ginzburg8.js

import {geoProjection as projection} from "d3-geo";
import {abs, epsilon} from "./math.js";

export function ginzburg8Raw(lambda, phi) {
  var lambda2 = lambda * lambda,
      phi2 = phi * phi;
  return [
    lambda * (1 - 0.162388 * phi2) * (0.87 - 0.000952426 * lambda2 * lambda2),
    phi * (1 + phi2 / 12)
  ];
}

ginzburg8Raw.invert = function(x, y) {
  var lambda = x,
      phi = y,
      i = 50, delta;
  do {
    var phi2 = phi * phi;
    phi -= delta = (phi * (1 + phi2 / 12) - y) / (1 + phi2 / 4);
  } while (abs(delta) > epsilon && --i > 0);
  i = 50;
  x /= 1 -0.162388 * phi2;
  do {
    var lambda4 = (lambda4 = lambda * lambda) * lambda4;
    lambda -= delta = (lambda * (0.87 - 0.000952426 * lambda4) - x) / (0.87 - 0.00476213 * lambda4);
  } while (abs(delta) > epsilon && --i > 0);
  return [lambda, phi];
};

export default function() {
  return projection(ginzburg8Raw)
      .scale(131.747);
}