source: node_modules/d3-geo/src/interpolate.js@ e4c61dd

import {asin, atan2, cos, degrees, haversin, radians, sin, sqrt} from "./math.js";

export default function(a, b) {
  var x0 = a[0] * radians,
      y0 = a[1] * radians,
      x1 = b[0] * radians,
      y1 = b[1] * radians,
      cy0 = cos(y0),
      sy0 = sin(y0),
      cy1 = cos(y1),
      sy1 = sin(y1),
      kx0 = cy0 * cos(x0),
      ky0 = cy0 * sin(x0),
      kx1 = cy1 * cos(x1),
      ky1 = cy1 * sin(x1),
      d = 2 * asin(sqrt(haversin(y1 - y0) + cy0 * cy1 * haversin(x1 - x0))),
      k = sin(d);

  var interpolate = d ? function(t) {
    var B = sin(t *= d) / k,
        A = sin(d - t) / k,
        x = A * kx0 + B * kx1,
        y = A * ky0 + B * ky1,
        z = A * sy0 + B * sy1;
    return [
      atan2(y, x) * degrees,
      atan2(z, sqrt(x * x + y * y)) * degrees
    ];
  } : function() {
    return [x0 * degrees, y0 * degrees];
  };

  interpolate.distance = d;

  return interpolate;
}