// Copyright (C) 2017 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.googlesource.gerrit.plugins.findowners;

import static java.util.concurrent.TimeUnit.SECONDS;

import com.google.common.cache.CacheBuilder;
import com.google.gerrit.server.query.change.ChangeData;
import com.google.gwtorm.server.OrmException;
import java.util.Collection;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import org.eclipse.jgit.lib.Repository;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/** Save OwnersDb in a cache for multiple calls to submit_filter. */
class Cache {
  private static final Logger log = LoggerFactory.getLogger(Cache.class);

  // The OwnersDb is created from OWNERS files in directories that
  // contain changed files of a patch set, which belongs to a project
  // and branch. OwnersDb can be cached if the head of a project branch
  // and the patch set are not changed.

  // Although the head of a project branch could be changed by other users,
  // it is better to assume the same for a patch set during a short period
  // of time. So multiple checks would have the same result. For example,
  // one client UI action can trigger multiple HTTP requests.
  // Each HTTP request has one StoredValues,
  // and can trigger multiple Prolog submit_filter.
  // Each submit_filter has one Prolog engine.
  // It would not be enough to keep the cache in a Prolog engine environment
  // or a StoredValues.

  private static Cache instance = null; // a singleton

  // When branch is "refs/heads/xyz" use only "xyz",
  // to share cached OwnersDb between these two branch names.
  private static final String REFS_HEADS = "refs/heads/";

  // Until we have a way to inject Java singleton into Prolog PRED_* constructors,
  // we keep the cache in a static object for all HTTP requests.
  private com.google.common.cache.Cache<String, OwnersDb> dbCache;

  private Cache() {
    init(Config.getMaxCacheAge(), Config.getMaxCacheSize());
  }

  long size() {
    return (dbCache == null) ? 0 : dbCache.size();
  }

  Cache init(int maxSeconds, int maxSize) {
    // This should be called once in normal configuration,
    // but could be called multiple times in unit or integration tests.
    if (dbCache != null) {
      dbCache.invalidateAll(); // release all cached objects
    }
    if (maxSeconds > 0) {
      log.info("Initialize Cache with maxSeconds=" + maxSeconds + " maxSize=" + maxSize);
      dbCache =
          CacheBuilder.newBuilder()
              .maximumSize(maxSize)
              .expireAfterWrite(maxSeconds, SECONDS)
              .build();
    } else {
      log.info("Cache disabled.");
      dbCache = null;
    }
    return this;
  }

  /** Returns a cached or new OwnersDb, for the current patchset. */
  OwnersDb get(Repository repo, ChangeData changeData) throws OrmException {
    return get(repo, changeData, changeData.currentPatchSet().getId().get());
  }

  /** Returns a cached or new OwnersDb, for the specified patchset. */
  OwnersDb get(Repository repository, ChangeData changeData, int patchset) throws OrmException {
    String branch = changeData.change().getDest().get();
    String dbKey = Cache.makeKey(changeData.getId().get(), patchset, branch);
    // TODO: get changed files of the given patchset?
    return get(dbKey, repository, branch, changeData.currentFilePaths());
  }

  /** Returns a cached or new OwnersDb, for the specified branch and changed files. */
  OwnersDb get(String key, Repository repository, String branch, Collection<String> files) {
    if (dbCache == null) { // Do not cache OwnersDb
      log.trace("Create new OwnersDb, key=" + key);
      return new OwnersDb(key, repository, branch, files);
    }
    try {
      log.trace("Get from cash " + dbCache + ", key=" + key + ", cache size=" + dbCache.size());
      return dbCache.get(
          key,
          new Callable<OwnersDb>() {
            @Override
            public OwnersDb call() {
              log.trace("Create new OwnersDb, key=" + key);
              return new OwnersDb(key, repository, branch, files);
            }
          });
    } catch (ExecutionException e) {
      log.error("Cache.get has exception: " + e);
      return new OwnersDb(key, repository, branch, files);
    }
  }

  public static String makeKey(int change, int patch, String branch) {
    if (branch.indexOf(REFS_HEADS) == 0) {
      branch = branch.substring(REFS_HEADS.length());
    }
    return change + ":" + patch + ":" + branch;
  }

  public static Cache getInstance() {
    if (instance == null) {
      instance = new Cache();
    }
    return instance;
  }
}