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

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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
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49
src/java.base/share/classes/java/util/Collections.java
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@ -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() {