PiAlgorithmStrategy.java

/*
 * Copyright (c) 2009-2015
 * 	IT-Consulting Stephan Schloepke (http://www.schloepke.de/)
 * 	klemm software consulting Mirko Klemm (http://www.klemm-scs.com/)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package org.jbasics.math.strategies;

import org.jbasics.math.AlgorithmStrategy;
import org.jbasics.math.IrationalNumber;
import org.jbasics.math.impl.MathImplConstants;
import org.jbasics.math.impl.SquareRootIrationalNumber;
import org.jbasics.math.impl.SquareRootReciprocalIrationalNumber;

import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;

public class PiAlgorithmStrategy implements AlgorithmStrategy<BigDecimal> {

	public BigDecimal calculate(MathContext mc, BigDecimal guess, BigDecimal... xn) {
		MathContext calcMC = new MathContext(mc.getPrecision() + 2, RoundingMode.HALF_EVEN);
		BigDecimal an = BigDecimal.ONE;
		IrationalNumber<BigDecimal> bn = SquareRootReciprocalIrationalNumber.SQUARE_ROOT_RECIPROCAL_OF_2;
		BigDecimal tn = MathImplConstants.QUARTER;
		BigDecimal pn = BigDecimal.ONE;
		BigDecimal anNext;
		BigDecimal anDiff;
		do {
			anNext = an.add(bn.valueToPrecision(calcMC)).divide(MathImplConstants.TWO, calcMC);
			bn = SquareRootIrationalNumber.valueOf(an.multiply(bn.valueToPrecision(mc), calcMC));
			anDiff = an.subtract(anNext);
			tn = tn.subtract(pn.multiply(anDiff.pow(2, calcMC), calcMC), calcMC);
			pn = pn.add(pn);
			an = anNext;
		} while (calcMC.getPrecision() - anDiff.scale() + anDiff.precision() - 1 > 0);
		BigDecimal pi = an.add(bn.valueToPrecision(calcMC)).pow(2, calcMC).divide(tn, calcMC).multiply(MathImplConstants.QUARTER, calcMC);
		if (xn == null || xn.length == 0) {
			return pi;
		} else {
			return pi.multiply(xn[0], mc);
		}
	}
}