// Copyright (C) 2020 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.plugins.codeowners.restapi;

import static com.google.common.collect.ImmutableList.toImmutableList;
import static com.google.common.collect.ImmutableSet.toImmutableSet;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.common.flogger.FluentLogger;
import com.google.gerrit.entities.Account;
import com.google.gerrit.entities.Project;
import com.google.gerrit.entities.RefNames;
import com.google.gerrit.exceptions.StorageException;
import com.google.gerrit.extensions.client.ListAccountsOption;
import com.google.gerrit.extensions.client.ListOption;
import com.google.gerrit.extensions.common.AccountVisibility;
import com.google.gerrit.extensions.restapi.AuthException;
import com.google.gerrit.extensions.restapi.BadRequestException;
import com.google.gerrit.extensions.restapi.Response;
import com.google.gerrit.extensions.restapi.RestReadView;
import com.google.gerrit.plugins.codeowners.api.CodeOwnerInfo;
import com.google.gerrit.plugins.codeowners.backend.CodeOwner;
import com.google.gerrit.plugins.codeowners.backend.CodeOwnerConfigHierarchy;
import com.google.gerrit.plugins.codeowners.backend.CodeOwnerResolver;
import com.google.gerrit.plugins.codeowners.backend.CodeOwnerResolverResult;
import com.google.gerrit.plugins.codeowners.backend.CodeOwnerScore;
import com.google.gerrit.plugins.codeowners.backend.CodeOwnerScoring;
import com.google.gerrit.plugins.codeowners.backend.config.CodeOwnersPluginConfiguration;
import com.google.gerrit.server.account.AccountControl;
import com.google.gerrit.server.account.AccountDirectory.FillOptions;
import com.google.gerrit.server.account.AccountLoader;
import com.google.gerrit.server.account.Accounts;
import com.google.gerrit.server.permissions.GlobalPermission;
import com.google.gerrit.server.permissions.PermissionBackend;
import com.google.gerrit.server.permissions.PermissionBackendException;
import com.google.gerrit.server.permissions.RefPermission;
import com.google.inject.Provider;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.List;
import java.util.Optional;
import java.util.Random;
import java.util.Set;
import java.util.stream.Stream;
import org.kohsuke.args4j.Option;

/**
 * Abstract base class for REST endpoints that get the code owners for an arbitrary path in a branch
 * or a revision of a change.
 */
public abstract class AbstractGetCodeOwnersForPath<R extends AbstractPathResource>
    implements RestReadView<R> {
  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  @VisibleForTesting public static final int DEFAULT_LIMIT = 10;

  private final AccountVisibility accountVisibility;
  private final Accounts accounts;
  private final AccountControl.Factory accountControlFactory;
  private final PermissionBackend permissionBackend;
  private final CodeOwnersPluginConfiguration codeOwnersPluginConfiguration;
  private final CodeOwnerConfigHierarchy codeOwnerConfigHierarchy;
  private final Provider<CodeOwnerResolver> codeOwnerResolver;
  private final CodeOwnerJson.Factory codeOwnerJsonFactory;
  private final EnumSet<ListAccountsOption> options;
  private final Set<String> hexOptions;

  private int limit = DEFAULT_LIMIT;
  private Optional<Long> seed = Optional.empty();

  @Option(
      name = "-o",
      usage = "Options to control which fields should be populated for the returned account")
  public void addOption(ListAccountsOption o) {
    options.add(o);
  }

  @Option(
      name = "-O",
      usage =
          "Options to control which fields should be populated for the returned account, in hex")
  void setOptionFlagsHex(String hex) {
    hexOptions.add(hex);
  }

  @Option(
      name = "--limit",
      aliases = {"-n"},
      metaVar = "CNT",
      usage = "maximum number of code owners to list (default = " + DEFAULT_LIMIT + ")")
  public void setLimit(int limit) {
    this.limit = limit;
  }

