stan/jdk-24
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

8214687: Optimize Collections.nCopies().hashCode() and equals()

Browse Source 
Reviewed-by: igerasim, smarks
This commit is contained in:
Tagir F. Valeev 2018-12-30 08:57:24 +07:00
parent 9d3cfddfb6
commit 42e7372b3e
2 changed files with 106 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

49
src/java.base/share/classes/java/util/Collections.java
View File

@ -459,7 +459,7 @@ public class Collections {
for (int i=size; i>1; i--)
swap(list, i-1, rnd.nextInt(i));
} else {
Object arr[] = list.toArray();
Object[] arr = list.toArray();
// Shuffle array
for (int i=size; i>1; i--)
@ -5101,6 +5101,53 @@ public class Collections {
return new CopiesList<>(toIndex - fromIndex, element);
}
@Override
public int hashCode() {
if (n == 0) return 1;
// hashCode of n repeating elements is 31^n + elementHash * Sum(31^k, k = 0..n-1)
// this implementation completes in O(log(n)) steps taking advantage of
// 31^(2*n) = (31^n)^2 and Sum(31^k, k = 0..(2*n-1)) = Sum(31^k, k = 0..n-1) * (31^n + 1)
int pow = 31;
int sum = 1;
for (int i = Integer.numberOfLeadingZeros(n) + 1; i < Integer.SIZE; i++) {
sum *= pow + 1;
pow *= pow;
if ((n << i) < 0) {
pow *= 31;
sum = sum * 31 + 1;
}
}
return pow + sum * (element == null ? 0 : element.hashCode());
}
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof CopiesList) {
CopiesList<?> other = (CopiesList<?>) o;
return n == other.n && (n == 0 || eq(element, other.element));
}
if (!(o instanceof List))
return false;
int remaining = n;
E e = element;
Iterator<?> itr = ((List<?>) o).iterator();
if (e == null) {
while (itr.hasNext() && remaining-- > 0) {
if (itr.next() != null)
return false;
}
} else {
while (itr.hasNext() && remaining-- > 0) {
if (!e.equals(itr.next()))
return false;
}
}
return remaining == 0 && !itr.hasNext();
}
// Override default methods in Collection
@Override
public Stream<E> stream() {

59
test/jdk/java/util/Collections/NCopies.java
View File

@ -1,5 +1,5 @@
/*
* Copyright (c) 2005, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2005, 2018, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
@ -28,8 +28,11 @@
* @author Martin Buchholz
*/
import java.util.ArrayList;
import java.util.Collections;
import java.util.AbstractList;
import java.util.List;
import java.util.Objects;
public class NCopies {
static volatile int passed = 0, failed = 0;
@ -82,6 +85,56 @@ public class NCopies {
checkEmpty(x.subList(x.size()/2, x.size()/2));
}
private static <T> List<T> referenceNCopies(int n, T o) {
// A simplest correct implementation of nCopies to compare with the actual optimized implementation
return new AbstractList<>() {
public int size() { return n; }
public T get(int index) {
Objects.checkIndex(index, n);
return o;
}
};
}
private static void checkHashCode() {
int[] sizes = {0, 1, 2, 3, 5, 10, 31, 32, 100, 1000};
String[] elements = {null, "non-null"};
for (int size : sizes) {
for (String element : elements) {
int expectedHashCode = referenceNCopies(size, element).hashCode();
int actualHashCode = Collections.nCopies(size, element).hashCode();
check(expectedHashCode == actualHashCode,
"Collections.nCopies(" + size + ", " + element + ").hashCode()");
}
}
}
private static void checkEquals() {
int[][] sizePairs = {{0, 0}, {0, 1}, {1, 0}, {1, 1}, {1, 2}, {2, 1}};
String[] elements = {null, "non-null"};
for (int[] pair : sizePairs) {
for (String element : elements) {
boolean equal = pair[0] == pair[1];
String msg = "[" + pair[0] + ", " + element + "] <=> [" + pair[1] + ", " + element + "]";
check(equal == Collections.nCopies(pair[0], element).equals(Collections.nCopies(pair[1], element)), msg);
check(equal == Collections.nCopies(pair[0], element).equals(referenceNCopies(pair[1], element)), msg);
check(equal == referenceNCopies(pair[0], element).equals(Collections.nCopies(pair[1], element)), msg);
}
}
List<String> nulls = Collections.nCopies(10, null);
List<String> nonNulls = Collections.nCopies(10, "non-null");
List<String> nullsButOne = new ArrayList<>(nulls);
nullsButOne.set(9, "non-null");
List<String> nonNullsButOne = new ArrayList<>(nonNulls);
nonNullsButOne.set(9, null);
check(!nulls.equals(nonNulls));
check(!nulls.equals(nullsButOne));
check(!nulls.equals(nonNullsButOne));
check(!nonNulls.equals(nonNullsButOne));
check(Collections.nCopies(0, null).equals(Collections.nCopies(0, "non-null")));
}
public static void main(String[] args) {
try {
List<String> empty = Collections.nCopies(0, "foo");
@ -92,6 +145,10 @@ public class NCopies {
check(foos.size() == 42);
checkFoos(foos.subList(foos.size()/2, foos.size()-1));
checkHashCode();
checkEquals();
} catch (Throwable t) { unexpected(t); }
System.out.printf("%nPassed = %d, failed = %d%n%n", passed, failed);