com.facebook.stats.cardinality.StaticModelUtil.java Source code

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Here is the source code for com.facebook.stats.cardinality.StaticModelUtil.java

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/*
 * Copyright (C) 2012 Facebook, Inc.
 *
 * 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.facebook.stats.cardinality;

import com.google.common.base.Preconditions;

import java.util.Arrays;

final class StaticModelUtil {
    public static final int COUNT_BITS = 12;
    public static final int MAX_COUNT = 1 << COUNT_BITS;

    // this is a little bigger than 1.0 / MAX_COUNT
    public static final double SMALLEST_PROBABILITY = 2.5e-4;

    private StaticModelUtil() {
    }

    public static double[] weightsToProbabilities(double[] weights) {
        return weightsToProbabilities(weights, Integer.MAX_VALUE);
    }

    public static double[] weightsToProbabilities(double[] weights, int iterationLimit) {
        Preconditions.checkNotNull(weights, "weights is null");
        Preconditions.checkArgument(weights.length > 0, "weights is empty");

        return weightsToProbabilities(Arrays.copyOf(weights, weights.length), sum(weights), iterationLimit);
    }

    private static strictfp double[] weightsToProbabilities(double[] weights, double sum, int iterationLimit) {
        int iterationCount = 0;

        do {
            for (int i = 0; i < weights.length; i++) {
                Preconditions.checkArgument(weights[i] >= 0, "weight %s value %s is not greater than zero", i,
                        weights[i]);
                weights[i] /= sum;

                // adjust probability is too small or too large
                // this number is large enough that when multiplied by MAX_TOTAL we get a whole number
                if (weights[i] < SMALLEST_PROBABILITY) {
                    weights[i] = SMALLEST_PROBABILITY;
                } else if (weights[i] > 0.999) {
                    // this generally leaves enough room for ever symbol to get a whole number part of MAX_TOTAL
                    weights[i] = 0.999;
                }
            }

            // keep normalizing until the total probability falls in a specific range
            sum = sum(weights);
            ++iterationCount;
        } while ((sum < 0.9999 || sum > 1.0001) && iterationCount < iterationLimit);

        return weights;
    }

    private static strictfp double sum(double[] values) {
        double sum = 0;
        for (double value : values) {
            sum += value;
        }
        return sum;
    }
}