  @Option(
      name = "--seed",
      usage = "seed that should be used to shuffle code owners that have the same score")
  public void setSeed(long seed) {
    this.seed = Optional.of(seed);
  }

  protected AbstractGetCodeOwnersForPath(
      AccountVisibility accountVisibility,
      Accounts accounts,
      AccountControl.Factory accountControlFactory,
      PermissionBackend permissionBackend,
      CodeOwnersPluginConfiguration codeOwnersPluginConfiguration,
      CodeOwnerConfigHierarchy codeOwnerConfigHierarchy,
      Provider<CodeOwnerResolver> codeOwnerResolver,
      CodeOwnerJson.Factory codeOwnerJsonFactory) {
    this.accountVisibility = accountVisibility;
    this.accounts = accounts;
    this.accountControlFactory = accountControlFactory;
    this.permissionBackend = permissionBackend;
    this.codeOwnersPluginConfiguration = codeOwnersPluginConfiguration;
    this.codeOwnerConfigHierarchy = codeOwnerConfigHierarchy;
    this.codeOwnerResolver = codeOwnerResolver;
    this.codeOwnerJsonFactory = codeOwnerJsonFactory;
    this.options = EnumSet.noneOf(ListAccountsOption.class);
    this.hexOptions = new HashSet<>();
  }

  protected Response<List<CodeOwnerInfo>> applyImpl(R rsrc)
      throws AuthException, BadRequestException, PermissionBackendException {
    parseHexOptions();
    validateLimit();

    if (!seed.isPresent()) {
      seed = getDefaultSeed(rsrc);
    }

    // The distance that applies to code owners that are defined in the root code owner
    // configuration.
    int rootDistance = rsrc.getPath().getNameCount();

    int defaultOwnersDistance = rootDistance + 1;
    int globalOwnersDistance = defaultOwnersDistance + 1;
    int maxDistance = globalOwnersDistance;

    CodeOwnerScoring.Builder distanceScoring = CodeOwnerScore.DISTANCE.createScoring(maxDistance);

    Set<CodeOwner> codeOwners = new HashSet<>();
    codeOwnerConfigHierarchy.visit(
        rsrc.getBranch(),
        rsrc.getRevision(),
        rsrc.getPath(),
        codeOwnerConfig -> {
          CodeOwnerResolverResult pathCodeOwners =
              codeOwnerResolver.get().resolvePathCodeOwners(codeOwnerConfig, rsrc.getPath());
          codeOwners.addAll(filterCodeOwners(rsrc, pathCodeOwners.codeOwners()));

          if (pathCodeOwners.ownedByAllUsers()) {
            fillUpWithRandomUsers(rsrc, codeOwners, limit);

            if (codeOwners.size() < limit) {
              logger.atFine().log(
                  "tried to fill up the suggestion list with random users,"
                      + " but didn't find enough visible accounts"
                      + " (wanted number of suggestions = %d, got = %d",
                  limit, codeOwners.size());
            }

            // We already found that the path is owned by all users. Hence we do not need to check
            // if there are further code owners in higher-level code owner configs.
            return false;
          }

          int distance =
              codeOwnerConfig.key().branchNameKey().branch().equals(RefNames.REFS_CONFIG)
                  ? defaultOwnersDistance
                  : rootDistance - codeOwnerConfig.key().folderPath().getNameCount();
          pathCodeOwners
              .codeOwners()
              .forEach(
                  localCodeOwner -> distanceScoring.putValueForCodeOwner(localCodeOwner, distance));

          // If codeOwners.size() >= limit we have gathered enough code owners and do not need to
          // look at further code owner configs.
          // We can abort here, since all further code owners will have a lower distance scoring
          // and hence they would appear at the end of the sorted code owners list and be dropped
          // due to the limit.
          return codeOwners.size() < limit;
        });

    if (codeOwners.size() < limit) {
      CodeOwnerResolverResult globalCodeOwners = getGlobalCodeOwners(rsrc.getBranch().project());
      globalCodeOwners
          .codeOwners()
          .forEach(
              codeOwner -> distanceScoring.putValueForCodeOwner(codeOwner, globalOwnersDistance));
      codeOwners.addAll(filterCodeOwners(rsrc, globalCodeOwners.codeOwners()));

      if (globalCodeOwners.ownedByAllUsers()) {
        fillUpWithRandomUsers(rsrc, codeOwners, limit);
      }
    }

    return Response.ok(
        codeOwnerJsonFactory
            .create(getFillOptions())
            .format(sortAndLimit(distanceScoring.build(), ImmutableSet.copyOf(codeOwners))));
  }

  private CodeOwnerResolverResult getGlobalCodeOwners(Project.NameKey projectName) {
    CodeOwnerResolverResult globalCodeOwners =
        codeOwnerResolver
            .get()
            .resolve(codeOwnersPluginConfiguration.getGlobalCodeOwners(projectName));
    logger.atFine().log("including global code owners = %s", globalCodeOwners);
    return globalCodeOwners;
  }

