io.questdb.std.Hash Maven / Gradle / Ivy

Show more of this group Show more artifacts with this name
Show all versions of core Show documentation
QuestDB is High Performance Time Series Database
The newest version!
/*******************************************************************************
 *     ___                  _   ____  ____
 *    / _ \ _   _  ___  ___| |_|  _ \| __ )
 *   | | | | | | |/ _ \/ __| __| | | |  _ \
 *   | |_| | |_| |  __/\__ \ |_| |_| | |_) |
 *    \__\_\\__,_|\___||___/\__|____/|____/
 *
 *  Copyright (c) 2014-2019 Appsicle
 *  Copyright (c) 2019-2020 QuestDB
 *
 *  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 io.questdb.std;

import java.math.RoundingMode;
import java.text.DecimalFormat;

public final class Hash {
    static final int HASH_BITS = 0x7fffffff;

    private Hash() {
    }

    /**
     * Restricts hashCode() of the underlying char sequence to be no greater than max.
     *
     * @param s   char sequence
     * @param max max value of hashCode()
     * @return power of 2 integer
     */
    public static int boundedHash(CharSequence s, int max) {
        return s == null ? -1 : (Chars.hashCode(s) & 0xFFFFFFF) & max;
    }

    /**
     * Calculates positive integer hash of memory pointer using Java hashcode() algorithm.
     *
     * @param p   memory pointer
     * @param len memory length in bytes
     * @return hash code
     */
    public static int hashMem(long p, int len) {
        long hash = 0;
        final long hi = p + len;
        while (hi - p > 7) {
            hash = (hash << 5) - hash + Unsafe.getUnsafe().getLong(p);
            p += Long.BYTES;
        }

        while (p < hi) {
            hash = (hash << 5) - hash + Unsafe.getUnsafe().getByte(p++);
        }

        return spread((int) hash);
    }

    /**
     * (copied from ConcurrentHashMap)
     * Spreads (XORs) higher bits of hash to lower and also forces top
     * bit to 0. Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a trade off between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     *
     * @param h hash code
     * @return adjusted hash code
     */
    public static int spread(int h) {
        return (h ^ (h >>> 16)) & HASH_BITS;
    }

    public static void main(String[] args) {
//        double d = -73.992588;
        double d = -73.99258800000001;
//        double d = 40.777057000000006;
//        double d = 40.728809000000005;
//        double d = -73.93972000000001+1e-15;
//        double d1 = -73.93972;

        DecimalFormat format = new DecimalFormat("#.########");
        format.setRoundingMode(RoundingMode.CEILING);
        System.out.println(format.format(d));

        System.out.println((Math.round(d * 1e8)) * 1e-8);
    }
}