org.wildfly.common.math.HashMath Maven / Gradle / Ivy
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* Copyright 2017 Red Hat, Inc., and individual contributors
* as indicated by the @author tags.
*
* 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 org.wildfly.common.math;
import org.wildfly.common.Assert;
/**
* Routines which are useful for hashcode computation, among other things.
*
* @author David M. Lloyd
*/
public final class HashMath {
private static final int PRESELECTED_PRIME = 1299827;
private HashMath() {
}
/**
* Round the given value up to the next positive power of two.
*
* @param value the value (must not be negative and must be less than or equal to {@code 2^31})
* @return the rounded power of two value
*/
public static int roundToPowerOfTwo(int value) {
Assert.checkMinimumParameter("value", 0, value);
Assert.checkMaximumParameter("value", 0x4000_0000, value);
return value <= 1 ? value : Integer.highestOneBit(value - 1) << 1;
}
/**
* A hash function which combines an accumulated hash with a next hash such that {@code f(f(k, p2, b), p1, a) ≠ₙ f(f(k, p1, a), p2, b)}.
* This function is suitable for object chains whose order affects the overall equality of the hash code.
*
* The exact algorithm is not specified and is therefore subject to change and should not be relied upon for hash
* codes that persist outside of the JVM process.
*
* @param accumulatedHash the accumulated hash code of the previous stage
* @param prime a prime multiplier
* @param nextHash the hash code of the next single item
* @return the new accumulated hash code
*/
public static int multiHashOrdered(int accumulatedHash, int prime, int nextHash) {
return multiplyWrap(accumulatedHash, prime) + nextHash;
}
/**
* A hash function which combines an accumulated hash with a next hash such that {@code f(f(k, p2, b), p1, a) = f(f(k, p1, a), p2, b)}.
* This function is suitable for object chains whose order does not affect the overall equality of the hash code.
*
* The exact algorithm is not specified and is therefore subject to change and should not be relied upon for hash
* codes that persist outside of the JVM process.
*
* @param accumulatedHash the accumulated hash code of the previous stage
* @param prime a prime multiplier
* @param nextHash the hash code of the next single item
* @return the new accumulated hash code
*/
public static int multiHashUnordered(int accumulatedHash, int prime, int nextHash) {
return multiplyWrap(nextHash, prime) + accumulatedHash;
}
/**
* A hash function which combines an accumulated hash with a next hash such that {@code f(f(k, b), a) ≠ₙ f(f(k, a), b)}.
* This function is suitable for object chains whose order affects the overall equality of the hash code.
*
* The exact algorithm is not specified and is therefore subject to change and should not be relied upon for hash
* codes that persist outside of the JVM process.
*
* @param accumulatedHash the accumulated hash code of the previous stage
* @param nextHash the hash code of the next single item
* @return the new accumulated hash code
*/
public static int multiHashOrdered(int accumulatedHash, int nextHash) {
return multiHashOrdered(accumulatedHash, PRESELECTED_PRIME, nextHash);
}
/**
* A hash function which combines an accumulated hash with a next hash such that {@code f(f(k, b), a) = f(f(k, a), b)}.
* This function is suitable for object chains whose order does not affect the overall equality of the hash code.
*
* The exact algorithm is not specified and is therefore subject to change and should not be relied upon for hash
* codes that persist outside of the JVM process.
*
* @param accumulatedHash the accumulated hash code of the previous stage
* @param nextHash the hash code of the next single item
* @return the new accumulated hash code
*/
public static int multiHashUnordered(int accumulatedHash, int nextHash) {
return multiHashUnordered(accumulatedHash, PRESELECTED_PRIME, nextHash);
}
/**
* Multiply two unsigned integers together. If the result overflows a 32-bit number, XOR the overflowed bits back into the result.
* This operation is commutative, i.e. if we designate the {@code ⨰} symbol to represent this operation, then {@code a ⨰ b = b ⨰ a}.
* This operation is not associative, i.e. {@code (a ⨰ b) ⨰ c ≠ₙ a ⨰ (b ⨰ c)} (the symbol {@code ≠ₙ} meaning "not necessarily equal to"),
* therefore this operation is suitable for ordered combinatorial hash functions.
*
* @param a the first number to multiply
* @param b the second number to multiply
* @return the wrapped multiply result
*/
public static int multiplyWrap(int a, int b) {
long r1 = (long) a * b;
return (int) r1 ^ (int) (r1 >>> 32);
}
}