// Copyright 2017 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,
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package com.google.devtools.build.lib.query2.engine;
import com.google.common.collect.ImmutableList;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
/**
* A helper for deduping values that have already been seen at certain "depths".
*
* <p>This is similar to {@link Uniquifier}.
*/
@ThreadSafe
public interface MinDepthUniquifier<T> {
/**
* Returns the subset of {@code newElements} that haven't been seen before at depths less than or
* equal to {@code depth}
*
* <p> There's a natural benign check-then-act race in all concurrent uses of this interface.
* Imagine we have an element e, two depths d1 and d2 (with d2 < d1), and two threads T1 and T2.
* T1 may think it's about to be the first one to process e at a depth less than or equal to d1.
* But before T1 finishes processing e, T2 may think _it's_ about to be first one to process an
* element at a depth less than or equal to than d2. T1's work is probably wasted.
*/
ImmutableList<T> uniqueAtDepthLessThanOrEqualTo(Iterable<T> newElements, int depth);
}