/**
 * Implementation of the net.sf.geographiclib.Geodesic class
 *
 * Copyright (c) Charles Karney (2013-2019) <charles@karney.com> and licensed
 * under the MIT/X11 License.  For more information, see
 * https://geographiclib.sourceforge.io/
 **********************************************************************/
package net.sf.geographiclib;

/**
 * Geodesic calculations.
 * <p>
 * The shortest path between two points on a ellipsoid at (<i>lat1</i>,
 * <i>lon1</i>) and (<i>lat2</i>, <i>lon2</i>) is called the geodesic.  Its
 * length is <i>s12</i> and the geodesic from point 1 to point 2 has azimuths
 * <i>azi1</i> and <i>azi2</i> at the two end points.  (The azimuth is the
 * heading measured clockwise from north.  <i>azi2</i> is the "forward"
 * azimuth, i.e., the heading that takes you beyond point 2 not back to point
 * 1.)
 * <p>
 * Given <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, and <i>s12</i>, we can
 * determine <i>lat2</i>, <i>lon2</i>, and <i>azi2</i>.  This is the
 * <i>direct</i> geodesic problem and its solution is given by the function
 * {@link #Direct Direct}.  (If <i>s12</i> is sufficiently large that the
 * geodesic wraps more than halfway around the earth, there will be another
 * geodesic between the points with a smaller <i>s12</i>.)
 * <p>
 * Given <i>lat1</i>, <i>lon1</i>, <i>lat2</i>, and <i>lon2</i>, we can
 * determine <i>azi1</i>, <i>azi2</i>, and <i>s12</i>.  This is the
 * <i>inverse</i> geodesic problem, whose solution is given by {@link #Inverse
 * Inverse}.  Usually, the solution to the inverse problem is unique.  In cases
 * where there are multiple solutions (all with the same <i>s12</i>, of
 * course), all the solutions can be easily generated once a particular
 * solution is provided.
 * <p>
 * The standard way of specifying the direct problem is the specify the
 * distance <i>s12</i> to the second point.  However it is sometimes useful
 * instead to specify the arc length <i>a12</i> (in degrees) on the auxiliary
 * sphere.  This is a mathematical construct used in solving the geodesic
 * problems.  The solution of the direct problem in this form is provided by
 * {@link #ArcDirect ArcDirect}.  An arc length in excess of 180&deg; indicates
 * that the geodesic is not a shortest path.  In addition, the arc length
 * between an equatorial crossing and the next extremum of latitude for a
 * geodesic is 90&deg;.
 * <p>
 * This class can also calculate several other quantities related to
 * geodesics.  These are:
 * <ul>
 * <li>
 *   <i>reduced length</i>.  If we fix the first point and increase
 *   <i>azi1</i> by <i>dazi1</i> (radians), the second point is displaced
 *   <i>m12</i> <i>dazi1</i> in the direction <i>azi2</i> + 90&deg;.  The
 *   quantity <i>m12</i> is called the "reduced length" and is symmetric under
 *   interchange of the two points.  On a curved surface the reduced length
 *   obeys a symmetry relation, <i>m12</i> + <i>m21</i> = 0.  On a flat
 *   surface, we have <i>m12</i> = <i>s12</i>.  The ratio <i>s12</i>/<i>m12</i>
 *   gives the azimuthal scale for an azimuthal equidistant projection.
 * <li>
 *   <i>geodesic scale</i>.  Consider a reference geodesic and a second
 *   geodesic parallel to this one at point 1 and separated by a small distance
 *   <i>dt</i>.  The separation of the two geodesics at point 2 is <i>M12</i>
 *   <i>dt</i> where <i>M12</i> is called the "geodesic scale".  <i>M21</i> is
 *   defined similarly (with the geodesics being parallel at point 2).  On a
 *   flat surface, we have <i>M12</i> = <i>M21</i> = 1.  The quantity
 *   1/<i>M12</i> gives the scale of the Cassini-Soldner projection.
 * <li>
 *   <i>area</i>.  The area between the geodesic from point 1 to point 2 and
 *   the equation is represented by <i>S12</i>; it is the area, measured
 *   counter-clockwise, of the geodesic quadrilateral with corners
 *   (<i>lat1</i>,<i>lon1</i>), (0,<i>lon1</i>), (0,<i>lon2</i>), and
 *   (<i>lat2</i>,<i>lon2</i>).  It can be used to compute the area of any
 *   geodesic polygon.
 * </ul>
 * <p>
 * The quantities <i>m12</i>, <i>M12</i>, <i>M21</i> which all specify the
 * behavior of nearby geodesics obey addition rules.  If points 1, 2, and 3 all
 * lie on a single geodesic, then the following rules hold:
 * <ul>
 * <li>
 *   <i>s13</i> = <i>s12</i> + <i>s23</i>
 * <li>
 *   <i>a13</i> = <i>a12</i> + <i>a23</i>
 * <li>
 *   <i>S13</i> = <i>S12</i> + <i>S23</i>
 * <li>
 *   <i>m13</i> = <i>m12</i> <i>M23</i> + <i>m23</i> <i>M21</i>
 * <li>
 *   <i>M13</i> = <i>M12</i> <i>M23</i> &minus; (1 &minus; <i>M12</i>
 *   <i>M21</i>) <i>m23</i> / <i>m12</i>
 * <li>
 *   <i>M31</i> = <i>M32</i> <i>M21</i> &minus; (1 &minus; <i>M23</i>
 *   <i>M32</i>) <i>m12</i> / <i>m23</i>
 * </ul>
 * <p>
 * The results of the geodesic calculations are bundled up into a {@link
 * GeodesicData} object which includes the input parameters and all the
 * computed results, i.e., <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, <i>lat2</i>,
 * <i>lon2</i>, <i>azi2</i>, <i>s12</i>, <i>a12</i>, <i>m12</i>, <i>M12</i>,
 * <i>M21</i>, <i>S12</i>.
 * <p>
 * The functions {@link #Direct(double, double, double, double, int) Direct},
 * {@link #ArcDirect(double, double, double, double, int) ArcDirect}, and
 * {@link #Inverse(double, double, double, double, int) Inverse} include an
 * optional final argument <i>outmask</i> which allows you specify which
 * results should be computed and returned.  If you omit <i>outmask</i>, then
 * the "standard" geodesic results are computed (latitudes, longitudes,
 * azimuths, and distance).  <i>outmask</i> is bitor'ed combination of {@link
 * GeodesicMask} values.  For example, if you wish just to compute the distance
 * between two points you would call, e.g.,
 * <pre>
 * {@code
 *  GeodesicData g = Geodesic.WGS84.Inverse(lat1, lon1, lat2, lon2,
 *                      GeodesicMask.DISTANCE); }</pre>
 * <p>
 * Additional functionality is provided by the {@link GeodesicLine} class,
 * which allows a sequence of points along a geodesic to be computed.
 * <p>
 * The shortest distance returned by the solution of the inverse problem is
 * (obviously) uniquely defined.  However, in a few special cases there are
 * multiple azimuths which yield the same shortest distance.  Here is a
 * catalog of those cases:
 * <ul>
 * <li>
 *   <i>lat1</i> = &minus;<i>lat2</i> (with neither point at a pole).  If
 *   <i>azi1</i> = <i>azi2</i>, the geodesic is unique.  Otherwise there are
 *   two geodesics and the second one is obtained by setting [<i>azi1</i>,
 *   <i>azi2</i>] &rarr; [<i>azi2</i>, <i>azi1</i>], [<i>M12</i>, <i>M21</i>]
 *   &rarr; [<i>M21</i>, <i>M12</i>], <i>S12</i> &rarr; &minus;<i>S12</i>.
 *   (This occurs when the longitude difference is near &plusmn;180&deg; for
 *   oblate ellipsoids.)
 * <li>
 *   <i>lon2</i> = <i>lon1</i> &plusmn; 180&deg; (with neither point at a
 *   pole).  If <i>azi1</i> = 0&deg; or &plusmn;180&deg;, the geodesic is
 *   unique.  Otherwise there are two geodesics and the second one is obtained
 *   by setting [ <i>azi1</i>, <i>azi2</i>] &rarr; [&minus;<i>azi1</i>,
 *   &minus;<i>azi2</i>], <i>S12</i> &rarr; &minus; <i>S12</i>.  (This occurs
 *   when <i>lat2</i> is near &minus;<i>lat1</i> for prolate ellipsoids.)
 * <li>
 *   Points 1 and 2 at opposite poles.  There are infinitely many geodesics
 *   which can be generated by setting [<i>azi1</i>, <i>azi2</i>] &rarr;
 *   [<i>azi1</i>, <i>azi2</i>] + [<i>d</i>, &minus;<i>d</i>], for arbitrary
 *   <i>d</i>.  (For spheres, this prescription applies when points 1 and 2 are
 *   antipodal.)
 * <li>
 *   <i>s12</i> = 0 (coincident points).  There are infinitely many geodesics
 *   which can be generated by setting [<i>azi1</i>, <i>azi2</i>] &rarr;
 *   [<i>azi1</i>, <i>azi2</i>] + [<i>d</i>, <i>d</i>], for arbitrary <i>d</i>.
 * </ul>
 * <p>
 * The calculations are accurate to better than 15 nm (15 nanometers) for the
 * WGS84 ellipsoid.  See Sec. 9 of
 * <a href="https://arxiv.org/abs/1102.1215v1">arXiv:1102.1215v1</a> for
 * details.  The algorithms used by this class are based on series expansions
 * using the flattening <i>f</i> as a small parameter.  These are only accurate
 * for |<i>f</i>| &lt; 0.02; however reasonably accurate results will be
 * obtained for |<i>f</i>| &lt; 0.2.  Here is a table of the approximate
 * maximum error (expressed as a distance) for an ellipsoid with the same
 * equatorial radius as the WGS84 ellipsoid and different values of the
 * flattening.<pre>
 *     |f|      error
 *     0.01     25 nm
 *     0.02     30 nm
 *     0.05     10 um
 *     0.1     1.5 mm
 *     0.2     300 mm </pre>
 * <p>
 * The algorithms are described in
 * <ul>
 * <li>C. F. F. Karney,
 *   <a href="https://doi.org/10.1007/s00190-012-0578-z">
 *   Algorithms for geodesics</a>,
 *   J. Geodesy <b>87</b>, 43&ndash;55 (2013)
 *   (<a href="https://geographiclib.sourceforge.io/geod-addenda.html">addenda</a>).
 * </ul>
 * <p>
 * Example of use:
 * <pre>
 * {@code
 * // Solve the direct geodesic problem.
 *
 * // This program reads in lines with lat1, lon1, azi1, s12 and prints
 * // out lines with lat2, lon2, azi2 (for the WGS84 ellipsoid).
 *
 * import java.util.*;
 * import net.sf.geographiclib.*;
 * public class Direct {
 *   public static void main(String[] args) {
 *     try {
 *       Scanner in = new Scanner(System.in);
 *       double lat1, lon1, azi1, s12;
 *       while (true) {
 *         lat1 = in.nextDouble(); lon1 = in.nextDouble();
 *         azi1 = in.nextDouble(); s12 = in.nextDouble();
 *         GeodesicData g = Geodesic.WGS84.Direct(lat1, lon1, azi1, s12);
 *         System.out.println(g.lat2 + " " + g.lon2 + " " + g.azi2);
 *       }
 *     }
 *     catch (Exception e) {}
 *   }
 * }}</pre>
 **********************************************************************/
public class Geodesic {

