/* this is my general purpose random function
 *
 * random numbers are always paramaterized between min and max.
 * however, the "focus" variable allows an interpolation between
 * a uniform and gaussian distribution (normal extends
 * "focus" standard deviations from center and a focus of 0
 * is a uniform distribution).
 */
function focusedRandom(min, max, focus, mean) {
  if(max === undefined) {
    max = min;
    min = 0;
  }
  if(focus === undefined) {
    focus = 1.0;
  }
  if(mean === undefined) {
    mean = (min + max) / 2.0;
  }
  if(focus == 0) {
    return d3.randomUniform(min, max)();
  }
  else if(focus < 0) {
    focus = -1 / focus;
  }
  sigma = (0.5 * (max - min)) / focus;
  val = d3.randomNormal(mean, sigma)();
  if (val > min && val < max) {
    return val;
  }
  return d3.randomUniform(min, max)();
}