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Random number generators, probability distributions, combinatorics and statistics for Java.
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// ============================================================================
// Copyright 2006-2010 Daniel W. Dyer
//
// 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 org.uncommons.maths.random;
import java.util.Random;
import org.uncommons.maths.number.ConstantGenerator;
import org.uncommons.maths.number.NumberGenerator;
/**
* Continuous random sequence that follows an
* exponential
* distribution.
* @author Daniel Dyer
* @since 1.0.2
*/
public class ExponentialGenerator implements NumberGenerator
{
private final NumberGenerator rate;
private final Random rng;
/**
* Creates a generator of exponentially-distributed values from a distribution
* with a rate controlled by the specified generator parameter. The mean of
* this distribution is {@literal 1 / rate}
* and the variance is {@literal 1 / rate^2}.
* @param rate A number generator that provides values to use as the rate for
* the exponential distribution. This generator must only return non-zero, positive
* values.
* @param rng The source of randomness used to generate the exponential values.
*/
public ExponentialGenerator(NumberGenerator rate,
Random rng)
{
this.rate = rate;
this.rng = rng;
}
/**
* Creates a generator of exponentially-distributed values from a distribution
* with the specified rate. The mean of this distribution is {@literal 1 / rate}
* and the variance is {@literal 1 / rate^2}.
* @param rate The rate (lamda) of the exponential distribution.
* @param rng The source of randomness used to generate the exponential values.
*/
public ExponentialGenerator(double rate,
Random rng)
{
this(new ConstantGenerator(rate), rng);
}
/**
* Generate the next exponential value from the current value of
* {@literal rate}.
* @return The next exponentially-distributed value.
*/
public Double nextValue()
{
double u;
do
{
// Get a uniformally-distributed random double between
// zero (inclusive) and 1 (exclusive)
u = rng.nextDouble();
} while (u == 0d); // Reject zero, u must be positive for this to work.
return (-Math.log(u)) / rate.nextValue();
}
}