  /**
   * The order of the expansions used by Geodesic.
   **********************************************************************/
  protected static final int GEOGRAPHICLIB_GEODESIC_ORDER = 6;

  protected static final int nA1_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nC1_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nC1p_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nA2_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nC2_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nA3_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nA3x_ = nA3_;
  protected static final int nC3_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nC3x_ = (nC3_ * (nC3_ - 1)) / 2;
  protected static final int nC4_ = GEOGRAPHICLIB_GEODESIC_ORDER;
  protected static final int nC4x_ = (nC4_ * (nC4_ + 1)) / 2;
  private static final int maxit1_ = 20;
  private static final int maxit2_ = maxit1_ + GeoMath.digits + 10;

  // Underflow guard.  We require
  //   tiny_ * epsilon() > 0
  //   tiny_ + epsilon() == epsilon()
  protected static final double tiny_ = Math.sqrt(Double.MIN_NORMAL);
  private static final double tol0_ = Math.ulp(1.0);
  // Increase multiplier in defn of tol1_ from 100 to 200 to fix inverse case
  // 52.784459512564 0 -52.784459512563990912 179.634407464943777557
  // which otherwise failed for Visual Studio 10 (Release and Debug)
  private static final double tol1_ = 200 * tol0_;
  private static final double tol2_ = Math.sqrt(tol0_);
  // Check on bisection interval
  private static final double tolb_ = tol0_ * tol2_;
  private static final double xthresh_ = 1000 * tol2_;

  protected double _a, _f, _f1, _e2, _ep2, _b, _c2;
  private double _n, _etol2;
  private double _A3x[], _C3x[], _C4x[];

  /**
   * Constructor for a ellipsoid with
   * <p>
   * @param a equatorial radius (meters).
   * @param f flattening of ellipsoid.  Setting <i>f</i> = 0 gives a sphere.
   *   Negative <i>f</i> gives a prolate ellipsoid.
   * @exception GeographicErr if <i>a</i> or (1 &minus; <i>f</i> ) <i>a</i> is
   *   not positive.
   **********************************************************************/
  public Geodesic(double a, double f) {
    _a = a;
    _f = f;
    _f1 = 1 - _f;
    _e2 = _f * (2 - _f);
    _ep2 = _e2 / GeoMath.sq(_f1); // e2 / (1 - e2)
    _n = _f / ( 2 - _f);
    _b = _a * _f1;
    _c2 = (GeoMath.sq(_a) + GeoMath.sq(_b) *
           (_e2 == 0 ? 1 :
            (_e2 > 0 ? GeoMath.atanh(Math.sqrt(_e2)) :
             Math.atan(Math.sqrt(-_e2))) /
            Math.sqrt(Math.abs(_e2))))/2; // authalic radius squared
    // The sig12 threshold for "really short".  Using the auxiliary sphere
    // solution with dnm computed at (bet1 + bet2) / 2, the relative error in
    // the azimuth consistency check is sig12^2 * abs(f) * min(1, 1-f/2) / 2.
    // (Error measured for 1/100 < b/a < 100 and abs(f) >= 1/1000.  For a
    // given f and sig12, the max error occurs for lines near the pole.  If
    // the old rule for computing dnm = (dn1 + dn2)/2 is used, then the error
    // increases by a factor of 2.)  Setting this equal to epsilon gives
    // sig12 = etol2.  Here 0.1 is a safety factor (error decreased by 100)
    // and max(0.001, abs(f)) stops etol2 getting too large in the nearly
    // spherical case.
    _etol2 = 0.1 * tol2_ /
              Math.sqrt( Math.max(0.001, Math.abs(_f)) *
                         Math.min(1.0, 1 - _f/2) / 2 );
    if (!(GeoMath.isfinite(_a) && _a > 0))
      throw new GeographicErr("Equatorial radius is not positive");
    if (!(GeoMath.isfinite(_b) && _b > 0))
      throw new GeographicErr("Polar semi-axis is not positive");
    _A3x = new double[nA3x_];
    _C3x = new double[nC3x_];
    _C4x = new double[nC4x_];

    A3coeff();
    C3coeff();
    C4coeff();
  }

  /**
   * Solve the direct geodesic problem where the length of the geodesic
   * is specified in terms of distance.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param s12 distance between point 1 and point 2 (meters); it can be
   *   negative.
   * @return a {@link GeodesicData} object with the following fields:
   *   <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, <i>lat2</i>, <i>lon2</i>,
   *   <i>azi2</i>, <i>s12</i>, <i>a12</i>.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].  The values
   * of <i>lon2</i> and <i>azi2</i> returned are in the range [&minus;180&deg;,
   * 180&deg;].
   * <p>
   * If either point is at a pole, the azimuth is defined by keeping the
   * longitude fixed, writing <i>lat</i> = &plusmn;(90&deg; &minus; &epsilon;),
   * and taking the limit &epsilon; &rarr; 0+.  An arc length greater that
   * 180&deg; signifies a geodesic which is not a shortest path.  (For a
   * prolate ellipsoid, an additional condition is necessary for a shortest
   * path: the longitudinal extent must not exceed of 180&deg;.)
   **********************************************************************/
  public GeodesicData Direct(double lat1, double lon1,
                             double azi1, double s12) {
    return Direct(lat1, lon1, azi1, false, s12, GeodesicMask.STANDARD);
  }
  /**
   * Solve the direct geodesic problem where the length of the geodesic is
   * specified in terms of distance and with a subset of the geodesic results
   * returned.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param s12 distance between point 1 and point 2 (meters); it can be
   *   negative.
   * @param outmask a bitor'ed combination of {@link GeodesicMask} values
   *   specifying which results should be returned.
   * @return a {@link GeodesicData} object with the fields specified by
   *   <i>outmask</i> computed.
   * <p>
   * <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, <i>s12</i>, and <i>a12</i> are
   * always included in the returned result.  The value of <i>lon2</i> returned
   * is in the range [&minus;180&deg;, 180&deg;], unless the <i>outmask</i>
   * includes the {@link GeodesicMask#LONG_UNROLL} flag.
   **********************************************************************/
  public GeodesicData Direct(double lat1, double lon1,
                             double azi1, double s12, int outmask) {
    return Direct(lat1, lon1, azi1, false, s12, outmask);
  }

  /**
   * Solve the direct geodesic problem where the length of the geodesic
   * is specified in terms of arc length.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param a12 arc length between point 1 and point 2 (degrees); it can
   *   be negative.
   * @return a {@link GeodesicData} object with the following fields:
   *   <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, <i>lat2</i>, <i>lon2</i>,
   *   <i>azi2</i>, <i>s12</i>, <i>a12</i>.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].  The values
   * of <i>lon2</i> and <i>azi2</i> returned are in the range [&minus;180&deg;,
   * 180&deg;].
   * <p>
   * If either point is at a pole, the azimuth is defined by keeping the
   * longitude fixed, writing <i>lat</i> = &plusmn;(90&deg; &minus; &epsilon;),
   * and taking the limit &epsilon; &rarr; 0+.  An arc length greater that
   * 180&deg; signifies a geodesic which is not a shortest path.  (For a
   * prolate ellipsoid, an additional condition is necessary for a shortest
   * path: the longitudinal extent must not exceed of 180&deg;.)
   **********************************************************************/
  public GeodesicData ArcDirect(double lat1, double lon1,
                                double azi1, double a12) {
    return Direct(lat1, lon1, azi1, true, a12, GeodesicMask.STANDARD);
  }

  /**
   * Solve the direct geodesic problem where the length of the geodesic is
   * specified in terms of arc length and with a subset of the geodesic results
   * returned.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param a12 arc length between point 1 and point 2 (degrees); it can
   *   be negative.
   * @param outmask a bitor'ed combination of {@link GeodesicMask} values
   *   specifying which results should be returned.
   * @return a {@link GeodesicData} object with the fields specified by
   *   <i>outmask</i> computed.
   * <p>
   * <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, and <i>a12</i> are always included
   * in the returned result.  The value of <i>lon2</i> returned is in the range
   * [&minus;180&deg;, 180&deg;], unless the <i>outmask</i> includes the {@link
   * GeodesicMask#LONG_UNROLL} flag.
   **********************************************************************/
  public GeodesicData ArcDirect(double lat1, double lon1,
                                double azi1, double a12, int outmask) {
    return Direct(lat1, lon1, azi1, true, a12, outmask);
  }

