// Copyright (C) 2014 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.ericsson.gerrit.plugins.highavailability.websession.file;

import com.ericsson.gerrit.plugins.highavailability.SharedDirectory;
import com.google.common.cache.Cache;
import com.google.common.cache.CacheStats;
import com.google.common.collect.ImmutableMap;
import com.google.gerrit.common.Nullable;
import com.google.gerrit.httpd.WebSessionManager;
import com.google.gerrit.httpd.WebSessionManager.Val;
import com.google.inject.Inject;
import com.google.inject.Singleton;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;
import java.time.Clock;
import java.time.Instant;
import java.time.ZoneId;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Singleton
public class FileBasedWebsessionCache implements Cache<String, WebSessionManager.Val> {
  /** Provides static methods to set the system clock for testing purposes only. */
  static class TimeMachine {
    private static Clock clock = Clock.systemDefaultZone();

    private TimeMachine() {
      throw new IllegalAccessError("Utility class. Not meant to be instantiated.");
    }

    static Instant now() {
      return Instant.now(getClock());
    }

    static void useFixedClockAt(Instant instant) {
      clock = Clock.fixed(instant, ZoneId.systemDefault());
    }

    static void useSystemDefaultZoneClock() {
      clock = Clock.systemDefaultZone();
    }

    private static Clock getClock() {
      return clock;
    }
  }

  private static final Logger log = LoggerFactory.getLogger(FileBasedWebsessionCache.class);

  private final Path websessionsDir;

  @Inject
  public FileBasedWebsessionCache(@SharedDirectory Path sharedDirectory) throws IOException {
    this.websessionsDir = sharedDirectory.resolve("websessions");
    Files.createDirectories(websessionsDir);
  }

  @Override
  public ConcurrentMap<String, Val> asMap() {
    ConcurrentMap<String, Val> map = new ConcurrentHashMap<>();
    for (Path path : listFiles()) {
      Val v = readFile(path);
      if (v != null) {
        map.put(path.getFileName().toString(), v);
      }
    }
    return map;
  }

  @Override
  public void cleanUp() {
    for (Path path : listFiles()) {
      Val val = readFile(path);
      if (val != null) {
        Instant expires = Instant.ofEpochMilli(val.getExpiresAt());
        if (expires.isBefore(TimeMachine.now())) {
          deleteFile(path);
        }
      }
    }
  }

  @Override
  public Val get(String key, Callable<? extends Val> valueLoader) throws ExecutionException {
    Val value = getIfPresent(key);
    if (value == null) {
      try {
        value = valueLoader.call();
      } catch (Exception e) {
        throw new ExecutionException(e);
      }
    }
    return value;
  }

  @Override
  public ImmutableMap<String, Val> getAllPresent(Iterable<?> keys) {
    ImmutableMap.Builder<String, Val> mapBuilder = new ImmutableMap.Builder<>();
    for (Object key : keys) {
      Val v = getIfPresent(key);
      if (v != null) {
        mapBuilder.put((String) key, v);
      }
    }
    return mapBuilder.build();
  }

  @Override
  @Nullable
  public Val getIfPresent(Object key) {
    if (key instanceof String) {
      Path path = websessionsDir.resolve((String) key);
      return readFile(path);
    }
    return null;
  }

  @Override
  public void invalidate(Object key) {
    if (key instanceof String) {
      deleteFile(websessionsDir.resolve((String) key));
    }
  }

  @Override
  public void invalidateAll() {
    for (Path path : listFiles()) {
      deleteFile(path);
    }
  }

  @Override
  public void invalidateAll(Iterable<?> keys) {
    for (Object key : keys) {
      invalidate(key);
    }
  }

  @Override
  public void put(String key, Val value) {
    try {
      Path tempFile = Files.createTempFile(websessionsDir, UUID.randomUUID().toString(), null);
      try (OutputStream fileStream = Files.newOutputStream(tempFile);
          ObjectOutputStream objStream = new ObjectOutputStream(fileStream)) {
        objStream.writeObject(value);
        Files.move(
            tempFile,
            tempFile.resolveSibling(key),
            StandardCopyOption.REPLACE_EXISTING,
            StandardCopyOption.ATOMIC_MOVE);
      }
    } catch (IOException e) {
      log.warn("Cannot put into cache {}", websessionsDir, e);
    }
  }

  @Override
  public void putAll(Map<? extends String, ? extends Val> keys) {
    for (Entry<? extends String, ? extends Val> e : keys.entrySet()) {
      put(e.getKey(), e.getValue());
    }
  }

  @Override
  public long size() {
    return listFiles().size();
  }

  @Override
  public CacheStats stats() {
    log.warn("stats() unimplemented");
    return null;
  }

  private Val readFile(Path path) {
    if (path.toFile().exists()) {
      try (InputStream fileStream = Files.newInputStream(path);
          ObjectInputStream objStream = new ObjectInputStream(fileStream)) {
        return (Val) objStream.readObject();
      } catch (ClassNotFoundException e) {
        log.warn(
            "Entry {} in cache {} has an incompatible class and can't be"
                + " deserialized. Invalidating entry.",
            path,
            websessionsDir);
        log.debug(e.getMessage(), e);
        invalidate(path.getFileName().toString());
      } catch (IOException e) {
        log.warn("Cannot read cache {}", websessionsDir, e);
      }
    }
    return null;
  }

  private void deleteFile(Path path) {
    try {
      Files.deleteIfExists(path);
    } catch (IOException e) {
      log.error("Error trying to delete {} from {}", path, websessionsDir, e);
    }
  }

  private List<Path> listFiles() {
    List<Path> files = new ArrayList<>();
    try (DirectoryStream<Path> dirStream = Files.newDirectoryStream(websessionsDir)) {
      for (Path path : dirStream) {
        files.add(path);
      }
    } catch (IOException e) {
      log.error("Cannot list files in cache {}", websessionsDir, e);
    }
    return files;
  }
}