Add a basic logging framework

Create a basic system to emit diagnostics to the user during operation.
Mimic the glog API for the most part, but use an output format similar
to that of coreutils and other established command-line tools.

Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

1 files changed, 96 insertions, 0 deletions

diff --git a/log.h b/log.h
new file mode 100644
index 0000000..fe2015b
--- /dev/null
+++ b/log.h
@@ -0,0 +1,96 @@
+// 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_