  /**
   * The general direct geodesic problem.  {@link #Direct Direct} and
   * {@link #ArcDirect ArcDirect} are defined in terms of this function.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param arcmode boolean flag determining the meaning of the
   *   <i>s12_a12</i>.
   * @param s12_a12 if <i>arcmode</i> is false, this is the distance between
   *   point 1 and point 2 (meters); otherwise it is the arc length between
   *   point 1 and point 2 (degrees); it can be negative.
   * @param outmask a bitor'ed combination of {@link GeodesicMask} values
   *   specifying which results should be returned.
   * @return a {@link GeodesicData} object with the fields specified by
   *   <i>outmask</i> computed.
   * <p>
   * The {@link GeodesicMask} values possible for <i>outmask</i> are
   * <ul>
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#LATITUDE} for the latitude
   *   <i>lat2</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#LONGITUDE} for the latitude
   *   <i>lon2</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#AZIMUTH} for the latitude
   *   <i>azi2</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#DISTANCE} for the distance
   *   <i>s12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#REDUCEDLENGTH} for the reduced
   *   length <i>m12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#GEODESICSCALE} for the geodesic
   *   scales <i>M12</i> and <i>M21</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#AREA} for the area <i>S12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#ALL} for all of the above;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#LONG_UNROLL}, if set then
   *   <i>lon1</i> is unchanged and <i>lon2</i> &minus; <i>lon1</i> indicates
   *   how many times and in what sense the geodesic encircles the ellipsoid.
   *   Otherwise <i>lon1</i> and <i>lon2</i> are both reduced to the range
   *   [&minus;180&deg;, 180&deg;].
   * </ul>
   * <p>
   * The function value <i>a12</i> is always computed and returned and this
   * equals <i>s12_a12</i> is <i>arcmode</i> is true.  If <i>outmask</i>
   * includes {@link GeodesicMask#DISTANCE} and <i>arcmode</i> is false, then
   * <i>s12</i> = <i>s12_a12</i>.  It is not necessary to include {@link
   * GeodesicMask#DISTANCE_IN} in <i>outmask</i>; this is automatically
   * included is <i>arcmode</i> is false.
   **********************************************************************/
  public GeodesicData Direct(double lat1, double lon1, double azi1,
                             boolean arcmode, double s12_a12, int outmask) {
    // Automatically supply DISTANCE_IN if necessary
    if (!arcmode) outmask |= GeodesicMask.DISTANCE_IN;
    return new GeodesicLine(this, lat1, lon1, azi1, outmask)
      .                         // Note the dot!
      Position(arcmode, s12_a12, outmask);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the direct geodesic problem
   * specified in terms of distance with all capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param s12 distance between point 1 and point 2 (meters); it can be
   *   negative.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the direct geodesic problem.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].
   **********************************************************************/
  public GeodesicLine DirectLine(double lat1, double lon1, double azi1,
                                 double s12) {
    return DirectLine(lat1, lon1, azi1, s12, GeodesicMask.ALL);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the direct geodesic problem
   * specified in terms of distance with a subset of the capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param s12 distance between point 1 and point 2 (meters); it can be
   *   negative.
   * @param caps bitor'ed combination of {@link GeodesicMask} values
   *   specifying the capabilities the GeodesicLine object should possess,
   *   i.e., which quantities can be returned in calls to
   *   {@link GeodesicLine#Position GeodesicLine.Position}.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the direct geodesic problem.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].
   **********************************************************************/
  public GeodesicLine DirectLine(double lat1, double lon1, double azi1,
                                 double s12, int caps) {
    return GenDirectLine(lat1, lon1, azi1, false, s12, caps);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the direct geodesic problem
   * specified in terms of arc length with all capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param a12 arc length between point 1 and point 2 (degrees); it can
   *   be negative.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the direct geodesic problem.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].
   **********************************************************************/
  public GeodesicLine ArcDirectLine(double lat1, double lon1, double azi1,
                                    double a12) {
    return ArcDirectLine(lat1, lon1, azi1, a12, GeodesicMask.ALL);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the direct geodesic problem
   * specified in terms of arc length with a subset of the capabilities
   * included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param a12 arc length between point 1 and point 2 (degrees); it can
   *   be negative.
   * @param caps bitor'ed combination of {@link GeodesicMask} values
   *   specifying the capabilities the GeodesicLine object should possess,
   *   i.e., which quantities can be returned in calls to
   *   {@link GeodesicLine#Position GeodesicLine.Position}.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the direct geodesic problem.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].
   **********************************************************************/
  public GeodesicLine ArcDirectLine(double lat1, double lon1, double azi1,
                                    double a12, int caps) {
    return GenDirectLine(lat1, lon1, azi1, true, a12, caps);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the direct geodesic problem
   * specified in terms of either distance or arc length with a subset of the
   * capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param arcmode boolean flag determining the meaning of the <i>s12_a12</i>.
   * @param s12_a12 if <i>arcmode</i> is false, this is the distance between
   *   point 1 and point 2 (meters); otherwise it is the arc length between
   *   point 1 and point 2 (degrees); it can be negative.
   * @param caps bitor'ed combination of {@link GeodesicMask} values
   *   specifying the capabilities the GeodesicLine object should possess,
   *   i.e., which quantities can be returned in calls to
   *   {@link GeodesicLine#Position GeodesicLine.Position}.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the direct geodesic problem.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].
   **********************************************************************/
  public GeodesicLine GenDirectLine(double lat1, double lon1, double azi1,
                                    boolean arcmode, double s12_a12, int caps)
  {
    azi1 = GeoMath.AngNormalize(azi1);
    double salp1, calp1;
    // Guard against underflow in salp0.  Also -0 is converted to +0.
    { Pair p = GeoMath.sincosd(GeoMath.AngRound(azi1));
      salp1 = p.first; calp1 = p.second; }
    // Automatically supply DISTANCE_IN if necessary
    if (!arcmode) caps |= GeodesicMask.DISTANCE_IN;
    return new GeodesicLine(this, lat1, lon1, azi1, salp1, calp1,
                        caps, arcmode, s12_a12);
  }

  /**
   * Solve the inverse geodesic problem.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param lat2 latitude of point 2 (degrees).
   * @param lon2 longitude of point 2 (degrees).
   * @return a {@link GeodesicData} object with the following fields:
   *   <i>lat1</i>, <i>lon1</i>, <i>azi1</i>, <i>lat2</i>, <i>lon2</i>,
   *   <i>azi2</i>, <i>s12</i>, <i>a12</i>.
   * <p>
   * <i>lat1</i> and <i>lat2</i> should be in the range [&minus;90&deg;,
   * 90&deg;].  The values of <i>azi1</i> and <i>azi2</i> returned are in the
   * range [&minus;180&deg;, 180&deg;].
   * <p>
   * If either point is at a pole, the azimuth is defined by keeping the
   * longitude fixed, writing <i>lat</i> = &plusmn;(90&deg; &minus; &epsilon;),
   * taking the limit &epsilon; &rarr; 0+.
   * <p>
   * The solution to the inverse problem is found using Newton's method.  If
   * this fails to converge (this is very unlikely in geodetic applications
   * but does occur for very eccentric ellipsoids), then the bisection method
   * is used to refine the solution.
   **********************************************************************/
  public GeodesicData Inverse(double lat1, double lon1,
                              double lat2, double lon2) {
    return Inverse(lat1, lon1, lat2, lon2, GeodesicMask.STANDARD);
  }

  private class InverseData {
    private GeodesicData g;
    private double salp1, calp1, salp2, calp2;
    private InverseData() {
      g = new GeodesicData();
      salp1 = calp1 = salp2 = calp2 = Double.NaN;
    }
  }

  private InverseData InverseInt(double lat1, double lon1,
                                 double lat2, double lon2, int outmask) {
    InverseData result = new InverseData();
    GeodesicData r = result.g;
    // Compute longitude difference (AngDiff does this carefully).  Result is
    // in [-180, 180] but -180 is only for west-going geodesics.  180 is for
    // east-going and meridional geodesics.
    r.lat1 = lat1 = GeoMath.LatFix(lat1); r.lat2 = lat2 = GeoMath.LatFix(lat2);
    // If really close to the equator, treat as on equator.
    lat1 = GeoMath.AngRound(lat1);
    lat2 = GeoMath.AngRound(lat2);
    double lon12, lon12s;
    {
        Pair p = GeoMath.AngDiff(lon1, lon2);
        lon12 = p.first; lon12s = p.second;
    }
    if ((outmask & GeodesicMask.LONG_UNROLL) != 0) {
      r.lon1 = lon1; r.lon2 = (lon1 + lon12) + lon12s;
    } else {
      r.lon1 = GeoMath.AngNormalize(lon1); r.lon2 = GeoMath.AngNormalize(lon2);
    }
    // Make longitude difference positive.
    int lonsign = lon12 >= 0 ? 1 : -1;
    // If very close to being on the same half-meridian, then make it so.
    lon12 = lonsign * GeoMath.AngRound(lon12);
    lon12s = GeoMath.AngRound((180 - lon12) - lonsign * lon12s);
    double
      lam12 = Math.toRadians(lon12), slam12, clam12;
    { Pair p = GeoMath.sincosd(lon12 > 90 ? lon12s : lon12);
      slam12 = p.first; clam12 = (lon12 > 90 ? -1 : 1) * p.second; }

    // Swap points so that point with higher (abs) latitude is point 1
    // If one latitude is a nan, then it becomes lat1.
    int swapp = Math.abs(lat1) < Math.abs(lat2) ? -1 : 1;
    if (swapp < 0) {
      lonsign *= -1;
      { double t = lat1; lat1 = lat2; lat2 = t; }
    }
    // Make lat1 <= 0
    int latsign = lat1 < 0 ? 1 : -1;
    lat1 *= latsign;
    lat2 *= latsign;
    // Now we have
    //
    //     0 <= lon12 <= 180
    //     -90 <= lat1 <= 0
    //     lat1 <= lat2 <= -lat1
    //
    // longsign, swapp, latsign register the transformation to bring the
    // coordinates to this canonical form.  In all cases, 1 means no change was
    // made.  We make these transformations so that there are few cases to
    // check, e.g., on verifying quadrants in atan2.  In addition, this
    // enforces some symmetries in the results returned.

    double sbet1, cbet1, sbet2, cbet2, s12x, m12x;
    s12x = m12x = Double.NaN;

    { Pair p = GeoMath.sincosd(lat1);
      sbet1 = _f1 * p.first; cbet1 = p.second; }
    // Ensure cbet1 = +epsilon at poles; doing the fix on beta means that sig12
    // will be <= 2*tiny for two points at the same pole.
    { Pair p = GeoMath.norm(sbet1, cbet1); sbet1 = p.first; cbet1 = p.second; }
    cbet1 = Math.max(tiny_, cbet1);

    { Pair p = GeoMath.sincosd(lat2);
      sbet2 = _f1 * p.first; cbet2 = p.second; }
    // Ensure cbet2 = +epsilon at poles
    { Pair p = GeoMath.norm(sbet2, cbet2); sbet2 = p.first; cbet2 = p.second; }
    cbet2 = Math.max(tiny_, cbet2);

    // If cbet1 < -sbet1, then cbet2 - cbet1 is a sensitive measure of the
    // |bet1| - |bet2|.  Alternatively (cbet1 >= -sbet1), abs(sbet2) + sbet1 is
    // a better measure.  This logic is used in assigning calp2 in Lambda12.
    // Sometimes these quantities vanish and in that case we force bet2 = +/-
    // bet1 exactly.  An example where is is necessary is the inverse problem
    // 48.522876735459 0 -48.52287673545898293 179.599720456223079643
    // which failed with Visual Studio 10 (Release and Debug)

    if (cbet1 < -sbet1) {
      if (cbet2 == cbet1)
        sbet2 = sbet2 < 0 ? sbet1 : -sbet1;
    } else {
      if (Math.abs(sbet2) == -sbet1)
        cbet2 = cbet1;
    }

    double
      dn1 = Math.sqrt(1 + _ep2 * GeoMath.sq(sbet1)),
      dn2 = Math.sqrt(1 + _ep2 * GeoMath.sq(sbet2));

    double a12, sig12, calp1, salp1, calp2, salp2;
    a12 = sig12 = calp1 = salp1 = calp2 = salp2 = Double.NaN;
    // index zero elements of these arrays are unused
    double C1a[] = new double[nC1_ + 1];
    double C2a[] = new double[nC2_ + 1];
    double C3a[] = new double[nC3_];

    boolean meridian = lat1 == -90 || slam12 == 0;

    if (meridian) {

      // Endpoints are on a single full meridian, so the geodesic might lie on
      // a meridian.