  /**
   * Filters out code owners that should not be suggested.
   *
   * <p>The following code owners are filtered out:
   *
   * <ul>
   *   <li>code owners that cannot see the branch: Code owners that cannot see the branch cannot
   *       approve paths in this branch. Hence returning them to the client is not useful.
   *   <li>code owners that are service users: Requesting a code owner approval from a service user
   *       normally doesn't make sense since they will not react to review requests.
   * </ul>
   */
  private ImmutableSet<CodeOwner> filterCodeOwners(R rsrc, ImmutableSet<CodeOwner> codeOwners) {
    return filterCodeOwners(rsrc, getVisibleCodeOwners(rsrc, codeOwners)).collect(toImmutableSet());
  }

  /**
   * To be overridden by subclasses to filter out additional code owners.
   *
   * @param rsrc resource on which the request is being performed
   * @param codeOwners stream of code owners that should be filtered
   * @return the filtered stream of code owners
   */
  protected Stream<CodeOwner> filterCodeOwners(R rsrc, Stream<CodeOwner> codeOwners) {
    return codeOwners;
  }

  /**
   * Returns the seed that should by default be used for sorting, if none was specified on the
   * request.
   *
   * <p>If {@link Optional#empty()} is returned, a random seed will be used.
   *
   * @param rsrc resource on which the request is being performed
   */
  protected Optional<Long> getDefaultSeed(R rsrc) {
    return Optional.empty();
  }

  private Stream<CodeOwner> getVisibleCodeOwners(R rsrc, ImmutableSet<CodeOwner> allCodeOwners) {
    return allCodeOwners.stream()
        .filter(
            codeOwner -> {
              if (isVisibleTo(rsrc, codeOwner)) {
                return true;
              }
              logger.atFine().log(
                  "Filtering out %s because this code owner cannot see the branch %s",
                  codeOwner, rsrc.getBranch().branch());
              return false;
            });
  }

  /** Whether the given resource is visible to the given code owner. */
  private boolean isVisibleTo(R rsrc, CodeOwner codeOwner) {
    // We always check for the visibility of the branch.
    // This is also correct for the GetCodeOwnersForPathInChange subclass where branch is the
    // destination branch of the change. For changes the intention of the visibility check is to
    // check whether the code owner could be added as reviewer to the change. For this it is
    // important whether the destination branch is visible to the code owner, rather than whether
    // the change is visible to the code owner. E.g. private changes are not visible to other users
    // unless they are added as a reviewer. This means, for private changes we want to suggest code
    // owners that cannot see the change, since adding them as a reviewer is possible. By adding the
    // code owner as a reviewer to the private change, the change becomes visible to them. This
    // behavior is consistent with the suggest reviewer implementation (see
    // SuggestChangeReviewers#getVisibility(ChangeControl).
    return permissionBackend
        .absentUser(codeOwner.accountId())
        .ref(rsrc.getBranch())
        .testOrFalse(RefPermission.READ);
  }

  private void parseHexOptions() throws BadRequestException {
    for (String hexOption : hexOptions) {
      try {
        options.addAll(
            ListOption.fromBits(ListAccountsOption.class, Integer.parseInt(hexOption, 16)));
      } catch (IllegalArgumentException e) {
        throw new BadRequestException(
            String.format("\"%s\" is not a valid value for \"-O\"", hexOption), e);
      }
    }
  }

  private void validateLimit() throws BadRequestException {
    if (limit <= 0) {
      throw new BadRequestException("limit must be positive");
    }
  }

  private Set<FillOptions> getFillOptions() throws AuthException, PermissionBackendException {
    Set<FillOptions> fillOptions = EnumSet.of(FillOptions.ID);
    if (options.contains(ListAccountsOption.DETAILS)) {
      fillOptions.addAll(AccountLoader.DETAILED_OPTIONS);
    }
    if (options.contains(ListAccountsOption.ALL_EMAILS)) {
      // Secondary emails are only visible to users that have the 'Modify Account' global
      // capability.
      permissionBackend.currentUser().check(GlobalPermission.MODIFY_ACCOUNT);
      fillOptions.add(FillOptions.EMAIL);
      fillOptions.add(FillOptions.SECONDARY_EMAILS);
    }
    return fillOptions;
  }

  private ImmutableList<CodeOwner> sortAndLimit(
      CodeOwnerScoring distanceScoring, ImmutableSet<CodeOwner> codeOwners) {
    return sortCodeOwners(seed, distanceScoring, codeOwners)
        .limit(limit)
        .collect(toImmutableList());
  }

