vanDerGrinten4.js

import {geoProjection as projection} from "d3-geo";
import {abs, epsilon, halfPi, min, pi, sign, sqrt} from "./math.js";

export function vanDerGrinten4Raw(lambda, phi) {
  if (!phi) return [lambda, 0];
  var phi0 = abs(phi);
  if (!lambda || phi0 === halfPi) return [0, phi];
  var B = phi0 / halfPi,
      B2 = B * B,
      C = (8 * B - B2 * (B2 + 2) - 5) / (2 * B2 * (B - 1)),
      C2 = C * C,
      BC = B * C,
      B_C2 = B2 + C2 + 2 * BC,
      B_3C = B + 3 * C,
      lambda0 = lambda / halfPi,
      lambda1 = lambda0 + 1 / lambda0,
      D = sign(abs(lambda) - halfPi) * sqrt(lambda1 * lambda1 - 4),
      D2 = D * D,
      F = B_C2 * (B2 + C2 * D2 - 1) + (1 - B2) * (B2 * (B_3C * B_3C + 4 * C2) + 12 * BC * C2 + 4 * C2 * C2),
      x1 = (D * (B_C2 + C2 - 1) + 2 * sqrt(F)) / (4 * B_C2 + D2);
  return [
    sign(lambda) * halfPi * x1,
    sign(phi) * halfPi * sqrt(1 + D * abs(x1) - x1 * x1)
  ];
}

vanDerGrinten4Raw.invert = function(x, y) {
  var delta;
  if (!x || !y) return [x, y];
  var sy = sign(y);
  y = abs(y) / pi;
  var x1 = sign(x) * x / halfPi,
      D = (x1 * x1 - 1 + 4 * y * y) / abs(x1),
      D2 = D * D,
      B = y * (2 - (y > 0.5 ? min(y, abs(x)) : 0)),
      r = x * x + y * y,
      i = 50;
  do {
    var B2 = B * B,
        C = (8 * B - B2 * (B2 + 2) - 5) / (2 * B2 * (B - 1)),
        C_ = (3 * B - B2 * B - 10) / (2 * B2 * B),
        C2 = C * C,
        BC = B * C,
        B_C = B + C,
        B_C2 = B_C * B_C,
        B_3C = B + 3 * C,
        F = B_C2 * (B2 + C2 * D2 - 1) + (1 - B2) * (B2 * (B_3C * B_3C + 4 * C2) + C2 * (12 * BC + 4 * C2)),
        F_ = -2 * B_C * (4 * BC * C2 + (1 - 4 * B2 + 3 * B2 * B2) * (1 + C_) + C2 * (-6 + 14 * B2 - D2 + (-8 + 8 * B2 - 2 * D2) * C_) + BC * (-8 + 12 * B2 + (-10 + 10 * B2 - D2) * C_)),
        sqrtF = sqrt(F),
        f = D * (B_C2 + C2 - 1) + 2 * sqrtF - x1 * (4 * B_C2 + D2),
        f_ = D * (2 * C * C_ + 2 * B_C * (1 + C_)) + F_ / sqrtF - 8 * B_C * (D * (-1 + C2 + B_C2) + 2 * sqrtF) * (1 + C_) / (D2 + 4 * B_C2);
    B -= delta = f / f_;
  } while (delta * r * r > epsilon && --i > 0);
  return [
    sign(x) * (sqrt(D * D + 4) + D) * pi / 4,
    sy * halfPi * B
  ];
};

export default function() {
  return projection(vanDerGrinten4Raw)
      .scale(127.16);
}