1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
|
// Copyright 2018 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.lib.collect;
import com.google.common.collect.ImmutableList;
import java.util.Comparator;
import java.util.PriorityQueue;
/**
* A stream aggregator that, given a {@code k}, aggregates a sequence of elements into the {@code k}
* most extreme.
*/
public class Extrema<T extends Comparable<T>> {
private final int k;
private final Comparator<T> extremaComparator;
private final PriorityQueue<T> priorityQueue;
/**
* Creates an {@link Extrema} that aggregates a sequence of elements into the {@code k} smallest.
*/
public static <T extends Comparable<T>> Extrema<T> min(int k) {
return new Extrema<>(k, Comparator.<T>naturalOrder());
}
/**
* Creates an {@link Extrema} that aggregates a sequence of elements into the {@code k} largest.
*/
public static <T extends Comparable<T>> Extrema<T> max(int k) {
return new Extrema<>(k, Comparator.<T>naturalOrder().reversed());
}
/**
* @param k the number of extreme elements to compute
* @param extremaComparator a comparator such that {@code extremaComparator(a, b) < 0} iff
* {@code a} is more extreme than {@code b}
*/
private Extrema(int k, Comparator<T> extremaComparator) {
this.k = k;
this.extremaComparator = extremaComparator;
this.priorityQueue = new PriorityQueue<>(
/*initialCapacity=*/ k,
// Our implementation strategy is to keep a priority queue of the k most extreme elements
// encountered, ordered backwards; this way we have constant-time access to the least
// extreme among these elements.
extremaComparator.reversed());
}
/**
* Aggregates the given element.
*
* <p>See {@link #getExtremeElements()}.
*/
public void aggregate(T element) {
if (priorityQueue.size() < k) {
priorityQueue.add(element);
} else {
if (extremaComparator.compare(element, priorityQueue.peek()) < 0) {
// Suppose the least extreme of the current k most extreme elements is e. If the new element
// is more extreme than e, then (i) it must be among the new k most extreme among the (2) e
// must not be.
priorityQueue.remove();
priorityQueue.add(element);
}
}
}
/**
* For an {@link Extrema} created with {@code k} and with {@code n} calls to {@link #aggregate}
* since the most recent call to {@link #clear}, returns the min(k, n) most extreme elements
* {@link #aggregate}'ed since the most recent call to {@link #clear}.
*/
public ImmutableList<T> getExtremeElements() {
return ImmutableList.sortedCopyOf(extremaComparator, priorityQueue);
}
/**
* Disregards all the elements {@link #aggregate}'ed already.
*
* <p>See {@link #getExtremeElements()}.
*/
public void clear() {
priorityQueue.clear();
}
}
|
