| /* |
| * 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.numbers.gamma; |
| |
| /** |
| * <a href="http://mathworld.wolfram.com/Erfc.html">Complementary error function</a>. |
| */ |
| public final class Erfc { |
| /** The threshold value for returning the extreme value. */ |
| private static final double EXTREME_VALUE_BOUND = 40; |
| |
| /** Private constructor. */ |
| private Erfc() { |
| // intentionally empty. |
| } |
| |
| /** |
| * <p> |
| * This implementation computes erfc(x) using the |
| * {@link RegularizedGamma.Q#value(double, double, double, int) regularized gamma function}, |
| * following <a href="http://mathworld.wolfram.com/Erf.html">Erf</a>, equation (3). |
| * </p> |
| * |
| * <p> |
| * The value returned is always between 0 and 2 (inclusive). |
| * If {@code abs(x) > 40}, then {@code erf(x)} is indistinguishable from |
| * either 0 or 2 at {@code double} precision, so the appropriate extreme |
| * value is returned. |
| * </p> |
| * |
| * @param x Value. |
| * @return the complementary error function. |
| * @throws ArithmeticException if the algorithm fails to converge. |
| * |
| * @see RegularizedGamma.Q#value(double, double, double, int) |
| */ |
| public static double value(double x) { |
| if (Math.abs(x) > EXTREME_VALUE_BOUND) { |
| return x > 0 ? 0 : 2; |
| } |
| final double ret = RegularizedGamma.Q.value(0.5, x * x, 1e-15, 10000); |
| return x < 0 ? |
| 2 - ret : |
| ret; |
| } |
| } |
| |