Hashing.java

/*
 *  Copyright 2014-2017 Real Logic Ltd.
 *
 * 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.agrona.collections;

/**
 * Hashing functions for applying to integers.
 */
public class Hashing
{
    /**
     * Default load factor to be used in open addressing hashed data structures.
     */
    public static final float DEFAULT_LOAD_FACTOR = 0.55f;

    /**
     * Generate a hash for an int value. This is a no op.
     *
     * @param value to be hashed.
     * @return the hashed value.
     */
    public static int hash(final int value)
    {
        return value * 31;
    }

    /**
     * Generate a hash for an long value.
     *
     * @param value to be hashed.
     * @return the hashed value.
     */
    public static int hash(final long value)
    {
        long hash = value * 31;
        hash = (int)hash ^ (int)(hash >>> 32);

        return (int)hash;
    }

    /**
     * Generate a hash for a int value.
     *
     * @param value to be hashed.
     * @param mask  mask to be applied that must be a power of 2 - 1.
     * @return the hash of the value.
     */
    public static int hash(final int value, final int mask)
    {
        final int hash = value * 31;

        return hash & mask;
    }

    /**
     * Generate a hash for a long value.
     *
     * @param value to be hashed.
     * @param mask  mask to be applied that must be a power of 2 - 1.
     * @return the hash of the value.
     */
    public static int hash(final long value, final int mask)
    {
        long hash = value * 31;
        hash = (int)hash ^ (int)(hash >>> 32);

        return (int)hash & mask;
    }

    /**
     * Generate an even hash for a int value.
     *
     * @param value to be hashed.
     * @param mask  mask to be applied that must be a power of 2 - 1.
     * @return the hash of the value which is always even.
     */
    public static int evenHash(final int value, final int mask)
    {
        final int hash = (value << 1) - (value << 8);

        return hash & mask;
    }

    /**
     * Generate an even hash for a long value.
     *
     * @param value to be hashed.
     * @param mask  mask to be applied that must be a power of 2 - 1.
     * @return the hash of the value which is always even.
     */
    public static int evenHash(final long value, final int mask)
    {
        int hash = (int)value ^ (int)(value >>> 32);
        hash = (hash << 1) - (hash << 8);

        return hash & mask;
    }

    /**
     * Combined two 32 bit keys into a 64-bit compound.
     *
     * @param keyPartA to make the upper bits
     * @param keyPartB to make the lower bits.
     * @return the compound key
     */
    public static long compoundKey(final int keyPartA, final int keyPartB)
    {
        return ((long)keyPartA << 32) | (keyPartB & 0xFFFF_FFFFL);
    }
}