// Copyright 2017 The Nomulus Authors. All Rights Reserved.
//
// 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
//
//     http://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.

package google.registry.proxy;

import com.google.common.collect.ImmutableMap;
import com.google.gson.Gson;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.logging.Formatter;
import java.util.logging.Level;
import java.util.logging.LogRecord;

/**
 * JUL formatter that formats log messages in a single-line JSON that Stackdriver logging can parse.
 *
 * <p>There is no clear documentation on how to achieve this or on the format of the JSON. This is
 * much a trial and error process, plus a lot of searching. To summarize, if the logs are printed to
 * {@code STDOUT} or {@code STDERR} in a single-line JSON, with the content in the {@code message}
 * field and the log level in the {@code severity} field, it will be picked up by Stackdriver
 * logging agent running in GKE containers and logged at correct level..
 *
 * @see <a
 *     href="https://medium.com/retailmenot-engineering/formatting-python-logs-for-stackdriver-5a5ddd80761c">
 *     Formatting Python Logs from Stackdriver</a> <a
 *     href="https://stackoverflow.com/questions/44164730/gke-stackdriver-java-logback-logging-format">
 *     GKE & Stackdriver: Java logback logging format?</a>
 */
class GcpJsonFormatter extends Formatter {

  /** JSON field that determines the log level. */
  private static final String SEVERITY = "severity";

  /**
   * JSON field that stores the calling class and function when the log occurs.
   *
   * <p>This field is not used by Stackdriver, but it is useful and can be found when the log
   * entries are expanded
   */
  private static final String SOURCE = "source";

  /** JSON field that contains the content, this will show up as the main entry in a log. */
  private static final String MESSAGE = "message";

  private static final Gson gson = new Gson();

  @Override
  public String format(LogRecord record) {
    // Add an extra newline before the message. Stackdriver does not show newlines correctly, and
    // treats them as whitespace. If you want to see correctly formatted log message, expand the
    // log and look for the jsonPayload.message field. This newline makes sure that the entire
    // message starts on its own line, so that indentation within the message is correct.

    String message = "\n" + record.getMessage();
    String severity = severityFor(record.getLevel());

    // The rest is mostly lifted from java.util.logging.SimpleFormatter.
    String stacktrace = "";
    if (record.getThrown() != null) {
      StringWriter sw = new StringWriter();
      try (PrintWriter pw = new PrintWriter(sw)) {
        pw.println();
        record.getThrown().printStackTrace(pw);
      }
      stacktrace = sw.toString();
    }

    String source;
    if (record.getSourceClassName() != null) {
      source = record.getSourceClassName();
      if (record.getSourceMethodName() != null) {
        source += " " + record.getSourceMethodName();
      }
    } else {
      source = record.getLoggerName();
    }

    return gson.toJson(
            ImmutableMap.of(SEVERITY, severity, SOURCE, source, MESSAGE, message + stacktrace))
        + '\n';
  }

  /**
   * Map {@link Level} to a severity string that Stackdriver understands.
   *
   * @see <a
   *     href="https://github.com/googleapis/google-cloud-java/blob/master/google-cloud-clients/google-cloud-logging/src/main/java/com/google/cloud/logging/LoggingHandler.java#L325">{@code LoggingHandler}</a>
   */
  private static String severityFor(Level level) {
    switch (level.intValue()) {
        // FINEST
      case 300:
        return "DEBUG";
        // FINER
      case 400:
        return "DEBUG";
        // FINE
      case 500:
        return "DEBUG";
        // CONFIG
      case 700:
        return "INFO";
        // INFO
      case 800:
        return "INFO";
        // WARNING
      case 900:
        return "WARNING";
        // SEVERE
      case 1000:
        return "ERROR";
      default:
        return "DEFAULT";
    }
  }
}