      calp1 = clam12; salp1 = slam12; // Head to the target longitude
      calp2 = 1; salp2 = 0;           // At the target we're heading north

      double
        // tan(bet) = tan(sig) * cos(alp)
        ssig1 = sbet1, csig1 = calp1 * cbet1,
        ssig2 = sbet2, csig2 = calp2 * cbet2;

      // sig12 = sig2 - sig1
      sig12 = Math.atan2(Math.max(0.0, csig1 * ssig2 - ssig1 * csig2),
                                       csig1 * csig2 + ssig1 * ssig2);
      {
        LengthsV v =
          Lengths(_n, sig12, ssig1, csig1, dn1,
                  ssig2, csig2, dn2, cbet1, cbet2,
                  outmask | GeodesicMask.DISTANCE | GeodesicMask.REDUCEDLENGTH,
                  C1a, C2a);
        s12x = v.s12b; m12x = v.m12b;
        if ((outmask & GeodesicMask.GEODESICSCALE) != 0) {
          r.M12 = v.M12; r.M21 = v.M21;
        }
      }
      // Add the check for sig12 since zero length geodesics might yield m12 <
      // 0.  Test case was
      //
      //    echo 20.001 0 20.001 0 | GeodSolve -i
      //
      // In fact, we will have sig12 > pi/2 for meridional geodesic which is
      // not a shortest path.
      if (sig12 < 1 || m12x >= 0) {
        // Need at least 2, to handle 90 0 90 180
        if (sig12 < 3 * tiny_)
          sig12 = m12x = s12x = 0;
        m12x *= _b;
        s12x *= _b;
        a12 = Math.toDegrees(sig12);
      } else
        // m12 < 0, i.e., prolate and too close to anti-podal
        meridian = false;
    }

    double omg12 = Double.NaN, somg12 = 2, comg12 = Double.NaN;
    if (!meridian &&
        sbet1 == 0 &&   // and sbet2 == 0
        // Mimic the way Lambda12 works with calp1 = 0
        (_f <= 0 || lon12s >= _f * 180)) {

      // Geodesic runs along equator
      calp1 = calp2 = 0; salp1 = salp2 = 1;
      s12x = _a * lam12;
      sig12 = omg12 = lam12 / _f1;
      m12x = _b * Math.sin(sig12);
      if ((outmask & GeodesicMask.GEODESICSCALE) != 0)
        r.M12 = r.M21 = Math.cos(sig12);
      a12 = lon12 / _f1;

    } else if (!meridian) {

      // Now point1 and point2 belong within a hemisphere bounded by a
      // meridian and geodesic is neither meridional or equatorial.

      // Figure a starting point for Newton's method
      double dnm;
      {
        InverseStartV v =
          InverseStart(sbet1, cbet1, dn1, sbet2, cbet2, dn2,
                       lam12, slam12, clam12,
                       C1a, C2a);
        sig12 = v.sig12;
        salp1 = v.salp1; calp1 = v.calp1;
        salp2 = v.salp2; calp2 = v.calp2;
        dnm = v.dnm;
      }

      if (sig12 >= 0) {
        // Short lines (InverseStart sets salp2, calp2, dnm)
        s12x = sig12 * _b * dnm;
        m12x = GeoMath.sq(dnm) * _b * Math.sin(sig12 / dnm);
        if ((outmask & GeodesicMask.GEODESICSCALE) != 0)
          r.M12 = r.M21 = Math.cos(sig12 / dnm);
        a12 = Math.toDegrees(sig12);
        omg12 = lam12 / (_f1 * dnm);
      } else {

        // Newton's method.  This is a straightforward solution of f(alp1) =
        // lambda12(alp1) - lam12 = 0 with one wrinkle.  f(alp) has exactly one
        // root in the interval (0, pi) and its derivative is positive at the
        // root.  Thus f(alp) is positive for alp > alp1 and negative for alp <
        // alp1.  During the course of the iteration, a range (alp1a, alp1b) is
        // maintained which brackets the root and with each evaluation of
        // f(alp) the range is shrunk, if possible.  Newton's method is
        // restarted whenever the derivative of f is negative (because the new
        // value of alp1 is then further from the solution) or if the new
        // estimate of alp1 lies outside (0,pi); in this case, the new starting
        // guess is taken to be (alp1a + alp1b) / 2.
        double ssig1, csig1, ssig2, csig2, eps, domg12;
        ssig1 = csig1 = ssig2 = csig2 = eps = domg12 = Double.NaN;
        int numit = 0;
        // Bracketing range
        double salp1a = tiny_, calp1a = 1, salp1b = tiny_, calp1b = -1;
        for (boolean tripn = false, tripb = false; numit < maxit2_; ++numit) {
          // the WGS84 test set: mean = 1.47, sd = 1.25, max = 16
          // WGS84 and random input: mean = 2.85, sd = 0.60
          double v, dv;
          {
            Lambda12V w =
              Lambda12(sbet1, cbet1, dn1, sbet2, cbet2, dn2, salp1, calp1,
                       slam12, clam12, numit < maxit1_, C1a, C2a, C3a);
            v = w.lam12;
            salp2 = w.salp2; calp2 = w.calp2;
            sig12 = w.sig12;
            ssig1 = w.ssig1; csig1 = w.csig1;
            ssig2 = w.ssig2; csig2 = w.csig2;
            eps = w.eps; domg12 = w.domg12;
            dv = w.dlam12;
          }
          // 2 * tol0 is approximately 1 ulp for a number in [0, pi].
          // Reversed test to allow escape with NaNs
          if (tripb || !(Math.abs(v) >= (tripn ? 8 : 1) * tol0_)) break;
          // Update bracketing values
          if (v > 0 && (numit > maxit1_ || calp1/salp1 > calp1b/salp1b))
            { salp1b = salp1; calp1b = calp1; }
          else if (v < 0 && (numit > maxit1_ || calp1/salp1 < calp1a/salp1a))
            { salp1a = salp1; calp1a = calp1; }
          if (numit < maxit1_ && dv > 0) {
            double
              dalp1 = -v/dv;
            double
              sdalp1 = Math.sin(dalp1), cdalp1 = Math.cos(dalp1),
              nsalp1 = salp1 * cdalp1 + calp1 * sdalp1;
            if (nsalp1 > 0 && Math.abs(dalp1) < Math.PI) {
              calp1 = calp1 * cdalp1 - salp1 * sdalp1;
              salp1 = nsalp1;
              { Pair p = GeoMath.norm(salp1, calp1);
                salp1 = p.first; calp1 = p.second; }
              // In some regimes we don't get quadratic convergence because
              // slope -> 0.  So use convergence conditions based on epsilon
              // instead of sqrt(epsilon).
              tripn = Math.abs(v) <= 16 * tol0_;
              continue;
            }
          }
          // Either dv was not positive or updated value was outside legal
          // range.  Use the midpoint of the bracket as the next estimate.
          // This mechanism is not needed for the WGS84 ellipsoid, but it does
          // catch problems with more eccentric ellipsoids.  Its efficacy is
          // such for the WGS84 test set with the starting guess set to alp1 =
          // 90deg:
          // the WGS84 test set: mean = 5.21, sd = 3.93, max = 24
          // WGS84 and random input: mean = 4.74, sd = 0.99
          salp1 = (salp1a + salp1b)/2;
          calp1 = (calp1a + calp1b)/2;
          { Pair p = GeoMath.norm(salp1, calp1);
            salp1 = p.first; calp1 = p.second; }
          tripn = false;
          tripb = (Math.abs(salp1a - salp1) + (calp1a - calp1) < tolb_ ||
                   Math.abs(salp1 - salp1b) + (calp1 - calp1b) < tolb_);
        }
        {
          // Ensure that the reduced length and geodesic scale are computed in
          // a "canonical" way, with the I2 integral.
          int lengthmask = outmask |
            ((outmask &
              (GeodesicMask.REDUCEDLENGTH | GeodesicMask.GEODESICSCALE)) != 0 ?
             GeodesicMask.DISTANCE : GeodesicMask.NONE);
          LengthsV v =
            Lengths(eps, sig12,
                    ssig1, csig1, dn1, ssig2, csig2, dn2, cbet1, cbet2,
                    lengthmask, C1a, C2a);
          s12x = v.s12b; m12x = v.m12b;
          if ((outmask & GeodesicMask.GEODESICSCALE) != 0) {
            r.M12 = v.M12; r.M21 = v.M21;
          }
        }
        m12x *= _b;
        s12x *= _b;
        a12 = Math.toDegrees(sig12);
        if ((outmask & GeodesicMask.AREA) != 0) {
          // omg12 = lam12 - domg12
          double sdomg12 = Math.sin(domg12), cdomg12 = Math.cos(domg12);
          somg12 = slam12 * cdomg12 - clam12 * sdomg12;
          comg12 = clam12 * cdomg12 + slam12 * sdomg12;
        }
      }
    }

    if ((outmask & GeodesicMask.DISTANCE) != 0)
      r.s12 = 0 + s12x;           // Convert -0 to 0

    if ((outmask & GeodesicMask.REDUCEDLENGTH) != 0)
      r.m12 = 0 + m12x;           // Convert -0 to 0

    if ((outmask & GeodesicMask.AREA) != 0) {
      double
        // From Lambda12: sin(alp1) * cos(bet1) = sin(alp0)
        salp0 = salp1 * cbet1,
        calp0 = Math.hypot(calp1, salp1 * sbet1); // calp0 > 0
      double alp12;
      if (calp0 != 0 && salp0 != 0) {
        double
          // From Lambda12: tan(bet) = tan(sig) * cos(alp)
          ssig1 = sbet1, csig1 = calp1 * cbet1,
          ssig2 = sbet2, csig2 = calp2 * cbet2,
          k2 = GeoMath.sq(calp0) * _ep2,
          eps = k2 / (2 * (1 + Math.sqrt(1 + k2)) + k2),
          // Multiplier = a^2 * e^2 * cos(alpha0) * sin(alpha0).
          A4 = GeoMath.sq(_a) * calp0 * salp0 * _e2;
        { Pair p = GeoMath.norm(ssig1, csig1);
          ssig1 = p.first; csig1 = p.second; }
        { Pair p = GeoMath.norm(ssig2, csig2);
          ssig2 = p.first; csig2 = p.second; }
        double C4a[] = new double[nC4_];
        C4f(eps, C4a);
        double
          B41 = SinCosSeries(false, ssig1, csig1, C4a),
          B42 = SinCosSeries(false, ssig2, csig2, C4a);
        r.S12 = A4 * (B42 - B41);
      } else
        // Avoid problems with indeterminate sig1, sig2 on equator
        r.S12 = 0;

