/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.tinkerpop.gremlin.algorithm.generator;

import java.util.Random;

/**
 * Generates values according to a normal distribution with the configured standard deviation.
 *
 * @author Matthias Broecheler (me@matthiasb.com)
 */
public class NormalDistribution implements Distribution {

    private final double stdDeviation;
    private final double mean;

    /**
     * Constructs a NormalDistribution with the given standard deviation.
     * <p/>
     * Setting the standard deviation to 0 makes this a constant distribution.
     *
     * @param stdDeviation Simple deviation of the distribution. Must be non-negative.
     */
    public NormalDistribution(final double stdDeviation) {
        this(stdDeviation, 0.0);
    }

    private NormalDistribution(final double stdDeviation, final double mean) {
        if (stdDeviation < 0)
            throw new IllegalArgumentException("Standard deviation must be non-negative: " + stdDeviation);
        if (mean < 0) throw new IllegalArgumentException("Mean must be positive: " + mean);
        this.stdDeviation = stdDeviation;
        this.mean = mean;
    }

    @Override
    public Distribution initialize(final int invocations, final int expectedTotal) {
        double mean = (expectedTotal * 1.0) / invocations; //TODO: account for truncated gaussian distribution
        return new NormalDistribution(stdDeviation, mean);
    }

    @Override
    public int nextValue(final Random random) {
        if (mean == 0.0) throw new IllegalStateException("Distribution has not been initialized");
        return (int) Math.round(random.nextGaussian() * stdDeviation + mean);
    }

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

    @Override
    public String toString() {
        return "NormalDistribution{" +
                "stdDeviation=" + stdDeviation +
                ", mean=" + mean +
                '}';
    }
}