RfFieldDuplication.java

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package org.hipparchus.special.elliptic.carlson;

import org.hipparchus.CalculusFieldElement;
import org.hipparchus.complex.Complex;
import org.hipparchus.complex.FieldComplex;
import org.hipparchus.util.FastMath;

/** Duplication algorithm for Carlson R<sub>F</sub> elliptic integral.
 * @param <T> type of the field elements (really {@link Complex} or {@link FieldComplex})
 * @since 2.0
 */
class RfFieldDuplication<T extends CalculusFieldElement<T>> extends FieldDuplication<T> {

    /** Simple constructor.
     * @param x first symmetric variable of the integral
     * @param y second symmetric variable of the integral
     * @param z third symmetric variable of the integral
     */
    RfFieldDuplication(final T x, final T y, final T z) {
        super(x, y, z);
    }

    /** {@inheritDoc} */
    @Override
    protected void initialMeanPoint(final T[] va) {
        va[3] = va[0].add(va[1]).add(va[2]).divide(3.0);
    }

    /** {@inheritDoc} */
    @Override
    protected T convergenceCriterion(final T r, final T max) {
        return max.divide(FastMath.sqrt(FastMath.sqrt(FastMath.sqrt(r.multiply(3.0)))));
    }

    /** {@inheritDoc} */
    @Override
    protected void update(final int m, final T[] vaM, final T[] sqrtM, final  double fourM) {

        // equation 2.3 in Carlson[1995]
        final T lambdaA = sqrtM[0].multiply(sqrtM[1]);
        final T lambdaB = sqrtM[0].multiply(sqrtM[2]);
        final T lambdaC = sqrtM[1].multiply(sqrtM[2]);

        // equations 2.3 and 2.4 in Carlson[1995]
        vaM[0] = vaM[0].linearCombination(0.25, vaM[0], 0.25, lambdaA, 0.25, lambdaB, 0.25, lambdaC); // xₘ
        vaM[1] = vaM[1].linearCombination(0.25, vaM[1], 0.25, lambdaA, 0.25, lambdaB, 0.25, lambdaC); // yₘ
        vaM[2] = vaM[2].linearCombination(0.25, vaM[2], 0.25, lambdaA, 0.25, lambdaB, 0.25, lambdaC); // zₘ
        vaM[3] = vaM[3].linearCombination(0.25, vaM[3], 0.25, lambdaA, 0.25, lambdaB, 0.25, lambdaC); // aₘ

    }

    /** {@inheritDoc} */
    @Override
    protected T evaluate(final T[] va0, final T aM, final  double fourM) {

        // compute symmetric differences
        final T inv  = aM.multiply(fourM).reciprocal();
        final T bigX = va0[3].subtract(va0[0]).multiply(inv);
        final T bigY = va0[3].subtract(va0[1]).multiply(inv);
        final T bigZ = bigX.add(bigY).negate();

        // compute elementary symmetric functions (we already know e1 = 0 by construction)
        final T e2  = bigX.multiply(bigY).subtract(bigZ.multiply(bigZ));
        final T e3  = bigX.multiply(bigY).multiply(bigZ);

        final T e2e2   = e2.multiply(e2);
        final T e2e3   = e2.multiply(e3);
        final T e3e3   = e3.multiply(e3);
        final T e2e2e2 = e2e2.multiply(e2);

        // evaluate integral using equation 19.36.1 in DLMF
        // (which add more terms than equation 2.7 in Carlson[1995])
        final T poly = e2e2e2.multiply(RfRealDuplication.E2_E2_E2).
                       add(e3e3.multiply(RfRealDuplication.E3_E3)).
                       add(e2e3.multiply(RfRealDuplication.E2_E3)).
                       add(e2e2.multiply(RfRealDuplication.E2_E2)).
                       add(e3.multiply(RfRealDuplication.E3)).
                       add(e2.multiply(RfRealDuplication.E2)).
                       add(RfRealDuplication.CONSTANT).
                       divide(RfRealDuplication.DENOMINATOR);
        return poly.divide(FastMath.sqrt(aM));

    }

    /** {@inheritDoc} */
    @Override
    public T integral() {
        final T x = getVi(0);
        final T y = getVi(1);
        final T z = getVi(2);
        if (x.isZero()) {
            return completeIntegral(y, z);
        } else if (y.isZero()) {
            return completeIntegral(x, z);
        } else if (z.isZero()) {
            return completeIntegral(x, y);
        } else {
            return super.integral();
        }
    }

    /** Compute Carlson complete elliptic integral R<sub>F</sub>(u, v, 0).
     * @param x first symmetric variable of the integral
     * @param y second symmetric variable of the integral
     * @return Carlson complete elliptic integral R<sub>F</sub>(u, v, 0)
     */
    private T completeIntegral(final T x, final T y) {

        T xM = x.sqrt();
        T yM = y.sqrt();

        // iterate down
        for (int i = 1; i < RfRealDuplication.AGM_MAX; ++i) {

            final T xM1 = xM;
            final T yM1 = yM;

            // arithmetic mean
            xM = xM1.add(yM1).multiply(0.5);

            // geometric mean
            yM = xM1.multiply(yM1).sqrt();

            // convergence (by the inequality of arithmetic and geometric means, this is non-negative)
            if (xM.subtract(yM).norm() <= 4 * FastMath.ulp(xM).getReal()) {
                // convergence has been reached
                break;
            }

        }

        return xM.add(yM).reciprocal().multiply(xM.getPi());

    }

}