// TODO breakAtDateLine?

d3.geo.circle = function() {

  var origin = [0, 0],

      degrees = 90 - 1e-2,

      radians = degrees * d3_geo_radians,

      arc = d3.geo.greatArc().target(Object);

  function circle() {

    // TODO render a circle as a Polygon

  }

  function visible(point) {

    return arc.distance(point) < radians;

  }

  circle.clip = function(d) {

    arc.source(typeof origin === "function" ? origin.apply(this, arguments) : origin);

    return clipType(d);

  };

  var clipType = d3_geo_type({

    FeatureCollection: function(o) {

      var features = o.features.map(clipType).filter(Object);

      return features && (o = Object.create(o), o.features = features, o);

    },

    Feature: function(o) {

      var geometry = clipType(o.geometry);

      return geometry && (o = Object.create(o), o.geometry = geometry, o);

    },

    Point: function(o) {

      return visible(o.coordinates) && o;

    },

    MultiPoint: function(o) {

      var coordinates = o.coordinates.filter(visible);

      return coordinates.length && {

        type: o.type,

        coordinates: coordinates

      };

    },

    LineString: function(o) {

      var coordinates = clip(o.coordinates);

      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);

    },

    MultiLineString: function(o) {

      var coordinates = o.coordinates.map(clip).filter(function(d) { return d.length; });

      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);

    },

    Polygon: function(o) {

      var coordinates = o.coordinates.map(clip);

      return coordinates[0].length && (o = Object.create(o), o.coordinates = coordinates, o);

    },

    MultiPolygon: function(o) {

      var coordinates = o.coordinates.map(function(d) { return d.map(clip); }).filter(function(d) { return d[0].length; });

      return coordinates.length && (o = Object.create(o), o.coordinates = coordinates, o);

    },

    GeometryCollection: function(o) {

      var geometries = o.geometries.map(clipType).filter(Object);

      return geometries.length && (o = Object.create(o), o.geometries = geometries, o);

    }

  });

  function clip(coordinates) {

    var i = -1,

        n = coordinates.length,

        clipped = [],

        p0,

        p1,

        p2,

        d0,

        d1;

    while (++i < n) {

      d1 = arc.distance(p2 = coordinates[i]);

      if (d1 < radians) {

        if (p1) clipped.push(d3_geo_greatArcInterpolate(p1, p2)((d0 - radians) / (d0 - d1)));

        clipped.push(p2);

        p0 = p1 = null;

      } else {

        p1 = p2;

        if (!p0 && clipped.length) {

          clipped.push(d3_geo_greatArcInterpolate(clipped[clipped.length - 1], p1)((radians - d0) / (d1 - d0)));

          p0 = p1;

        }

      }

      d0 = d1;

    }

    if (p1 && clipped.length) {

      d1 = arc.distance(p2 = clipped[0]);

      clipped.push(d3_geo_greatArcInterpolate(p1, p2)((d0 - radians) / (d0 - d1)));

    }

    return resample(clipped);

  }

  // Resample coordinates, creating great arcs between each.

  function resample(coordinates) {

    var i = 0,

        n = coordinates.length,

        j,

        m,

        resampled = n ? [coordinates[0]] : coordinates,

        resamples,

        origin = arc.source();

    while (++i < n) {

      resamples = arc.source(coordinates[i - 1])(coordinates[i]).coordinates;

      for (j = 0, m = resamples.length; ++j < m;) resampled.push(resamples[j]);

    }

    arc.source(origin);

    return resampled;

  }

  circle.origin = function(x) {

    if (!arguments.length) return origin;

    origin = x;

    return circle;

  };

  circle.angle = function(x) {

    if (!arguments.length) return degrees;

    radians = (degrees = +x) * d3_geo_radians;

    return circle;

  };

  // Precision is specified in degrees.

  circle.precision = function(x) {

    if (!arguments.length) return arc.precision();

    arc.precision(x);

    return circle;

  };

  return circle;

}