commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/ConstantContinuousDistribution.java

/*
 * 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.commons.statistics.distribution;

import org.apache.commons.rng.UniformRandomProvider;

/**
 * Implementation of the constant real distribution.
 */
public class ConstantContinuousDistribution extends AbstractContinuousDistribution {
    /** Constant value of the distribution. */
    private final double value;

    /**
     * Create a constant real distribution with the given value.
     *
     * @param value Value of this distribution.
     */
    public ConstantContinuousDistribution(double value) {
        this.value = value;
    }

    /** {@inheritDoc} */
    @Override
    public double density(double x) {
        return x == value ? 1 : 0;
    }

    /** {@inheritDoc} */
    @Override
    public double cumulativeProbability(double x)  {
        return x < value ? 0 : 1;
    }

    /** {@inheritDoc} */
    @Override
    public double inverseCumulativeProbability(final double p) {
        if (p < 0 ||
            p > 1) {
            throw new DistributionException(DistributionException.INVALID_PROBABILITY, p);
        }
        return value;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public double getMean() {
        return value;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public double getVariance() {
        return 0;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public double getSupportLowerBound() {
        return value;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public double getSupportUpperBound() {
        return value;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean isSupportConnected() {
        return true;
    }

    /**
     * {@inheritDoc}
     *
     * @param rng Not used: distribution contains a single value.
     * @return the value of the distribution.
     */
    @Override
    public ContinuousDistribution.Sampler createSampler(final UniformRandomProvider rng) {
        return this::getSupportLowerBound;
    }
}