// 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]);

};