// Copyright (C) 2012 The Android Open Source Project
//
// 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 com.google.gerrit.server.project;

import com.google.gerrit.server.config.ConfigUtil;
import com.google.gerrit.server.config.GerritServerConfig;
import com.google.inject.Inject;
import com.google.inject.Singleton;

import org.eclipse.jgit.lib.Config;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/** Ticks periodically to force refresh events for {@link ProjectCacheImpl}. */
@Singleton
public class ProjectCacheClock {
  private volatile long generation;

  @Inject
  public ProjectCacheClock(@GerritServerConfig Config serverConfig) {
    this(TimeUnit.MILLISECONDS.convert(
        ConfigUtil.getTimeUnit(serverConfig,
            "cache", "projects", "checkFrequency",
            5, TimeUnit.MINUTES), TimeUnit.MINUTES));
  }

  public ProjectCacheClock(long checkFrequencyMillis) {
    if (10 < checkFrequencyMillis) {
      // Start with generation 1 (to avoid magic 0 below).
      generation = 1;
      ThreadFactory factory = new ThreadFactory() {
        private final AtomicInteger id = new AtomicInteger();

        @Override
        public Thread newThread(Runnable runnable) {
          Thread thread = Executors.defaultThreadFactory().newThread(runnable);
          thread.setName(String.format("ProjectCacheClock-%d", id.incrementAndGet()));
          thread.setDaemon(true);
          thread.setPriority(Thread.MIN_PRIORITY);
          return thread;
        }
      };
      ScheduledExecutorService executor = Executors.newScheduledThreadPool(1, factory);
      executor.scheduleAtFixedRate(new Runnable() {
        @Override
        public void run() {
          // This is not exactly thread-safe, but is OK for our use.
          // The only thread that writes the volatile is this task.
          generation = generation + 1;
        }
      }, checkFrequencyMillis, checkFrequencyMillis, TimeUnit.MILLISECONDS);
    } else {
      // Magic generation 0 triggers ProjectState to always
      // check on each needsRefresh() request we make to it.
      generation = 0;
    }
  }

  long read() {
    return generation;
  }
}