// Copyright 2014 The Bazel 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 com.google.devtools.build.skyframe;

import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
import java.util.Map;
import javax.annotation.Nullable;

/**
 * A graph that exposes its entries and structure, for use by classes that must traverse it.
 *
 * <p>Certain graph implementations can throw {@link InterruptedException} when trying to retrieve
 * node entries. Such exceptions should not be caught locally -- they should be allowed to propagate
 * up.
 */
@ThreadSafe
public interface QueryableGraph {
  /**
   * Returns the node with the given {@code key}, or {@code null} if the node does not exist.
   *
   * @param requestor if non-{@code null}, the node on behalf of which {@code key} is being
   *     requested.
   * @param reason the reason the node is being requested.
   */
  @Nullable
  NodeEntry get(@Nullable SkyKey requestor, Reason reason, SkyKey key) throws InterruptedException;

  /**
   * Fetches all the given nodes. Returns a map {@code m} such that, for all {@code k} in {@code
   * keys}, {@code m.get(k).equals(e)} iff {@code get(k) == e} and {@code e != null}, and {@code
   * !m.containsKey(k)} iff {@code get(k) == null}.
   *
   * @param requestor if non-{@code null}, the node on behalf of which the given {@code keys} are
   *     being requested.
   * @param reason the reason the nodes are being requested.
   */
  Map<SkyKey, ? extends NodeEntry> getBatch(
      @Nullable SkyKey requestor, Reason reason, Iterable<SkyKey> keys) throws InterruptedException;

  /**
   * Examines all the given keys. Returns an iterable of keys whose corresponding nodes are
   * currently available to be fetched.
   *
   * <p>Note: An unavailable node does not mean it is not in the graph. It only means it's not ready
   * to be fetched immediately.
   *
   * @param reason the reason the nodes are being requested.
   */
  Iterable<SkyKey> getCurrentlyAvailableNodes(Iterable<SkyKey> keys, Reason reason);

  /**
   * The reason that a node is being looked up in the Skyframe graph.
   *
   * <p>Alternate graph implementations may wish to make use of this information.
   */
  enum Reason {
    /**
     * The node is being fetched in order to see if it needs to be evaluated or because it was just
     * evaluated, but *not* because it was just requested during evaluation of a SkyFunction
     * (see {@link #DEP_REQUESTED}).
     */
    PRE_OR_POST_EVALUATION,

    /**
     * The node is being looked up as part of the prefetch step before evaluation of a SkyFunction.
     */
    PREFETCH,

    /**
     * The node is being fetched because it is about to be evaluated, but *not* because it was just
     * requested during evaluation of a SkyFunction (see {@link #DEP_REQUESTED}).
     */
    EVALUATION,

    /** The node is being looked up because it was requested during evaluation of a SkyFunction. */
    DEP_REQUESTED,

    /** The node is being looked up during the invalidation phase of Skyframe evaluation. */
    INVALIDATION,

    /** The node is being looked up during the cycle checking phase of Skyframe evaluation. */
    CYCLE_CHECKING,

    /** The node is being looked up so that an rdep can be added to it. */
    RDEP_ADDITION,

    /** The node is being looked up so that an rdep can be removed from it. */
    RDEP_REMOVAL,

    /** The node is being looked up so it can be enqueued for evaluation or change pruning. */
    ENQUEUING_CHILD,

    /**
     * The node is being looked up so that it can be signaled that a dependency is now complete.
     */
    SIGNAL_DEP,

    /**
     * The node is being looking up as part of the error bubbling phase of fail-fast Skyframe
     * evaluation.
     */
    ERROR_BUBBLING,

    /** The node is being looked up merely for an existence check. */
    EXISTENCE_CHECKING,

    /**
     * The node is being looked up to service {@link WalkableGraph#getValue},
     * {@link WalkableGraph#getException}, {@link WalkableGraph#getMissingAndExceptions}, or
     * {@link WalkableGraph#getSuccessfulValues}.
     */
    WALKABLE_GRAPH_VALUE,

    /** The node is being looked up to service {@link WalkableGraph#getDirectDeps}. */
    WALKABLE_GRAPH_DEPS,

    /** The node is being looked up to service {@link WalkableGraph#getReverseDeps}. */
    WALKABLE_GRAPH_RDEPS,

    /** Some other reason than one of the above. */
    OTHER,
  }
}