      if (!meridian && somg12 > 1) {
        somg12 = Math.sin(omg12); comg12 = Math.cos(omg12);
      }

      if (!meridian &&
          comg12 > -0.7071 &&     // Long difference not too big
          sbet2 - sbet1 < 1.75) { // Lat difference not too big
        // Use tan(Gamma/2) = tan(omg12/2)
        // * (tan(bet1/2)+tan(bet2/2))/(1+tan(bet1/2)*tan(bet2/2))
        // with tan(x/2) = sin(x)/(1+cos(x))
        double
          domg12 = 1 + comg12, dbet1 = 1 + cbet1, dbet2 = 1 + cbet2;
        alp12 = 2 * Math.atan2( somg12 * ( sbet1 * dbet2 + sbet2 * dbet1 ),
                           domg12 * ( sbet1 * sbet2 + dbet1 * dbet2 ) );
      } else {
        // alp12 = alp2 - alp1, used in atan2 so no need to normalize
        double
          salp12 = salp2 * calp1 - calp2 * salp1,
          calp12 = calp2 * calp1 + salp2 * salp1;
        // The right thing appears to happen if alp1 = +/-180 and alp2 = 0, viz
        // salp12 = -0 and alp12 = -180.  However this depends on the sign
        // being attached to 0 correctly.  The following ensures the correct
        // behavior.
        if (salp12 == 0 && calp12 < 0) {
          salp12 = tiny_ * calp1;
          calp12 = -1;
        }
        alp12 = Math.atan2(salp12, calp12);
      }
      r.S12 += _c2 * alp12;
      r.S12 *= swapp * lonsign * latsign;
      // Convert -0 to 0
      r.S12 += 0;
    }

    // Convert calp, salp to azimuth accounting for lonsign, swapp, latsign.
    if (swapp < 0) {
      { double t = salp1; salp1 = salp2; salp2 = t; }
      { double t = calp1; calp1 = calp2; calp2 = t; }
      if ((outmask & GeodesicMask.GEODESICSCALE) != 0)
        { double t = r.M12; r.M12 = r.M21; r.M21 = t; }
    }

    salp1 *= swapp * lonsign; calp1 *= swapp * latsign;
    salp2 *= swapp * lonsign; calp2 *= swapp * latsign;

    // Returned value in [0, 180]
    r.a12 = a12;
    result.salp1 = salp1; result.calp1 = calp1;
    result.salp2 = salp2; result.calp2 = calp2;
    return result;
  }

  /**
   * Solve the inverse geodesic problem with a subset of the geodesic results
   * returned.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param lat2 latitude of point 2 (degrees).
   * @param lon2 longitude of point 2 (degrees).
   * @param outmask a bitor'ed combination of {@link GeodesicMask} values
   *   specifying which results should be returned.
   * @return a {@link GeodesicData} object with the fields specified by
   *   <i>outmask</i> computed.
   * <p>
   * The {@link GeodesicMask} values possible for <i>outmask</i> are
   * <ul>
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#DISTANCE} for the distance
   *   <i>s12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#AZIMUTH} for the latitude
   *   <i>azi2</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#REDUCEDLENGTH} for the reduced
   *   length <i>m12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#GEODESICSCALE} for the geodesic
   *   scales <i>M12</i> and <i>M21</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#AREA} for the area <i>S12</i>;
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#ALL} for all of the above.
   * <li>
   *   <i>outmask</i> |= {@link GeodesicMask#LONG_UNROLL}, if set then
   *   <i>lon1</i> is unchanged and <i>lon2</i> &minus; <i>lon1</i> indicates
   *   whether the geodesic is east going or west going.  Otherwise <i>lon1</i>
   *   and <i>lon2</i> are both reduced to the range [&minus;180&deg;,
   *   180&deg;].
   * </ul>
   * <p>
   * <i>lat1</i>, <i>lon1</i>, <i>lat2</i>, <i>lon2</i>, and <i>a12</i> are
   * always included in the returned result.
   **********************************************************************/
  public GeodesicData Inverse(double lat1, double lon1,
                              double lat2, double lon2, int outmask) {
    outmask &= GeodesicMask.OUT_MASK;
    InverseData result = InverseInt(lat1, lon1, lat2, lon2, outmask);
    GeodesicData r = result.g;
    if ((outmask & GeodesicMask.AZIMUTH) != 0) {
      r.azi1 = GeoMath.atan2d(result.salp1, result.calp1);
      r.azi2 = GeoMath.atan2d(result.salp2, result.calp2);
    }
    return r;
  }

  /**
   * Define a {@link GeodesicLine} in terms of the inverse geodesic problem
   * with all capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param lat2 latitude of point 2 (degrees).
   * @param lon2 longitude of point 2 (degrees).
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the inverse geodesic problem.
   * <p>
   * <i>lat1</i> and <i>lat2</i> should be in the range [&minus;90&deg;,
   * 90&deg;].
   **********************************************************************/
  public GeodesicLine InverseLine(double lat1, double lon1,
                                  double lat2, double lon2)  {
    return InverseLine(lat1, lon1, lat2, lon2, GeodesicMask.ALL);
  }

  /**
   * Define a {@link GeodesicLine} in terms of the inverse geodesic problem
   * with a subset of the capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param lat2 latitude of point 2 (degrees).
   * @param lon2 longitude of point 2 (degrees).
   * @param caps bitor'ed combination of {@link GeodesicMask} values specifying
   *   the capabilities the GeodesicLine object should possess, i.e., which
   *   quantities can be returned in calls to
   *   {@link GeodesicLine#Position GeodesicLine.Position}.
   * @return a {@link GeodesicLine} object.
   * <p>
   * This function sets point 3 of the GeodesicLine to correspond to point 2
   * of the inverse geodesic problem.
   * <p>
   * <i>lat1</i> and <i>lat2</i> should be in the range [&minus;90&deg;,
   * 90&deg;].
   **********************************************************************/
  public GeodesicLine InverseLine(double lat1, double lon1,
                                  double lat2, double lon2, int caps)  {
    InverseData result = InverseInt(lat1, lon1, lat2, lon2, 0);
    double salp1 = result.salp1, calp1 = result.calp1,
      azi1 = GeoMath.atan2d(salp1, calp1), a12 = result.g.a12;
    // Ensure that a12 can be converted to a distance
    if ((caps & (GeodesicMask.OUT_MASK & GeodesicMask.DISTANCE_IN)) != 0)
      caps |= GeodesicMask.DISTANCE;
    return new GeodesicLine(this, lat1, lon1, azi1, salp1, calp1, caps,
                            true, a12);
  }

  /**
   * Set up to compute several points on a single geodesic with all
   * capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @return a {@link GeodesicLine} object.
   * <p>
   * <i>lat1</i> should be in the range [&minus;90&deg;, 90&deg;].  The full
   * set of capabilities is included.
   * <p>
   * If the point is at a pole, the azimuth is defined by keeping the
   * <i>lon1</i> fixed, writing <i>lat1</i> = &plusmn;(90 &minus; &epsilon;),
   * taking the limit &epsilon; &rarr; 0+.
   **********************************************************************/
  public GeodesicLine Line(double lat1, double lon1, double azi1) {
    return Line(lat1, lon1, azi1, GeodesicMask.ALL);
  }
  /**
   * Set up to compute several points on a single geodesic with a subset of the
   * capabilities included.
   * <p>
   * @param lat1 latitude of point 1 (degrees).
   * @param lon1 longitude of point 1 (degrees).
   * @param azi1 azimuth at point 1 (degrees).
   * @param caps bitor'ed combination of {@link GeodesicMask} values specifying
   *   the capabilities the {@link GeodesicLine} object should possess, i.e.,
   *   which quantities can be returned in calls to {@link
   *   GeodesicLine#Position GeodesicLine.Position}.
   * @return a {@link GeodesicLine} object.
   * <p>
   * The {@link GeodesicMask} values are
   * <ul>
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#LATITUDE} for the latitude
   *   <i>lat2</i>; this is added automatically;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#LONGITUDE} for the latitude
   *   <i>lon2</i>;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#AZIMUTH} for the azimuth <i>azi2</i>;
   *   this is added automatically;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#DISTANCE} for the distance
   *   <i>s12</i>;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#REDUCEDLENGTH} for the reduced length
   *   <i>m12</i>;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#GEODESICSCALE} for the geodesic
   *   scales <i>M12</i> and <i>M21</i>;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#AREA} for the area <i>S12</i>;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#DISTANCE_IN} permits the length of
   *   the geodesic to be given in terms of <i>s12</i>; without this capability
   *   the length can only be specified in terms of arc length;
   * <li>
   *   <i>caps</i> |= {@link GeodesicMask#ALL} for all of the above.
   * </ul>
   * <p>
   * If the point is at a pole, the azimuth is defined by keeping <i>lon1</i>
   * fixed, writing <i>lat1</i> = &plusmn;(90 &minus; &epsilon;), and taking
   * the limit &epsilon; &rarr; 0+.
   **********************************************************************/
  public GeodesicLine Line(double lat1, double lon1, double azi1, int caps) {
    return new GeodesicLine(this, lat1, lon1, azi1, caps);
  }

  /**
   * @return <i>a</i> the equatorial radius of the ellipsoid (meters).  This is
   *   the value used in the constructor.
   **********************************************************************/
  public double EquatorialRadius() { return _a; }

  /**
   * @return <i>f</i> the  flattening of the ellipsoid.  This is the
   *   value used in the constructor.
   **********************************************************************/
  public double Flattening() { return _f; }

  /**
   * @return total area of ellipsoid in meters<sup>2</sup>.  The area of a
   *   polygon encircling a pole can be found by adding EllipsoidArea()/2 to
   *   the sum of <i>S12</i> for each side of the polygon.
   **********************************************************************/
  public double EllipsoidArea() { return 4 * Math.PI * _c2; }

  /**
   * @deprecated An old name for {@link #EquatorialRadius()}.
   * @return <i>a</i> the equatorial radius of the ellipsoid (meters).
   **********************************************************************/
  // @Deprecated
  public double MajorRadius() { return EquatorialRadius(); }

