PolyEquationImpl.java

/*******************************************************************************
 * $Id: $
 * Copyright (c) 2009-2010 Tim Tiemens.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the GNU Lesser Public License v2.1
 * which accompanies this distribution, and is available at
 * http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 *
 * Contributors:
 *     Tim Tiemens - initial API and implementation
 ******************************************************************************/
package com.aegiswallet.helpers.secretshare.math;

import com.aegiswallet.helpers.secretshare.SecretShareException;

import java.math.BigInteger;

/**
 * Polynomial equation implementation.
 * 8*x^3  + 5*x^2  + 13*x   + 432
 * [term3]  [term2]  [term1]  [term0]
 * <p/>
 * Arguments to the constructor are in this order:
 * 432     + 13*x    + 5*x^2 + 8*x^3
 * [term0]   [term1]  [term2]  [term3]
 * <p/>
 * <p/>
 * Provides support ONLY for integer exponent values.
 * Provides support ONLY for BigInteger values of "x".
 * Provides support ONLY for BigInteger values of "coefficients"
 * <p/>
 * It may be a "short distance" between BigInteger and BigDecimal,
 * but that distance is still too big to justify making
 * this implementation more complex.
 *
 * @author tiemens
 */
public class PolyEquationImpl {

    // ==================================================
    // class static data
    // ==================================================

    // ==================================================
    // class static methods
    // ==================================================

    // ==================================================
    // instance data
    // ==================================================

    // [0] is x^0, i.e. "*1" the "constant term"
    // [1] is x^1, i.e. "*x" the "x term"
    // [2] is x^2, i.e. the "*x^2 term"
    private BigInteger[] coefficients;

    // ==================================================
    // factories
    // ==================================================

    /**
     * Helper factory to create instances.
     * Accepts 'int', converts them to BigInteger, and calls the constructor.
     *
     * @param args as int values
     * @return instance
     */
    public static PolyEquationImpl create(final int... args) {
        BigInteger[] bigints = new BigInteger[args.length];
        for (int i = 0, n = args.length; i < n; i++) {
            bigints[i] = BigInteger.valueOf(args[i]);
        }
        return new PolyEquationImpl(bigints);
    }

    // ==================================================
    // constructors
    // ==================================================
    public PolyEquationImpl(final BigInteger[] inCoeffs) {
        if (inCoeffs.length <= 0) {
            throw new SecretShareException("Must have at least 1 coefficient");
        }
        coefficients = new BigInteger[inCoeffs.length];

        for (int i = 0, n = inCoeffs.length; i < n; i++) {
            if (inCoeffs[i] == null) {
                throw new SecretShareException("Coefficient index=" + i +
                        " is null, and cannot be null");
            }
            coefficients[i] = inCoeffs[i];
        }

        // Constraint check:
        for (Number n : coefficients) {
            if (n == null) {
                throw new SecretShareException("Programmer error - internal coefficient is null");
            }
        }
    }
    // ==================================================
    // public methods
    // ==================================================

    public BigInteger calculateFofX(final BigInteger x) {
        // pick up the 0th term directly:
        BigInteger ret = coefficients[0];

        // for each of the other terms:
        for (int term = 1, n = coefficients.length; term < n; term++) {
            // the index of term N is N:
            final int indexOfTerm = term;

            // the exponent of term N is N:
            final int powerOfTerm = term;

            BigInteger base = x;
            BigInteger power = base.pow(powerOfTerm);

            BigInteger add = coefficients[indexOfTerm].multiply(power);
            ret = ret.add(add);
        }
        return ret;
    }

    public String debugDump() {
        String ret = "PolyEqImpl[\n";
        for (int i = 0, n = coefficients.length; i < n; i++) {
            ret += "  x^" + i + " * " + coefficients[i].toString() + " + \n";
        }
        ret += "]";
        return ret;
    }

}