/*
* 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.math3.complex;
import java.io.Serializable;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.exception.MathIllegalStateException;
import org.apache.commons.math3.exception.OutOfRangeException;
import org.apache.commons.math3.exception.ZeroException;
import org.apache.commons.math3.exception.util.LocalizedFormats;
/**
* A helper class for the computation and caching of the {@code n}-th roots of
* unity.
*
* @since 3.0
*/
public class RootsOfUnity implements Serializable {
/** Serializable version id. */
private static final long serialVersionUID = 20120201L;
/** Number of roots of unity. */
private int omegaCount;
/** Real part of the roots. */
private double[] omegaReal;
/**
* Imaginary part of the {@code n}-th roots of unity, for positive values
* of {@code n}. In this array, the roots are stored in counter-clockwise
* order.
*/
private double[] omegaImaginaryCounterClockwise;
/**
* Imaginary part of the {@code n}-th roots of unity, for negative values
* of {@code n}. In this array, the roots are stored in clockwise order.
*/
private double[] omegaImaginaryClockwise;
/**
* {@code true} if {@link #computeRoots(int)} was called with a positive
* value of its argument {@code n}. In this case, counter-clockwise ordering
* of the roots of unity should be used.
*/
private boolean isCounterClockWise;
/**
* Build an engine for computing the {@code n}-th roots of unity.
*/
public RootsOfUnity() {
omegaCount = 0;
omegaReal = null;
omegaImaginaryCounterClockwise = null;
omegaImaginaryClockwise = null;
isCounterClockWise = true;
}
/**
* Returns {@code true} if {@link #computeRoots(int)} was called with a
* positive value of its argument {@code n}. If {@code true}, then
* counter-clockwise ordering of the roots of unity should be used.
*
* @return {@code true} if the roots of unity are stored in
* counter-clockwise order
* @throws MathIllegalStateException if no roots of unity have been computed
* yet
*/
public synchronized boolean isCounterClockWise()
throws MathIllegalStateException {
if (omegaCount == 0) {
throw new MathIllegalStateException(
LocalizedFormats.ROOTS_OF_UNITY_NOT_COMPUTED_YET);
}
return isCounterClockWise;
}
/**
* <p>
* Computes the {@code n}-th roots of unity. The roots are stored in
* {@code omega[]}, such that {@code omega[k] = w ^ k}, where
* {@code k = 0, ..., n - 1}, {@code w = exp(2 * pi * i / n)} and
* {@code i = sqrt(-1)}.
* </p>
* <p>
* Note that {@code n} can be positive of negative
* </p>
* <ul>
* <li>{@code abs(n)} is always the number of roots of unity.</li>
* <li>If {@code n > 0}, then the roots are stored in counter-clockwise order.</li>
* <li>If {@code n < 0}, then the roots are stored in clockwise order.</p>
* </ul>
*
* @param n the (signed) number of roots of unity to be computed
* @throws ZeroException if {@code n = 0}
*/
public synchronized void computeRoots(int n) throws ZeroException {
if (n == 0) {
throw new ZeroException(
LocalizedFormats.CANNOT_COMPUTE_0TH_ROOT_OF_UNITY);
}
isCounterClockWise = n > 0;
// avoid repetitive calculations
final int absN = Math.abs(n);
if (absN == omegaCount) {
return;
}
// calculate everything from scratch
final double t = 2.0 * Math.PI / absN;
final double cosT = Math.cos(t);
final double sinT = Math.sin(t);
omegaReal = new double[absN];
omegaImaginaryCounterClockwise = new double[absN];
omegaImaginaryClockwise = new double[absN];
omegaReal[0] = 1.0;
omegaImaginaryCounterClockwise[0] = 0.0;
omegaImaginaryClockwise[0] = 0.0;
for (int i = 1; i < absN; i++) {
omegaReal[i] = omegaReal[i - 1] * cosT -
omegaImaginaryCounterClockwise[i - 1] * sinT;
omegaImaginaryCounterClockwise[i] = omegaReal[i - 1] * sinT +
omegaImaginaryCounterClockwise[i - 1] * cosT;
omegaImaginaryClockwise[i] = -omegaImaginaryCounterClockwise[i];
}
omegaCount = absN;
}
/**
* Get the real part of the {@code k}-th {@code n}-th root of unity.
*
* @param k index of the {@code n}-th root of unity
* @return real part of the {@code k}-th {@code n}-th root of unity
* @throws MathIllegalStateException if no roots of unity have been
* computed yet
* @throws MathIllegalArgumentException if {@code k} is out of range
*/
public synchronized double getReal(int k)
throws MathIllegalStateException, MathIllegalArgumentException {
if (omegaCount == 0) {
throw new MathIllegalStateException(
LocalizedFormats.ROOTS_OF_UNITY_NOT_COMPUTED_YET);
}
if ((k < 0) || (k >= omegaCount)) {
throw new OutOfRangeException(
LocalizedFormats.OUT_OF_RANGE_ROOT_OF_UNITY_INDEX,
Integer.valueOf(k),
Integer.valueOf(0),
Integer.valueOf(omegaCount - 1));
}
return omegaReal[k];
}
/**
* Get the imaginary part of the {@code k}-th {@code n}-th root of unity.
*
* @param k index of the {@code n}-th root of unity
* @return imaginary part of the {@code k}-th {@code n}-th root of unity
* @throws MathIllegalStateException if no roots of unity have been
* computed yet
* @throws OutOfRangeException if {@code k} is out of range
*/
public synchronized double getImaginary(int k)
throws MathIllegalStateException, OutOfRangeException {
if (omegaCount == 0) {
throw new MathIllegalStateException(
LocalizedFormats.ROOTS_OF_UNITY_NOT_COMPUTED_YET);
}
if ((k < 0) || (k >= omegaCount)) {
throw new OutOfRangeException(
LocalizedFormats.OUT_OF_RANGE_ROOT_OF_UNITY_INDEX,
Integer.valueOf(k),
Integer.valueOf(0),
Integer.valueOf(omegaCount - 1));
}
return isCounterClockWise ? omegaImaginaryCounterClockwise[k] :
omegaImaginaryClockwise[k];
}
/**
* Returns the number of roots of unity currently stored. If
* {@link #computeRoots(int)} was called with {@code n}, then this method
* returns {@code abs(n)}. If no roots of unity have been computed yet, this
* method returns 0.
*
* @return the number of roots of unity currently stored
*/
public synchronized int getNumberOfRoots() {
return omegaCount;
}
}