// Note: requires coordinates to be counterclockwise and convex!
d3.geom.polygon = function(coordinates) {

  coordinates.area = function() {
    var i = 0,
        n = coordinates.length,
        a = coordinates[n - 1][0] * coordinates[0][1],
        b = coordinates[n - 1][1] * coordinates[0][0];
    while (++i < n) {
      a += coordinates[i - 1][0] * coordinates[i][1];
      b += coordinates[i - 1][1] * coordinates[i][0];
    }
    return (b - a) * .5;
  };

  coordinates.centroid = function(k) {
    var i = -1,
        n = coordinates.length - 1,
        x = 0,
        y = 0,
        a,
        b,
        c;
    if (!arguments.length) k = 1 / (6 * coordinates.area());
    while (++i < n) {
      a = coordinates[i];
      b = coordinates[i + 1];
      c = a[0] * b[1] - b[0] * a[1];
      x += (a[0] + b[0]) * c;
      y += (a[1] + b[1]) * c;
    }
    return [x * k, y * k];
  };

  // The Sutherland-Hodgman clipping algorithm.
  coordinates.clip = function(subject) {
    var input,
        i = -1,
        n = coordinates.length,
        j,
        m,
        a = coordinates[n - 1],
        b,
        c,
        d;
    while (++i < n) {
      input = subject.slice();
      subject.length = 0;
      b = coordinates[i];
      c = input[(m = input.length) - 1];
      j = -1;
      while (++j < m) {
        d = input[j];
        if (d3_geom_polygonInside(d, a, b)) {
          if (!d3_geom_polygonInside(c, a, b)) {
            subject.push(d3_geom_polygonIntersect(c, d, a, b));
          }
          subject.push(d);
        } else if (d3_geom_polygonInside(c, a, b)) {
          subject.push(d3_geom_polygonIntersect(c, d, a, b));
        }
        c = d;
      }
      a = b;
    }
    return subject;
  };

  return coordinates;
};

function d3_geom_polygonInside(p, a, b) {
  return (b[0] - a[0]) * (p[1] - a[1]) < (b[1] - a[1]) * (p[0] - a[0]);
}

// Intersect two infinite lines cd and ab.
function d3_geom_polygonIntersect(c, d, a, b) {
  var x1 = c[0], x2 = d[0], x3 = a[0], x4 = b[0],
      y1 = c[1], y2 = d[1], y3 = a[1], y4 = b[1],
      x13 = x1 - x3,
      x21 = x2 - x1,
      x43 = x4 - x3,
      y13 = y1 - y3,
      y21 = y2 - y1,
      y43 = y4 - y3,
      ua = (x43 * y13 - y43 * x13) / (y43 * x21 - x43 * y21);
  return [x1 + ua * x21, y1 + ua * y21];
}