  /**
   * A global instantiation of Geodesic with the parameters for the WGS84
   * ellipsoid.
   **********************************************************************/
  public static final Geodesic WGS84 =
    new Geodesic(Constants.WGS84_a, Constants.WGS84_f);

  // This is a reformulation of the geodesic problem.  The notation is as
  // follows:
  // - at a general point (no suffix or 1 or 2 as suffix)
  //   - phi = latitude
  //   - beta = latitude on auxiliary sphere
  //   - omega = longitude on auxiliary sphere
  //   - lambda = longitude
  //   - alpha = azimuth of great circle
  //   - sigma = arc length along great circle
  //   - s = distance
  //   - tau = scaled distance (= sigma at multiples of pi/2)
  // - at northwards equator crossing
  //   - beta = phi = 0
  //   - omega = lambda = 0
  //   - alpha = alpha0
  //   - sigma = s = 0
  // - a 12 suffix means a difference, e.g., s12 = s2 - s1.
  // - s and c prefixes mean sin and cos

  protected static double SinCosSeries(boolean sinp,
                                       double sinx, double cosx,
                                       double c[]) {
    // Evaluate
    // y = sinp ? sum(c[i] * sin( 2*i    * x), i, 1, n) :
    //            sum(c[i] * cos((2*i+1) * x), i, 0, n-1)
    // using Clenshaw summation.  N.B. c[0] is unused for sin series
    // Approx operation count = (n + 5) mult and (2 * n + 2) add
    int
      k = c.length,             // Point to one beyond last element
      n = k - (sinp ? 1 : 0);
    double
      ar = 2 * (cosx - sinx) * (cosx + sinx), // 2 * cos(2 * x)
      y0 = (n & 1) != 0 ? c[--k] : 0, y1 = 0;        // accumulators for sum
    // Now n is even
    n /= 2;
    while (n-- != 0) {
      // Unroll loop x 2, so accumulators return to their original role
      y1 = ar * y0 - y1 + c[--k];
      y0 = ar * y1 - y0 + c[--k];
    }
    return sinp
      ? 2 * sinx * cosx * y0    // sin(2 * x) * y0
      : cosx * (y0 - y1);       // cos(x) * (y0 - y1)
  }

  private class LengthsV {
    private double s12b, m12b, m0, M12, M21;
    private LengthsV() {
      s12b = m12b = m0 = M12 = M21 = Double.NaN;
    }
  }

  private LengthsV Lengths(double eps, double sig12,
                           double ssig1, double csig1, double dn1,
                           double ssig2, double csig2, double dn2,
                           double cbet1, double cbet2,
                           int outmask,
                           // Scratch areas of the right size
                           double C1a[], double C2a[]) {
    // Return m12b = (reduced length)/_b; also calculate s12b = distance/_b,
    // and m0 = coefficient of secular term in expression for reduced length.
    outmask &= GeodesicMask.OUT_MASK;
    LengthsV v = new LengthsV(); // To hold s12b, m12b, m0, M12, M21;

    double m0x = 0, J12 = 0, A1 = 0, A2 = 0;
    if ((outmask & (GeodesicMask.DISTANCE | GeodesicMask.REDUCEDLENGTH |
                    GeodesicMask.GEODESICSCALE)) != 0) {
      A1 = A1m1f(eps);
      C1f(eps, C1a);
      if ((outmask & (GeodesicMask.REDUCEDLENGTH |
                      GeodesicMask.GEODESICSCALE)) != 0) {
        A2 = A2m1f(eps);
        C2f(eps, C2a);
        m0x = A1 - A2;
        A2 = 1 + A2;
      }
      A1 = 1 + A1;
    }
    if ((outmask & GeodesicMask.DISTANCE) != 0) {
      double B1 = SinCosSeries(true, ssig2, csig2, C1a) -
        SinCosSeries(true, ssig1, csig1, C1a);
      // Missing a factor of _b
      v.s12b = A1 * (sig12 + B1);
      if ((outmask & (GeodesicMask.REDUCEDLENGTH |
                      GeodesicMask.GEODESICSCALE)) != 0) {
        double B2 = SinCosSeries(true, ssig2, csig2, C2a) -
          SinCosSeries(true, ssig1, csig1, C2a);
        J12 = m0x * sig12 + (A1 * B1 - A2 * B2);
      }
    } else if ((outmask & (GeodesicMask.REDUCEDLENGTH |
                           GeodesicMask.GEODESICSCALE)) != 0) {
      // Assume here that nC1_ >= nC2_
      for (int l = 1; l <= nC2_; ++l)
        C2a[l] = A1 * C1a[l] - A2 * C2a[l];
      J12 = m0x * sig12 + (SinCosSeries(true, ssig2, csig2, C2a) -
                           SinCosSeries(true, ssig1, csig1, C2a));
    }
    if ((outmask & GeodesicMask.REDUCEDLENGTH) != 0) {
      v.m0 = m0x;
      // Missing a factor of _b.
      // Add parens around (csig1 * ssig2) and (ssig1 * csig2) to ensure
      // accurate cancellation in the case of coincident points.
      v.m12b = dn2 * (csig1 * ssig2) - dn1 * (ssig1 * csig2) -
        csig1 * csig2 * J12;
    }
    if ((outmask & GeodesicMask.GEODESICSCALE) != 0) {
      double csig12 = csig1 * csig2 + ssig1 * ssig2;
      double t = _ep2 * (cbet1 - cbet2) * (cbet1 + cbet2) / (dn1 + dn2);
      v.M12 = csig12 + (t * ssig2 - csig2 * J12) * ssig1 / dn1;
      v.M21 = csig12 - (t * ssig1 - csig1 * J12) * ssig2 / dn2;
    }
    return v;
  }

  private static double Astroid(double x, double y) {
    // Solve k^4+2*k^3-(x^2+y^2-1)*k^2-2*y^2*k-y^2 = 0 for positive root k.
    // This solution is adapted from Geocentric::Reverse.
    double k;
    double
      p = GeoMath.sq(x),
      q = GeoMath.sq(y),
      r = (p + q - 1) / 6;
    if ( !(q == 0 && r <= 0) ) {
      double
        // Avoid possible division by zero when r = 0 by multiplying equations
        // for s and t by r^3 and r, resp.
        S = p * q / 4,            // S = r^3 * s
        r2 = GeoMath.sq(r),
        r3 = r * r2,
        // The discriminant of the quadratic equation for T3.  This is zero on
        // the evolute curve p^(1/3)+q^(1/3) = 1
        disc = S * (S + 2 * r3);
      double u = r;
      if (disc >= 0) {
        double T3 = S + r3;
        // Pick the sign on the sqrt to maximize abs(T3).  This minimizes loss
        // of precision due to cancellation.  The result is unchanged because
        // of the way the T is used in definition of u.
        T3 += T3 < 0 ? -Math.sqrt(disc) : Math.sqrt(disc); // T3 = (r * t)^3
        // N.B. cbrt always returns the double root.  cbrt(-8) = -2.
        double T = Math.cbrt(T3); // T = r * t
        // T can be zero; but then r2 / T -> 0.
        u += T + (T != 0 ? r2 / T : 0);
      } else {
        // T is complex, but the way u is defined the result is double.
        double ang = Math.atan2(Math.sqrt(-disc), -(S + r3));
        // There are three possible cube roots.  We choose the root which
        // avoids cancellation.  Note that disc < 0 implies that r < 0.
        u += 2 * r * Math.cos(ang / 3);
      }
      double
        v = Math.sqrt(GeoMath.sq(u) + q),    // guaranteed positive
        // Avoid loss of accuracy when u < 0.
        uv = u < 0 ? q / (v - u) : u + v, // u+v, guaranteed positive
        w = (uv - q) / (2 * v);           // positive?
      // Rearrange expression for k to avoid loss of accuracy due to
      // subtraction.  Division by 0 not possible because uv > 0, w >= 0.
      k = uv / (Math.sqrt(uv + GeoMath.sq(w)) + w);   // guaranteed positive
    } else {               // q == 0 && r <= 0
      // y = 0 with |x| <= 1.  Handle this case directly.
      // for y small, positive root is k = abs(y)/sqrt(1-x^2)
      k = 0;
    }
    return k;
  }

  private class InverseStartV {
    private double sig12, salp1, calp1,
    // Only updated if return val >= 0
      salp2, calp2,
    // Only updated for short lines
      dnm;
    private InverseStartV() {
      sig12 = salp1 = calp1 = salp2 = calp2 = dnm = Double.NaN;
    }
  }

  private InverseStartV InverseStart(double sbet1, double cbet1, double dn1,
                                     double sbet2, double cbet2, double dn2,
                                     double lam12,
                                     double slam12, double clam12,
                                     // Scratch areas of the right size
                                     double C1a[], double C2a[]) {
    // Return a starting point for Newton's method in salp1 and calp1 (function
    // value is -1).  If Newton's method doesn't need to be used, return also
    // salp2 and calp2 and function value is sig12.

    // To hold sig12, salp1, calp1, salp2, calp2, dnm.
    InverseStartV w = new InverseStartV();
    w.sig12 = -1;               // Return value
    double
      // bet12 = bet2 - bet1 in [0, pi); bet12a = bet2 + bet1 in (-pi, 0]
      sbet12 = sbet2 * cbet1 - cbet2 * sbet1,
      cbet12 = cbet2 * cbet1 + sbet2 * sbet1;
    double sbet12a = sbet2 * cbet1 + cbet2 * sbet1;
    boolean shortline = cbet12 >= 0 && sbet12 < 0.5 &&
      cbet2 * lam12 < 0.5;
    double somg12, comg12;
    if (shortline) {
      double sbetm2 = GeoMath.sq(sbet1 + sbet2);
      // sin((bet1+bet2)/2)^2
      // =  (sbet1 + sbet2)^2 / ((sbet1 + sbet2)^2 + (cbet1 + cbet2)^2)
      sbetm2 /= sbetm2 + GeoMath.sq(cbet1 + cbet2);
      w.dnm = Math.sqrt(1 + _ep2 * sbetm2);
      double omg12 = lam12 / (_f1 * w.dnm);
      somg12 = Math.sin(omg12); comg12 = Math.cos(omg12);
    } else {
      somg12 = slam12; comg12 = clam12;
    }

    w.salp1 = cbet2 * somg12;
    w.calp1 = comg12 >= 0 ?
      sbet12 + cbet2 * sbet1 * GeoMath.sq(somg12) / (1 + comg12) :
      sbet12a - cbet2 * sbet1 * GeoMath.sq(somg12) / (1 - comg12);

