HashBase.java

/*
 * Copyright 2014-2015 the original author or authors
 *
 * 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.wplatform.ddal.util;


/**
 * The base for other hash classes.
 */
public abstract class HashBase {

    /**
     * The maximum load, in percent.
     * declared as long so we do long arithmetic so we don't overflow.
     */
    private static final long MAX_LOAD = 90;

    /**
     * The bit mask to get the index from the hash code.
     */
    protected int mask;

    /**
     * The number of slots in the table.
     */
    protected int len;

    /**
     * The number of occupied slots, excluding the zero key (if any).
     */
    protected int size;

    /**
     * The number of deleted slots.
     */
    protected int deletedCount;

    /**
     * The level. The number of slots is 2 ^ level.
     */
    protected int level;

    /**
     * Whether the zero key is used.
     */
    protected boolean zeroKey;

    private int maxSize, minSize, maxDeleted;

    public HashBase() {
        reset(2);
    }

    /**
     * Increase the size of the underlying table and re-distribute the elements.
     *
     * @param newLevel the new level
     */
    protected abstract void rehash(int newLevel);

    /**
     * Get the size of the map.
     *
     * @return the size
     */
    public int size() {
        return size + (zeroKey ? 1 : 0);
    }

    /**
     * Check the size before adding an entry. This method resizes the map if
     * required.
     */
    void checkSizePut() {
        if (deletedCount > size) {
            rehash(level);
        }
        if (size + deletedCount >= maxSize) {
            rehash(level + 1);
        }
    }

    /**
     * Check the size before removing an entry. This method resizes the map if
     * required.
     */
    protected void checkSizeRemove() {
        if (size < minSize && level > 0) {
            rehash(level - 1);
        } else if (deletedCount > maxDeleted) {
            rehash(level);
        }
    }

    /**
     * Clear the map and reset the level to the specified value.
     *
     * @param newLevel the new level
     */
    protected void reset(int newLevel) {
        // can't exceed 30 or we will generate a negative value
        // for the "len" field
        if (newLevel > 30) {
            throw new IllegalStateException("exceeded max size of hash table");
        }
        size = 0;
        level = newLevel;
        len = 2 << level;
        mask = len - 1;
        minSize = (int) ((1 << level) * MAX_LOAD / 100);
        maxSize = (int) (len * MAX_LOAD / 100);
        deletedCount = 0;
        maxDeleted = 20 + len / 2;
    }

    /**
     * Calculate the index for this hash code.
     *
     * @param hash the hash code
     * @return the index
     */
    protected int getIndex(int hash) {
        return hash & mask;
    }

}