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+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+// A basic logging system built with command-line utilities in mind.
+
+#ifndef GSRSUP_LOG_H_
+#define GSRSUP_LOG_H_
+
+#include <limits>
+#include <ostream>
+#include <string>
+
+#include <absl/strings/str_cat.h>
+#include <absl/strings/string_view.h>
+#include <stdlib.h>
+
+namespace gsrsup {
+
+// A logger object backed by a std::ostream (or by nothing, if the ostream is
+// null).
+//
+// This class inherits the thread-safety properties of its underlying ostream.
+class Log final {
+ public:
+  enum Level : int {
+    kDebug = 0,
+    kInfo = 1,
+    kWarning = 2,
+    kError = 3,
+    kNeverPrint = std::numeric_limits<int>::max(),
+  };
+
+  explicit Log(absl::string_view program, Level minimum_level,
+               std::ostream* out)
+      : prefix_(program), minimum_level_(minimum_level), out_(out) {}
+
+  explicit Log(absl::string_view program, Level minimum_level,
+               std::ostream& out)
+      : Log(program, minimum_level, &out) {}
+
+  explicit Log(absl::string_view program, std::ostream* out)
+      : Log(program, kInfo, out) {}
+
+  explicit Log(absl::string_view program, std::ostream& out)
+      : Log(program, &out) {}
+
+  Log(const Log&) = default;
+  Log& operator=(const Log&) = default;
+
+  template <typename... Args>
+  void Message(Level, Args...);
+
+ private:
+  std::string prefix_;
+  Level minimum_level_;
+  std::ostream* out_;
+};
+
+template <typename... Args>
+void Log::Message(Level level, Args... args) {
+  if (level < minimum_level_ || out_ == nullptr) {
+    return;
+  }
+
+  absl::string_view level_str;
+  switch (level) {
+    case kDebug:
+      level_str = "debug: ";
+      break;
+    case kWarning:
+      level_str = "warning: ";
+      break;
+    case kError:
+      level_str = "error: ";
+      break;
+    default:
+      break;
+  }
+  std::string message = absl::StrCat(prefix_, ": ", level_str, args..., "\n");
+  out_->write(message.data(), message.size());
+}
+
+}  // namespace gsrsup
+
+#endif  // GSRSUP_LOG_H_