// TODO clip input coordinates on opposite hemisphere
d3.geo.azimuthal = function() {
  var mode = "orthographic", // or stereographic, gnomonic, equidistant or equalarea
      origin,
      scale = 200,
      translate = [480, 250],
      x0,
      y0,
      cy0,
      sy0;

  function azimuthal(coordinates) {
    var x1 = coordinates[0] * d3_geo_radians - x0,
        y1 = coordinates[1] * d3_geo_radians,
        cx1 = Math.cos(x1),
        sx1 = Math.sin(x1),
        cy1 = Math.cos(y1),
        sy1 = Math.sin(y1),
        cc = mode !== "orthographic" ? sy0 * sy1 + cy0 * cy1 * cx1 : null,
        c,
        k = mode === "stereographic" ? 1 / (1 + cc)
          : mode === "gnomonic" ? 1 / cc
          : mode === "equidistant" ? (c = Math.acos(cc), c ? c / Math.sin(c) : 0)
          : mode === "equalarea" ? Math.sqrt(2 / (1 + cc))
          : 1,
        x = k * cy1 * sx1,
        y = k * (sy0 * cy1 * cx1 - cy0 * sy1);
    return [
      scale * x + translate[0],
      scale * y + translate[1]
    ];
  }

  azimuthal.invert = function(coordinates) {
    var x = (coordinates[0] - translate[0]) / scale,
        y = (coordinates[1] - translate[1]) / scale,
        p = Math.sqrt(x * x + y * y),
        c = mode === "stereographic" ? 2 * Math.atan(p)
          : mode === "gnomonic" ? Math.atan(p)
          : mode === "equidistant" ? p
          : mode === "equalarea" ? 2 * Math.asin(.5 * p)
          : Math.asin(p),
        sc = Math.sin(c),
        cc = Math.cos(c);
    return [
      (x0 + Math.atan2(x * sc, p * cy0 * cc + y * sy0 * sc)) / d3_geo_radians,
      Math.asin(cc * sy0 - (p ? (y * sc * cy0) / p : 0)) / d3_geo_radians
    ];
  };

  azimuthal.mode = function(x) {
    if (!arguments.length) return mode;
    mode = x + "";
    return azimuthal;
  };

  azimuthal.origin = function(x) {
    if (!arguments.length) return origin;
    origin = x;
    x0 = origin[0] * d3_geo_radians;
    y0 = origin[1] * d3_geo_radians;
    cy0 = Math.cos(y0);
    sy0 = Math.sin(y0);
    return azimuthal;
  };

  azimuthal.scale = function(x) {
    if (!arguments.length) return scale;
    scale = +x;
    return azimuthal;
  };

  azimuthal.translate = function(x) {
    if (!arguments.length) return translate;
    translate = [+x[0], +x[1]];
    return azimuthal;
  };

  return azimuthal.origin([0, 0]);
};