Extract make metric name from ranking metric.

- Extract the metric parsing routine from ranking.
- Add test.

R-package/src/Makevars.in

@@ -90,6 +90,7 @@ OBJECTS= \
     $(PKGROOT)/src/common/stats.o \
     $(PKGROOT)/src/common/survival_util.o \
     $(PKGROOT)/src/common/threading_utils.o \
+    $(PKGROOT)/src/common/ranking_utils.o \
     $(PKGROOT)/src/common/timer.o \
     $(PKGROOT)/src/common/version.o \
     $(PKGROOT)/src/c_api/c_api.o \

R-package/src/Makevars.win

@@ -90,6 +90,7 @@ OBJECTS= \
     $(PKGROOT)/src/common/stats.o \
     $(PKGROOT)/src/common/survival_util.o \
     $(PKGROOT)/src/common/threading_utils.o \
+    $(PKGROOT)/src/common/ranking_utils.o \
     $(PKGROOT)/src/common/timer.o \
     $(PKGROOT)/src/common/version.o \
     $(PKGROOT)/src/c_api/c_api.o \

include/xgboost/string_view.h

@@ -1,15 +1,15 @@
-/*!
- * Copyright 2021 by XGBoost Contributors
+/**
+ * Copyright 2021-2023 by XGBoost Contributors
  */
 #ifndef XGBOOST_STRING_VIEW_H_
 #define XGBOOST_STRING_VIEW_H_
-#include <xgboost/logging.h>
-#include <xgboost/span.h>
+#include <xgboost/logging.h>  // CHECK_LT
+#include <xgboost/span.h>     // Span
 
-#include <algorithm>
-#include <iterator>
-#include <ostream>
-#include <string>
+#include <algorithm>          // std::equal,std::min
+#include <iterator>           // std::reverse_iterator
+#include <ostream>            // std::ostream
+#include <string>             // std::char_traits,std::string
 
 namespace xgboost {
 struct StringView {
@@ -28,29 +28,31 @@ struct StringView {
 
  public:
   constexpr StringView() = default;
-  constexpr StringView(CharT const* str, size_t size) : str_{str}, size_{size} {}
+  constexpr StringView(CharT const* str, std::size_t size) : str_{str}, size_{size} {}
   explicit StringView(std::string const& str) : str_{str.c_str()}, size_{str.size()} {}
-  StringView(CharT const* str) : str_{str}, size_{Traits::length(str)} {}  // NOLINT
+  constexpr StringView(CharT const* str)  // NOLINT
+      : str_{str}, size_{str == nullptr ? 0ul : Traits::length(str)} {}
 
   CharT const& operator[](size_t p) const { return str_[p]; }
   CharT const& at(size_t p) const {  // NOLINT
     CHECK_LT(p, size_);
     return str_[p];
   }
-  constexpr size_t size() const { return size_; }  // NOLINT
-  StringView substr(size_t beg, size_t n) const {  // NOLINT
+  constexpr std::size_t size() const { return size_; }  // NOLINT
+  constexpr bool empty() const { return size() == 0; }  // NOLINT
+  StringView substr(size_t beg, size_t n) const {       // NOLINT
     CHECK_LE(beg, size_);
     size_t len = std::min(n, size_ - beg);
     return {str_ + beg, len};
   }
-  CharT const* c_str() const { return str_; }  // NOLINT
+  CharT const* c_str() const { return str_; }                    // NOLINT
 
   constexpr CharT const* cbegin() const { return str_; }         // NOLINT
   constexpr CharT const* cend() const { return str_ + size(); }  // NOLINT
   constexpr CharT const* begin() const { return str_; }          // NOLINT
   constexpr CharT const* end() const { return str_ + size(); }   // NOLINT
 
-  const_reverse_iterator rbegin() const noexcept {  // NOLINT
+  const_reverse_iterator rbegin() const noexcept {               // NOLINT
     return const_reverse_iterator(this->end());
   }
   const_reverse_iterator crbegin() const noexcept {  // NOLINT

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_

src/metric/rank_metric.cc

@@ -28,6 +28,7 @@
 
 #include "../collective/communicator-inl.h"
 #include "../common/math.h"
+#include "../common/ranking_utils.h"  // MakeMetricName
 #include "../common/threading_utils.h"
 #include "metric_common.h"
 #include "xgboost/host_device_vector.h"
@@ -232,23 +233,7 @@ struct EvalRank : public Metric, public EvalRankConfig {
 
  protected:
   explicit EvalRank(const char* name, const char* param) {
-    using namespace std;  // NOLINT(*)
-
-    if (param != nullptr) {
-      std::ostringstream os;
-      if (sscanf(param, "%u[-]?", &topn) == 1) {
-        os << name << '@' << param;
-        this->name = os.str();
-      } else {
-        os << name << param;
-        this->name = os.str();
-      }
-      if (param[strlen(param) - 1] == '-') {
-        minus = true;
-      }
-    } else {
-      this->name = name;
-    }
+    this->name = ltr::MakeMetricName(name, param, &topn, &minus);
   }
 
   virtual double EvalGroup(PredIndPairContainer *recptr) const = 0;

tests/cpp/common/test_ranking_utils.cc

@@ -0,0 +1,37 @@
+/**
+ * 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
+
+  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