io.airlift.stats.cardinality.Utils.java Source code

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Here is the source code for io.airlift.stats.cardinality.Utils.java

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/*
 * 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.airlift.stats.cardinality;

import com.google.common.base.Preconditions;

final class Utils {
    private Utils() {
    }

    public static double alpha(int indexBitLength) {
        switch (indexBitLength) {
        case 4:
            return 0.673;
        case 5:
            return 0.697;
        case 6:
            return 0.709;
        default:
            return (0.7213 / (1 + 1.079 / numberOfBuckets(indexBitLength)));
        }
    }

    public static boolean isPowerOf2(long value) {
        Preconditions.checkArgument(value > 0, "value must be positive");
        return (value & (value - 1)) == 0;
    }

    public static int indexBitLength(int numberOfBuckets) {
        Preconditions.checkArgument(isPowerOf2(numberOfBuckets), "numberOfBuckets must be a power of 2, actual: %s",
                numberOfBuckets);
        return (int) (Math.log(numberOfBuckets) / Math.log(2));
    }

    public static int numberOfBuckets(int indexBitLength) {
        return 1 << indexBitLength;
    }

    public static int computeIndex(long hash, int indexBitLength) {
        return (int) (hash >>> (Long.SIZE - indexBitLength));
    }

    public static int numberOfLeadingZeros(long hash, int indexBitLength) {
        long value = (hash << indexBitLength) | (1L << (indexBitLength - 1)); // place a 1 in the LSB to preserve the original number of leading zeros if the hash happens to be 0
        return Long.numberOfLeadingZeros(value);
    }

    public static int computeValue(long hash, int indexBitLength) {
        return numberOfLeadingZeros(hash, indexBitLength) + 1;
    }

    public static double linearCounting(int zeroBuckets, int totalBuckets) {
        return totalBuckets * Math.log(totalBuckets * 1.0 / zeroBuckets);
    }
}