src/geo/conic-equal-area.js

import "../math/trigonometry";
import "geo";
import "conic";
import "projection";

function d3_geo_conicEqualArea(φ0, φ1) {
  var sinφ0 = Math.sin(φ0),
      n = (sinφ0 + Math.sin(φ1)) / 2,
      C = 1 + sinφ0 * (2 * n - sinφ0),
      ρ0 = Math.sqrt(C) / n;

  function forward(λ, φ) {
    var ρ = Math.sqrt(C - 2 * n * Math.sin(φ)) / n;
    return [
      ρ * Math.sin(λ *= n),
      ρ0 - ρ * Math.cos(λ)
    ];
  }

  forward.invert = function(x, y) {
    var ρ0_y = ρ0 - y;
    return [
      Math.atan2(x, ρ0_y) / n,
      d3_asin((C - (x * x + ρ0_y * ρ0_y) * n * n) / (2 * n))
    ];
  };

  return forward;
}

(d3.geo.conicEqualArea = function() {
  return d3_geo_conic(d3_geo_conicEqualArea);
}).raw = d3_geo_conicEqualArea;