// 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;
}