    double
      ssig12 = Math.hypot(w.salp1, w.calp1),
      csig12 = sbet1 * sbet2 + cbet1 * cbet2 * comg12;

    if (shortline && ssig12 < _etol2) {
      // really short lines
      w.salp2 = cbet1 * somg12;
      w.calp2 = sbet12 - cbet1 * sbet2 *
        (comg12 >= 0 ? GeoMath.sq(somg12) / (1 + comg12) : 1 - comg12);
      { Pair p = GeoMath.norm(w.salp2, w.calp2);
        w.salp2 = p.first; w.calp2 = p.second; }
      // Set return value
      w.sig12 = Math.atan2(ssig12, csig12);
    } else if (Math.abs(_n) > 0.1 || // Skip astroid calc if too eccentric
               csig12 >= 0 ||
               ssig12 >= 6 * Math.abs(_n) * Math.PI * GeoMath.sq(cbet1)) {
      // Nothing to do, zeroth order spherical approximation is OK
    } else {
      // Scale lam12 and bet2 to x, y coordinate system where antipodal point
      // is at origin and singular point is at y = 0, x = -1.
      double y, lamscale, betscale;
      // In C++ volatile declaration needed to fix inverse case
      // 56.320923501171 0 -56.320923501171 179.664747671772880215
      // which otherwise fails with g++ 4.4.4 x86 -O3
      double x;
      double lam12x = Math.atan2(-slam12, -clam12); // lam12 - pi
      if (_f >= 0) {            // In fact f == 0 does not get here
        // x = dlong, y = dlat
        {
          double
            k2 = GeoMath.sq(sbet1) * _ep2,
            eps = k2 / (2 * (1 + Math.sqrt(1 + k2)) + k2);
          lamscale = _f * cbet1 * A3f(eps) * Math.PI;
        }
        betscale = lamscale * cbet1;

        x = lam12x / lamscale;
        y = sbet12a / betscale;
      } else {                  // _f < 0
        // x = dlat, y = dlong
        double
          cbet12a = cbet2 * cbet1 - sbet2 * sbet1,
          bet12a = Math.atan2(sbet12a, cbet12a);
        double m12b, m0;
        // In the case of lon12 = 180, this repeats a calculation made in
        // Inverse.
        LengthsV v =
          Lengths(_n, Math.PI + bet12a,
                  sbet1, -cbet1, dn1, sbet2, cbet2, dn2,
                  cbet1, cbet2, GeodesicMask.REDUCEDLENGTH, C1a, C2a);
        m12b = v.m12b; m0 = v.m0;

        x = -1 + m12b / (cbet1 * cbet2 * m0 * Math.PI);
        betscale = x < -0.01 ? sbet12a / x :
          -_f * GeoMath.sq(cbet1) * Math.PI;
        lamscale = betscale / cbet1;
        y = lam12x / lamscale;
      }

      if (y > -tol1_ && x > -1 - xthresh_) {
        // strip near cut
        if (_f >= 0) {
          w.salp1 = Math.min(1.0, -x);
          w.calp1 = - Math.sqrt(1 - GeoMath.sq(w.salp1));
        } else {
          w.calp1 = Math.max(x > -tol1_ ? 0.0 : -1.0, x);
          w.salp1 = Math.sqrt(1 - GeoMath.sq(w.calp1));
        }
      } else {
        // Estimate alp1, by solving the astroid problem.
        //
        // Could estimate alpha1 = theta + pi/2, directly, i.e.,
        //   calp1 = y/k; salp1 = -x/(1+k);  for _f >= 0
        //   calp1 = x/(1+k); salp1 = -y/k;  for _f < 0 (need to check)
        //
        // However, it's better to estimate omg12 from astroid and use
        // spherical formula to compute alp1.  This reduces the mean number of
        // Newton iterations for astroid cases from 2.24 (min 0, max 6) to 2.12
        // (min 0 max 5).  The changes in the number of iterations are as
        // follows:
        //
        // change percent
        //    1       5
        //    0      78
        //   -1      16
        //   -2       0.6
        //   -3       0.04
        //   -4       0.002
        //
        // The histogram of iterations is (m = number of iterations estimating
        // alp1 directly, n = number of iterations estimating via omg12, total
        // number of trials = 148605):
        //
        //  iter    m      n
        //    0   148    186
        //    1 13046  13845
        //    2 93315 102225
        //    3 36189  32341
        //    4  5396      7
        //    5   455      1
        //    6    56      0
        //
        // Because omg12 is near pi, estimate work with omg12a = pi - omg12
        double k = Astroid(x, y);
        double
          omg12a = lamscale * ( _f >= 0 ? -x * k/(1 + k) : -y * (1 + k)/k );
        somg12 = Math.sin(omg12a); comg12 = -Math.cos(omg12a);
        // Update spherical estimate of alp1 using omg12 instead of lam12
        w.salp1 = cbet2 * somg12;
        w.calp1 = sbet12a - cbet2 * sbet1 * GeoMath.sq(somg12) / (1 - comg12);
      }
    }
    // Sanity check on starting guess.  Backwards check allows NaN through.
    if (!(w.salp1 <= 0))
      { Pair p = GeoMath.norm(w.salp1, w.calp1);
        w.salp1 = p.first; w.calp1 = p.second; }
    else {
      w.salp1 = 1; w.calp1 = 0;
    }
    return w;
  }

  private class Lambda12V {
    private double lam12, salp2, calp2, sig12, ssig1, csig1, ssig2, csig2,
      eps, domg12, dlam12;
    private Lambda12V() {
      lam12 = salp2 = calp2 = sig12 = ssig1 = csig1 = ssig2 = csig2
        = eps = domg12 = dlam12 = Double.NaN;
    }
  }

  private Lambda12V Lambda12(double sbet1, double cbet1, double dn1,
                             double sbet2, double cbet2, double dn2,
                             double salp1, double calp1,
                             double slam120, double clam120,
                             boolean diffp,
                             // Scratch areas of the right size
                             double C1a[], double C2a[], double C3a[]) {
    // Object to hold lam12, salp2, calp2, sig12, ssig1, csig1, ssig2, csig2,
    // eps, domg12, dlam12;

    Lambda12V w = new Lambda12V();

    if (sbet1 == 0 && calp1 == 0)
      // Break degeneracy of equatorial line.  This case has already been
      // handled.
      calp1 = -tiny_;

    double
      // sin(alp1) * cos(bet1) = sin(alp0)
      salp0 = salp1 * cbet1,
      calp0 = Math.hypot(calp1, salp1 * sbet1); // calp0 > 0

    double somg1, comg1, somg2, comg2, somg12, comg12;
    // tan(bet1) = tan(sig1) * cos(alp1)
    // tan(omg1) = sin(alp0) * tan(sig1) = tan(omg1)=tan(alp1)*sin(bet1)
    w.ssig1 = sbet1; somg1 = salp0 * sbet1;
    w.csig1 = comg1 = calp1 * cbet1;
    { Pair p = GeoMath.norm(w.ssig1, w.csig1);
      w.ssig1 = p.first; w.csig1 = p.second; }
    // GeoMath.norm(somg1, comg1); -- don't need to normalize!

    // Enforce symmetries in the case abs(bet2) = -bet1.  Need to be careful
    // about this case, since this can yield singularities in the Newton
    // iteration.
    // sin(alp2) * cos(bet2) = sin(alp0)
    w.salp2 = cbet2 != cbet1 ? salp0 / cbet2 : salp1;
    // calp2 = sqrt(1 - sq(salp2))
    //       = sqrt(sq(calp0) - sq(sbet2)) / cbet2
    // and subst for calp0 and rearrange to give (choose positive sqrt
    // to give alp2 in [0, pi/2]).
    w.calp2 = cbet2 != cbet1 || Math.abs(sbet2) != -sbet1 ?
      Math.sqrt(GeoMath.sq(calp1 * cbet1) +
           (cbet1 < -sbet1 ?
            (cbet2 - cbet1) * (cbet1 + cbet2) :
            (sbet1 - sbet2) * (sbet1 + sbet2))) / cbet2 :
      Math.abs(calp1);
    // tan(bet2) = tan(sig2) * cos(alp2)
    // tan(omg2) = sin(alp0) * tan(sig2).
    w.ssig2 = sbet2; somg2 = salp0 * sbet2;
    w.csig2 = comg2 = w.calp2 * cbet2;
    { Pair p = GeoMath.norm(w.ssig2, w.csig2);
      w.ssig2 = p.first; w.csig2 = p.second; }
    // GeoMath.norm(somg2, comg2); -- don't need to normalize!

    // sig12 = sig2 - sig1, limit to [0, pi]
    w.sig12 = Math.atan2(Math.max(0.0, w.csig1 * w.ssig2 - w.ssig1 * w.csig2),
                                       w.csig1 * w.csig2 + w.ssig1 * w.ssig2);

    // omg12 = omg2 - omg1, limit to [0, pi]
    somg12 = Math.max(0.0, comg1 * somg2 - somg1 * comg2);
    comg12 =               comg1 * comg2 + somg1 * somg2;
    // eta = omg12 - lam120
    double eta = Math.atan2(somg12 * clam120 - comg12 * slam120,
                            comg12 * clam120 + somg12 * slam120);
    double B312;
    double k2 = GeoMath.sq(calp0) * _ep2;
    w.eps = k2 / (2 * (1 + Math.sqrt(1 + k2)) + k2);
    C3f(w.eps, C3a);
    B312 = (SinCosSeries(true, w.ssig2, w.csig2, C3a) -
            SinCosSeries(true, w.ssig1, w.csig1, C3a));
    w.domg12 = -_f * A3f(w.eps) * salp0 * (w.sig12 + B312);
    w.lam12 = eta + w.domg12;

    if (diffp) {
      if (w.calp2 == 0)
        w.dlam12 = - 2 * _f1 * dn1 / sbet1;
      else {
        LengthsV v =
          Lengths(w.eps, w.sig12, w.ssig1, w.csig1, dn1, w.ssig2, w.csig2, dn2,
                  cbet1, cbet2, GeodesicMask.REDUCEDLENGTH, C1a, C2a);
        w.dlam12 = v.m12b;
        w.dlam12 *= _f1 / (w.calp2 * cbet2);
      }
    }

    return w;
  }

  protected double A3f(double eps) {
    // Evaluate A3
    return GeoMath.polyval(nA3_ - 1, _A3x, 0, eps);
  }

  protected void C3f(double eps, double c[]) {
    // Evaluate C3 coeffs
    // Elements c[1] thru c[nC3_ - 1] are set
    double mult = 1;
    int o = 0;
    for (int l = 1; l < nC3_; ++l) { // l is index of C3[l]
      int m = nC3_ - l - 1;          // order of polynomial in eps
      mult *= eps;
      c[l] = mult * GeoMath.polyval(m, _C3x, o, eps);
      o += m + 1;
    }
  }

  protected void C4f(double eps, double c[]) {
    // Evaluate C4 coeffs
    // Elements c[0] thru c[nC4_ - 1] are set
    double mult = 1;
    int o = 0;
    for (int l = 0; l < nC4_; ++l) { // l is index of C4[l]
      int m = nC4_ - l - 1;          // order of polynomial in eps
      c[l] = mult * GeoMath.polyval(m, _C4x, o, eps);
      o += m + 1;
      mult *= eps;
    }
  }

