}  // namespace xgboost

tests/cpp/common/test_ranking_utils.cu

@@ -1,3 +1,6 @@
+/**
+ * Copyright 2021 by XGBoost Contributors
+ */
 #include <gtest/gtest.h>
 #include "../../../src/common/ranking_utils.cuh"
 #include "../../../src/common/device_helpers.cuh"

tests/cpp/common/test_string_view.cc

@@ -1,9 +1,13 @@
-/*!
- * Copyright (c) by XGBoost Contributors 2021
+/**
+ * Copyright 2021-2023 by XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/string_view.h>
-#include <string_view>
+
+#include <algorithm>  // std::equal
+#include <sstream>    // std::stringstream
+#include <string>     // std::string
+
 namespace xgboost {
 TEST(StringView, Basic) {
   StringView str{"This is a string."};
@@ -24,5 +28,16 @@ TEST(StringView, Basic) {
   ASSERT_FALSE(substr == "i");
 
   ASSERT_TRUE(std::equal(substr.crbegin(), substr.crend(), StringView{"si"}.cbegin()));
+
+  {
+    StringView empty{nullptr};
+    ASSERT_TRUE(empty.empty());
+  }
+  {
+    StringView empty{""};
+    ASSERT_TRUE(empty.empty());
+    StringView empty2{nullptr};
+    ASSERT_EQ(empty, empty2);
+  }
 }
 }  // namespace xgboost