/*
* Copyright 2012-present Facebook, Inc.
*
* 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.facebook.buck.rules;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import java.util.Comparator;
/**
* A utility to create a {@link Comparator} based on a list such that when the elements are
* compared, the original list order is preserved.
*/
final class RetainOrderComparator {
/** Utility class: do not instantiate. */
private RetainOrderComparator() {}
/**
* This is meant to be used for testing when a list of objects created via EasyMock need to be
* added to a SortedSet:
*
* <pre>
* Iterable<T> iterable;
* Comparator<T> comparator = RetainOrderComparator.createComparator(iterable);
* ImmutableSortedSet<T> sortedElements = ImmutableSortedSet.copyOf(comparator, iterable);
* </pre>
*/
public static <T> Comparator<T> createComparator(Iterable<T> iterable) {
final ImmutableList<T> items = ImmutableList.copyOf(iterable);
return (a, b) -> {
int indexA = -1;
int indexB = -1;
int index = 0;
for (T item : items) {
// Note that == is used rather than .equals() because this is often used with a list of
// objects created via EasyMock, which means it would be a pain to mock out all of the
// calls to .equals(). Fortunately, most lists used during are short, so this is not
// prohibitively expensive even though it is O(N).
if (a == item) {
indexA = index;
}
if (b == item) {
indexB = index;
}
++index;
}
Preconditions.checkState(indexA >= 0, "The first element must be in the collection");
Preconditions.checkState(indexB >= 0, "The second element must be in the collection");
return indexA - indexB;
};
}
}