// 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.model.server;

import static google.registry.model.ofy.ObjectifyService.ofy;
import static google.registry.model.transaction.TransactionManagerFactory.tm;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.primitives.Longs;
import com.googlecode.objectify.annotation.Entity;
import com.googlecode.objectify.annotation.Unindex;
import google.registry.model.annotations.NotBackedUp;
import google.registry.model.annotations.NotBackedUp.Reason;
import google.registry.model.common.CrossTldSingleton;
import java.nio.ByteBuffer;
import java.util.UUID;
import java.util.concurrent.ExecutionException;

/** A secret number used for generating tokens (such as XSRF tokens). */
@Entity
@Unindex
@NotBackedUp(reason = Reason.AUTO_GENERATED)
// TODO(b/27427316): Replace this with an entry in KMSKeyring
public class ServerSecret extends CrossTldSingleton {

  /**
   * Cache of the singleton ServerSecret instance that creates it if not present.
   *
   * <p>The key is meaningless since there is only one instance; this is essentially a memoizing
   * Supplier that can be reset for testing purposes.
   */
  private static final LoadingCache<Class<ServerSecret>, ServerSecret> CACHE =
      CacheBuilder.newBuilder().build(
          new CacheLoader<Class<ServerSecret>, ServerSecret>() {
            @Override
            public ServerSecret load(Class<ServerSecret> unused) {
              // Fast path - non-transactional load to hit memcache.
              ServerSecret secret = ofy().load().entity(new ServerSecret()).now();
              if (secret != null) {
                return secret;
              }
              // Slow path - transactionally create a new ServerSecret (once per app setup).
              return tm().transact(() -> {
                // Check again for an existing secret within the transaction to avoid races.
                ServerSecret secret1 = ofy().load().entity(new ServerSecret()).now();
                if (secret1 == null) {
                  UUID uuid = UUID.randomUUID();
                  secret1 = create(uuid.getMostSignificantBits(), uuid.getLeastSignificantBits());
                  ofy().saveWithoutBackup().entity(secret1).now();
                }
                return secret1;
              });
            }
          });

  /** Returns the global ServerSecret instance, creating it if one isn't already in Datastore. */
  public static ServerSecret get() {
    try {
      return CACHE.get(ServerSecret.class);
    } catch (ExecutionException e) {
      throw new RuntimeException(e);
    }
  }

  /** Most significant 8 bytes of the UUID value. */
  long mostSignificant;

  /** Least significant 8 bytes of the UUID value. */
  long leastSignificant;

  @VisibleForTesting
  static ServerSecret create(long mostSignificant, long leastSignificant) {
    ServerSecret secret = new ServerSecret();
    secret.mostSignificant = mostSignificant;
    secret.leastSignificant = leastSignificant;
    return secret;
  }

  /** Returns the value of this ServerSecret as a UUID. */
  public UUID asUuid() {
    return new UUID(mostSignificant, leastSignificant);
  }

  /** Returns the value of this ServerSecret as a byte array. */
  public byte[] asBytes() {
    return ByteBuffer.allocate(Longs.BYTES * 2)
        .putLong(mostSignificant)
        .putLong(leastSignificant)
        .array();
  }

  @VisibleForTesting
  static void resetCache() {
    CACHE.invalidateAll();
  }
}