  /**
   * Sorts the code owners.
   *
   * <p>Code owners with higher distance score are returned first.
   *
   * <p>The order of code owners with the same distance score is random.
   *
   * @param seed seed that should be used to randomize the order
   * @param distanceScoring the distance scorings for the code owners
   * @param codeOwners the code owners that should be sorted
   * @return the sorted code owners
   */
  private static Stream<CodeOwner> sortCodeOwners(
      Optional<Long> seed, CodeOwnerScoring distanceScoring, ImmutableSet<CodeOwner> codeOwners) {
    return randomizeOrder(seed, codeOwners).sorted(distanceScoring.comparingByScoring());
  }

  /**
   * Returns the entries from the given set in a random order.
   *
   * @param seed seed that should be used to randomize the order
   * @param set the set for which the entries should be returned in a random order
   * @return the entries from the given set in a random order
   */
  private static <T> Stream<T> randomizeOrder(Optional<Long> seed, Set<T> set) {
    List<T> randomlyOrderedCodeOwners = new ArrayList<>(set);
    Collections.shuffle(
        randomlyOrderedCodeOwners, seed.isPresent() ? new Random(seed.get()) : new Random());
    return randomlyOrderedCodeOwners.stream();
  }

  /**
   * If the limit is not reached yet, add random visible users as code owners to the given code
   * owner set.
   *
   * <p>Must be only used to complete the suggestion list when it is found that the path is owned by
   * all user.
   */
  private void fillUpWithRandomUsers(R rsrc, Set<CodeOwner> codeOwners, int limit) {
    if (codeOwners.size() >= limit) {
      // limit is already reach, we don't need to add further suggestions
      return;
    }

    logger.atFine().log("filling up with random users");
    codeOwners.addAll(
        filterCodeOwners(
                rsrc,
                // ask for 2 times the number of users that we need so that we still have enough
                // suggestions when some users are removed by the filterCodeOwners call or if the
                // returned users were already present in codeOwners
                getRandomVisibleUsers(2 * limit - codeOwners.size())
                    .map(CodeOwner::create)
                    .collect(toImmutableSet()))
            .stream()
            .filter(codeOwner -> !codeOwners.contains(codeOwner))
            .limit(limit - codeOwners.size())
            .collect(toImmutableSet()));
  }

  /**
   * Returns random visible users, at most as many as specified by the limit.
   *
   * <p>It's possible that this method returns less users than the limit although further visible
   * users exist. This is because we may inspect only a random set of users, instead of all users,
   * for performance reasons.
   *
   * @param limit the max number of users that should be returned
   * @return random visible users
   */
  private Stream<Account.Id> getRandomVisibleUsers(int limit) {
    try {
      if (permissionBackend.currentUser().test(GlobalPermission.VIEW_ALL_ACCOUNTS)) {
        return getRandomUsers(limit);
      }

      switch (accountVisibility) {
        case ALL:
          return getRandomUsers(limit);
        case SAME_GROUP:
        case VISIBLE_GROUP:
          // We cannot afford to inspect all relevant users and test their visibility for
          // performance reasons, hence we use a random sample of users that is 3 times the limit.
          return getRandomUsers(3 * limit)
              .filter(accountId -> accountControlFactory.get().canSee(accountId))
              .limit(limit);
        case NONE:
          return Stream.of();
      }

      throw new IllegalStateException("unknown account visibility setting: " + accountVisibility);
    } catch (IOException | PermissionBackendException e) {
      throw new StorageException("failed to get visible users", e);
    }
  }

  /**
   * Returns random users, at most as many as specified by the limit.
   *
   * <p>No visibility check is performed.
   */
  private Stream<Account.Id> getRandomUsers(int limit) throws IOException {
    return randomizeOrder(seed, accounts.allIds()).limit(limit);
  }
}