function getDistance(p1, p2, props) {
  if (!props) {
    props = PROPS_XY;
  }
  var x = p2[props[0]] - p1[props[0]];
  var y = p2[props[1]] - p1[props[1]];

  return Math.sqrt(x * x + y * y);
}
@private
calculate the absolute distance between two points
@param {Object} p1 {x, y}
@param {Object} p2 {x, y}
@param {Array} [props] containing x and y keys
@return {Number} distance