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

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

src/common/ranking_utils.cc

@@ -0,0 +1,34 @@
+/**
+ * Copyright 2023 by XGBoost contributors
+ */
+#include "ranking_utils.h"
+
+#include <cstdint>                // std::uint32_t
+#include <sstream>                // std::ostringstream
+#include <string>                 // std::string,std::sscanf
+
+#include "xgboost/string_view.h"  // StringView
+
+namespace xgboost {
+namespace ltr {
+std::string MakeMetricName(StringView name, StringView param, std::uint32_t* topn, bool* minus) {
+  std::string out_name;
+  if (!param.empty()) {
+    std::ostringstream os;
+    if (std::sscanf(param.c_str(), "%u[-]?", topn) == 1) {
+      os << name << '@' << param;
+      out_name = os.str();
+    } else {
+      os << name << param;
+      out_name = os.str();
+    }
+    if (*param.crbegin() == '-') {
+      *minus = true;
+    }
+  } else {
+    out_name = name.c_str();
+  }
+  return out_name;
+}
+}  // namespace ltr
+}  // namespace xgboost

src/common/ranking_utils.h

@@ -0,0 +1,29 @@
+/**
+ * Copyright 2023 by XGBoost contributors
+ */
+#ifndef XGBOOST_COMMON_RANKING_UTILS_H_
+#define XGBOOST_COMMON_RANKING_UTILS_H_
+
+#include <cstddef>                // std::size_t
+#include <cstdint>                // std::uint32_t
+#include <string>                 // std::string
+
+#include "xgboost/string_view.h"  // StringView
+
+namespace xgboost {
+namespace ltr {
+/**
+ * \brief Construct name for ranking metric given parameters.
+ *
+ * \param [in] name   Null terminated string for metric name
+ * \param [in] param  Null terminated string for parameter like the `3-` in `ndcg@3-`.
+ * \param [out] topn  Top n documents parsed from param. Unchanged if it's not specified.
+ * \param [out] minus Whether we should turn the score into loss. Unchanged if it's not
+ *                    specified.
+ *
+ * \return The name of the metric.
+ */
+std::string MakeMetricName(StringView name, StringView param, std::uint32_t* topn, bool* minus);
+}  // namespace ltr
+}  // namespace xgboost
+#endif  // XGBOOST_COMMON_RANKING_UTILS_H_