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package com.tinkerpop.gremlin.algorithm.generator;

import java.util.Random;

/**
 * Generates values according to a scale-free distribution with the configured gamma value.
 *
 * @author Matthias Broecheler ([email protected])
 */
public class PowerLawDistribution implements Distribution {

    private final double gamma;
    private final double multiplier;

    /**
     * Constructs a new scale-free distribution for the provided gamma value.
     */
    public PowerLawDistribution(final double gamma) {
        this(gamma, 0.0);
    }

    private PowerLawDistribution(final double gamma, final double multiplier) {
        if (gamma <= 2.0) throw new IllegalArgumentException("Beta must be bigger than 2: " + gamma);
        if (multiplier < 0) throw new IllegalArgumentException("Invalid multiplier value: " + multiplier);
        this.gamma = gamma;
        this.multiplier = multiplier;
    }

    @Override
    public Distribution initialize(final int invocations, final int expectedTotal) {
        double multiplier = expectedTotal / ((gamma - 1) / (gamma - 2) * invocations) * 2; //times two because we are generating stubs
        assert multiplier > 0;
        return new PowerLawDistribution(gamma, multiplier);
    }

    @Override
    public int nextValue(final Random random) {
        if (multiplier == 0.0) throw new IllegalStateException("Distribution has not been initialized");
        return getValue(random, multiplier, gamma);
    }

    @Override
    public int nextConditionalValue(final Random random, final int otherValue) {
        return nextValue(random);
    }

    @Override
    public String toString() {
        return "PowerLawDistribution{" +
                "gamma=" + gamma +
                ", multiplier=" + multiplier +
                '}';
    }

    public static int getValue(final Random random, final double multiplier, final double beta) {
        return (int) Math.round(multiplier * (Math.pow(1.0 / (1.0 - random.nextDouble()), 1.0 / (beta - 1.0)) - 1.0));
    }
}