  // The scale factor A1-1 = mean value of (d/dsigma)I1 - 1
  protected static double A1m1f(double eps) {
    final double coeff[] = {
      // (1-eps)*A1-1, polynomial in eps2 of order 3
      1, 4, 64, 0, 256,
    };
    int m = nA1_/2;
    double t = GeoMath.polyval(m, coeff, 0, GeoMath.sq(eps)) / coeff[m + 1];
    return (t + eps) / (1 - eps);
  }

  // The coefficients C1[l] in the Fourier expansion of B1
  protected static void C1f(double eps, double c[]) {
    final double coeff[] = {
      // C1[1]/eps^1, polynomial in eps2 of order 2
      -1, 6, -16, 32,
      // C1[2]/eps^2, polynomial in eps2 of order 2
      -9, 64, -128, 2048,
      // C1[3]/eps^3, polynomial in eps2 of order 1
      9, -16, 768,
      // C1[4]/eps^4, polynomial in eps2 of order 1
      3, -5, 512,
      // C1[5]/eps^5, polynomial in eps2 of order 0
      -7, 1280,
      // C1[6]/eps^6, polynomial in eps2 of order 0
      -7, 2048,
    };
    double
      eps2 = GeoMath.sq(eps),
      d = eps;
    int o = 0;
    for (int l = 1; l <= nC1_; ++l) { // l is index of C1p[l]
      int m = (nC1_ - l) / 2;         // order of polynomial in eps^2
      c[l] = d * GeoMath.polyval(m, coeff, o, eps2) / coeff[o + m + 1];
      o += m + 2;
      d *= eps;
    }
  }

  // The coefficients C1p[l] in the Fourier expansion of B1p
  protected static void C1pf(double eps, double c[]) {
    final double coeff[] = {
      // C1p[1]/eps^1, polynomial in eps2 of order 2
      205, -432, 768, 1536,
      // C1p[2]/eps^2, polynomial in eps2 of order 2
      4005, -4736, 3840, 12288,
      // C1p[3]/eps^3, polynomial in eps2 of order 1
      -225, 116, 384,
      // C1p[4]/eps^4, polynomial in eps2 of order 1
      -7173, 2695, 7680,
      // C1p[5]/eps^5, polynomial in eps2 of order 0
      3467, 7680,
      // C1p[6]/eps^6, polynomial in eps2 of order 0
      38081, 61440,
    };
    double
      eps2 = GeoMath.sq(eps),
      d = eps;
    int o = 0;
    for (int l = 1; l <= nC1p_; ++l) { // l is index of C1p[l]
      int m = (nC1p_ - l) / 2;         // order of polynomial in eps^2
      c[l] = d * GeoMath.polyval(m, coeff, o, eps2) / coeff[o + m + 1];
      o += m + 2;
      d *= eps;
    }
  }

  // The scale factor A2-1 = mean value of (d/dsigma)I2 - 1
  protected static double A2m1f(double eps) {
    final double coeff[] = {
      // (eps+1)*A2-1, polynomial in eps2 of order 3
      -11, -28, -192, 0, 256,
    };
    int m = nA2_/2;
    double t = GeoMath.polyval(m, coeff, 0, GeoMath.sq(eps)) / coeff[m + 1];
    return (t - eps) / (1 + eps);
  }

  // The coefficients C2[l] in the Fourier expansion of B2
  protected static void C2f(double eps, double c[]) {
    final double coeff[] = {
      // C2[1]/eps^1, polynomial in eps2 of order 2
      1, 2, 16, 32,
      // C2[2]/eps^2, polynomial in eps2 of order 2
      35, 64, 384, 2048,
      // C2[3]/eps^3, polynomial in eps2 of order 1
      15, 80, 768,
      // C2[4]/eps^4, polynomial in eps2 of order 1
      7, 35, 512,
      // C2[5]/eps^5, polynomial in eps2 of order 0
      63, 1280,
      // C2[6]/eps^6, polynomial in eps2 of order 0
      77, 2048,
    };
    double
      eps2 = GeoMath.sq(eps),
      d = eps;
    int o = 0;
    for (int l = 1; l <= nC2_; ++l) { // l is index of C2[l]
      int m = (nC2_ - l) / 2;         // order of polynomial in eps^2
      c[l] = d * GeoMath.polyval(m, coeff, o, eps2) / coeff[o + m + 1];
      o += m + 2;
      d *= eps;
    }
  }

  // The scale factor A3 = mean value of (d/dsigma)I3
  protected void A3coeff() {
    final double coeff[] = {
      // A3, coeff of eps^5, polynomial in n of order 0
      -3, 128,
      // A3, coeff of eps^4, polynomial in n of order 1
      -2, -3, 64,
      // A3, coeff of eps^3, polynomial in n of order 2
      -1, -3, -1, 16,
      // A3, coeff of eps^2, polynomial in n of order 2
      3, -1, -2, 8,
      // A3, coeff of eps^1, polynomial in n of order 1
      1, -1, 2,
      // A3, coeff of eps^0, polynomial in n of order 0
      1, 1,
    };
    int o = 0, k = 0;
    for (int j = nA3_ - 1; j >= 0; --j) { // coeff of eps^j
      int m = Math.min(nA3_ - j - 1, j);  // order of polynomial in n
      _A3x[k++] = GeoMath.polyval(m, coeff, o, _n) / coeff[o + m + 1];
      o += m + 2;
    }
  }

  // The coefficients C3[l] in the Fourier expansion of B3
  protected void C3coeff() {
    final double coeff[] = {
      // C3[1], coeff of eps^5, polynomial in n of order 0
      3, 128,
      // C3[1], coeff of eps^4, polynomial in n of order 1
      2, 5, 128,
      // C3[1], coeff of eps^3, polynomial in n of order 2
      -1, 3, 3, 64,
      // C3[1], coeff of eps^2, polynomial in n of order 2
      -1, 0, 1, 8,
      // C3[1], coeff of eps^1, polynomial in n of order 1
      -1, 1, 4,
      // C3[2], coeff of eps^5, polynomial in n of order 0
      5, 256,
      // C3[2], coeff of eps^4, polynomial in n of order 1
      1, 3, 128,
      // C3[2], coeff of eps^3, polynomial in n of order 2
      -3, -2, 3, 64,
      // C3[2], coeff of eps^2, polynomial in n of order 2
      1, -3, 2, 32,
      // C3[3], coeff of eps^5, polynomial in n of order 0
      7, 512,
      // C3[3], coeff of eps^4, polynomial in n of order 1
      -10, 9, 384,
      // C3[3], coeff of eps^3, polynomial in n of order 2
      5, -9, 5, 192,
      // C3[4], coeff of eps^5, polynomial in n of order 0
      7, 512,
      // C3[4], coeff of eps^4, polynomial in n of order 1
      -14, 7, 512,
      // C3[5], coeff of eps^5, polynomial in n of order 0
      21, 2560,
    };
    int o = 0, k = 0;
    for (int l = 1; l < nC3_; ++l) {        // l is index of C3[l]
      for (int j = nC3_ - 1; j >= l; --j) { // coeff of eps^j
        int m = Math.min(nC3_ - j - 1, j);  // order of polynomial in n
        _C3x[k++] = GeoMath.polyval(m, coeff, o, _n) / coeff[o + m + 1];
        o += m + 2;
      }
    }
  }

  protected void C4coeff() {
    final double coeff[] = {
      // C4[0], coeff of eps^5, polynomial in n of order 0
      97, 15015,
      // C4[0], coeff of eps^4, polynomial in n of order 1
      1088, 156, 45045,
      // C4[0], coeff of eps^3, polynomial in n of order 2
      -224, -4784, 1573, 45045,
      // C4[0], coeff of eps^2, polynomial in n of order 3
      -10656, 14144, -4576, -858, 45045,
      // C4[0], coeff of eps^1, polynomial in n of order 4
      64, 624, -4576, 6864, -3003, 15015,
      // C4[0], coeff of eps^0, polynomial in n of order 5
      100, 208, 572, 3432, -12012, 30030, 45045,
      // C4[1], coeff of eps^5, polynomial in n of order 0
      1, 9009,
      // C4[1], coeff of eps^4, polynomial in n of order 1
      -2944, 468, 135135,
      // C4[1], coeff of eps^3, polynomial in n of order 2
      5792, 1040, -1287, 135135,
      // C4[1], coeff of eps^2, polynomial in n of order 3
      5952, -11648, 9152, -2574, 135135,
      // C4[1], coeff of eps^1, polynomial in n of order 4
      -64, -624, 4576, -6864, 3003, 135135,
      // C4[2], coeff of eps^5, polynomial in n of order 0
      8, 10725,
      // C4[2], coeff of eps^4, polynomial in n of order 1
      1856, -936, 225225,
      // C4[2], coeff of eps^3, polynomial in n of order 2
      -8448, 4992, -1144, 225225,
      // C4[2], coeff of eps^2, polynomial in n of order 3
      -1440, 4160, -4576, 1716, 225225,
      // C4[3], coeff of eps^5, polynomial in n of order 0
      -136, 63063,
      // C4[3], coeff of eps^4, polynomial in n of order 1
      1024, -208, 105105,
      // C4[3], coeff of eps^3, polynomial in n of order 2
      3584, -3328, 1144, 315315,
      // C4[4], coeff of eps^5, polynomial in n of order 0
      -128, 135135,
      // C4[4], coeff of eps^4, polynomial in n of order 1
      -2560, 832, 405405,
      // C4[5], coeff of eps^5, polynomial in n of order 0
      128, 99099,
    };
    int o = 0, k = 0;
    for (int l = 0; l < nC4_; ++l) {        // l is index of C4[l]
      for (int j = nC4_ - 1; j >= l; --j) { // coeff of eps^j
        int m = nC4_ - j - 1;               // order of polynomial in n
        _C4x[k++] = GeoMath.polyval(m, coeff, o, _n) / coeff[o + m + 1];
        o += m + 2;
      }
    }
  }
}
