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

export function eckert2Raw(lambda, phi) {
  var alpha = sqrt(4 - 3 * sin(abs(phi)));
  return [
    2 / sqrt(6 * pi) * lambda * alpha,
    sign(phi) * sqrt(2 * pi / 3) * (2 - alpha)
  ];
}

eckert2Raw.invert = function(x, y) {
  var alpha = 2 - abs(y) / sqrt(2 * pi / 3);
  return [
    x * sqrt(6 * pi) / (2 * alpha),
    sign(y) * asin((4 - alpha * alpha) / 3)
  ];
};

export default function() {
  return projection(eckert2Raw)
      .scale(165.664);
}