src/main/java/com/google/devtools/build/skyframe/CycleDeduper.java

// 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.common.base.Preconditions;
import com.google.common.collect.HashMultimap;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.SetMultimap;

/**
 * Dedupes C candidate cycles of size O(L) in O(CL) time and memory in the common case and
 * O(C^2 * L) time and O(CL) memory in the extreme case.
 *
 * Two cycles are considered duplicates if they are exactly the same except for the entry point.
 * For example, 'a' -> 'b' -> 'c' -> 'a' is the considered the same as 'b' -> 'c' -> 'a' -> 'b'.
 */
class CycleDeduper<T> {
  private final SetMultimap<ImmutableSet<T>, ImmutableList<T>> knownCyclesByMembers =
      HashMultimap.create();

  /**
   * Marks a non-empty list representing a cycle of unique values as being seen and returns true iff
   * the cycle has been seen before, accounting for logical equivalence of cycles.
   *
   * <p>For example, the cycle 'a' -> 'b' -> 'c' -> 'a' is represented by the list ['a', 'b', 'c']
   * and is logically equivalent to the cycle represented by the list ['b', 'c', 'a'].
   */
  public boolean alreadySeen(ImmutableList<T> cycle) {
    ImmutableSet<T> cycleMembers = ImmutableSet.copyOf(cycle);
    Preconditions.checkState(!cycle.isEmpty());
    Preconditions.checkState(cycle.size() == cycleMembers.size(),
        "cycle doesn't have unique members: " + cycle);

    if (knownCyclesByMembers.containsEntry(cycleMembers, cycle)) {
      return true;
    }

    // Of the C cycles, suppose there are D cycles that have the same members (but are in an
    // incompatible order). This code path takes O(D * L) time. The common case is that D is
    // very small.
    boolean found = false;
    for (ImmutableList<T> candidateCycle : knownCyclesByMembers.get(cycleMembers)) {
      int startPos = candidateCycle.indexOf(cycle.get(0));
      // The use of a multimap keyed by cycle members guarantees that the first element of 'cycle'
      // is present in 'candidateCycle'.
      Preconditions.checkState(startPos >= 0);
      if (equalsWithSingleLoopFrom(cycle, candidateCycle, startPos)) {
        found = true;
        break;
      }
    }
    // We add the cycle even if it's a duplicate so that future exact copies of this can be
    // processed in O(L) time. We are already using O(CL) memory, and this optimization doesn't
    // change that.
    knownCyclesByMembers.put(cycleMembers, cycle);
    return found;
  }

  /**
   * Returns true iff
   *   listA[0], listA[1], ..., listA[listA.size()]
   * is the same as
   *   listB[start], listB[start+1], ..., listB[listB.size()-1], listB[0], ..., listB[start-1]
   */
  private boolean equalsWithSingleLoopFrom(ImmutableList<T> listA, ImmutableList<T> listB,
      int start) {
    if (listA.size() != listB.size()) {
      return false;
    }
    int length = listA.size();
    for (int i = 0; i < length; i++) {
      if (!listA.get(i).equals(listB.get((i + start) % length))) {
        return false;
      }
    }
    return true;
  }
}