Add quantile metric. (#8761)

src/common/quantile_loss_utils.cc

@@ -0,0 +1,74 @@
+/**
+ * Copyright 2023 by XGBoost contributors
+ */
+#include "quantile_loss_utils.h"
+
+#include <cctype>             // std::isspace
+#include <istream>            // std::istream
+#include <ostream>            // std::ostream
+#include <string>             // std::string
+#include <vector>             // std::vector
+
+#include "xgboost/json.h"     // F32Array,TypeCheck,get,Number
+#include "xgboost/json_io.h"  // JsonWriter
+
+namespace xgboost {
+namespace common {
+std::ostream& operator<<(std::ostream& os, const ParamFloatArray& array) {
+  auto const& t = array.Get();
+  xgboost::F32Array arr{t.size()};
+  for (std::size_t i = 0; i < t.size(); ++i) {
+    arr.Set(i, t[i]);
+  }
+  std::vector<char> stream;
+  xgboost::JsonWriter writer{&stream};
+  arr.Save(&writer);
+  for (auto c : stream) {
+    os << c;
+  }
+  return os;
+}
+
+std::istream& operator>>(std::istream& is, ParamFloatArray& array) {
+  auto& t = array.Get();
+  t.clear();
+  std::string str;
+  while (!is.eof()) {
+    std::string tmp;
+    is >> tmp;
+    str += tmp;
+  }
+  std::size_t head{0};
+  // unify notation for parsing.
+  while (std::isspace(str[head])) {
+    ++head;
+  }
+  if (str[head] == '(') {
+    str[head] = '[';
+  }
+  auto tail = str.size() - 1;
+  while (std::isspace(str[tail])) {
+    --tail;
+  }
+  if (str[tail] == ')') {
+    str[tail] = ']';
+  }
+
+  auto jarr = xgboost::Json::Load(xgboost::StringView{str});
+  // return if there's only one element
+  if (xgboost::IsA<xgboost::Number>(jarr)) {
+    t.emplace_back(xgboost::get<xgboost::Number const>(jarr));
+    return is;
+  }
+
+  auto jvec = xgboost::get<xgboost::Array const>(jarr);
+  for (auto v : jvec) {
+    xgboost::TypeCheck<xgboost::Number>(v, "alpha");
+    t.emplace_back(get<xgboost::Number const>(v));
+  }
+  return is;
+}
+
+DMLC_REGISTER_PARAMETER(QuantileLossParam);
+}  // namespace common
+}  // namespace xgboost