antimeridian.js

import clip from "./index";
import {abs, atan, cos, epsilon, halfPi, pi, sin} from "../math";

export default clip(
  function() { return true; },
  clipAntimeridianLine,
  clipAntimeridianInterpolate,
  [-pi, -halfPi]
);

// Takes a line and cuts into visible segments. Return values: 0 - there were
// intersections or the line was empty; 1 - no intersections; 2 - there were
// intersections, and the first and last segments should be rejoined.
function clipAntimeridianLine(stream) {
  var lambda0 = NaN,
      phi0 = NaN,
      sign0 = NaN,
      clean; // no intersections

  return {
    lineStart: function() {
      stream.lineStart();
      clean = 1;
    },
    point: function(lambda1, phi1) {
      var sign1 = lambda1 > 0 ? pi : -pi,
          delta = abs(lambda1 - lambda0);
      if (abs(delta - pi) < epsilon) { // line crosses a pole
        stream.point(lambda0, phi0 = (phi0 + phi1) / 2 > 0 ? halfPi : -halfPi);
        stream.point(sign0, phi0);
        stream.lineEnd();
        stream.lineStart();
        stream.point(sign1, phi0);
        stream.point(lambda1, phi0);
        clean = 0;
      } else if (sign0 !== sign1 && delta >= pi) { // line crosses antimeridian
        if (abs(lambda0 - sign0) < epsilon) lambda0 -= sign0 * epsilon; // handle degeneracies
        if (abs(lambda1 - sign1) < epsilon) lambda1 -= sign1 * epsilon;
        phi0 = clipAntimeridianIntersect(lambda0, phi0, lambda1, phi1);
        stream.point(sign0, phi0);
        stream.lineEnd();
        stream.lineStart();
        stream.point(sign1, phi0);
        clean = 0;
      }
      stream.point(lambda0 = lambda1, phi0 = phi1);
      sign0 = sign1;
    },
    lineEnd: function() {
      stream.lineEnd();
      lambda0 = phi0 = NaN;
    },
    clean: function() {
      return 2 - clean; // if intersections, rejoin first and last segments
    }
  };
}

function clipAntimeridianIntersect(lambda0, phi0, lambda1, phi1) {
  var cosPhi0,
      cosPhi1,
      sinLambda0Lambda1 = sin(lambda0 - lambda1);
  return abs(sinLambda0Lambda1) > epsilon
      ? atan((sin(phi0) * (cosPhi1 = cos(phi1)) * sin(lambda1)
          - sin(phi1) * (cosPhi0 = cos(phi0)) * sin(lambda0))
          / (cosPhi0 * cosPhi1 * sinLambda0Lambda1))
      : (phi0 + phi1) / 2;
}

function clipAntimeridianInterpolate(from, to, direction, stream) {
  var phi;
  if (from == null) {
    phi = direction * halfPi;
    stream.point(-pi, phi);
    stream.point(0, phi);
    stream.point(pi, phi);
    stream.point(pi, 0);
    stream.point(pi, -phi);
    stream.point(0, -phi);
    stream.point(-pi, -phi);
    stream.point(-pi, 0);
    stream.point(-pi, phi);
  } else if (abs(from[0] - to[0]) > epsilon) {
    var lambda = from[0] < to[0] ? pi : -pi;
    phi = direction * lambda / 2;
    stream.point(-lambda, phi);
    stream.point(0, phi);
    stream.point(lambda, phi);
  } else {
    stream.point(to[0], to[1]);
  }
}