shouldTrigger() implementation from the Median Delta Breaker

MedianDeltaBreaker.sol

/**
 * @title   Median Delta Breaker
 * @notice  Breaker contract that will trigger when an updated oracle median rate changes
 *          more than a configured relative threshold from the previous one. If this
 *          breaker is triggered for a rate feed it should be set to no trading mode.
 */
contract MedianDeltaBreaker is IBreaker, WithCooldown, WithThreshold, Ownable {
  using SafeMath for uint256;
  using FixidityLib for FixidityLib.Fraction;

  /* ==================== Events ==================== */
  event SmoothingFactorSet(address rateFeedId, uint256 smoothingFactor);

  /* ==================== State Variables ==================== */
  // Address of the Mento SortedOracles contract
  ISortedOracles public sortedOracles;

  // Default smoothing factor for EMA as a Fixidity value
  uint256 public constant DEFAULT_SMOOTHING_FACTOR = 1e24;

  // Smoothing factor per rate feed
  mapping(address => FixidityLib.Fraction) public smoothingFactors;

  // EMA of the median rates per rate feed
  mapping(address => uint256) public medianRatesEMA;

  // -- Code omitted for brevity -- //

  /**
   * @notice  Check if the current median report rate for a rate feed change, relative
   *          to the last median report, is greater than the configured threshold.
   *          If the change is greater than the threshold the breaker will be triggered.
   * @param   rateFeedID The rate feed to be checked.
   * @return  triggerBreaker  A bool indicating whether or not this breaker
   *                          should be tripped for the rate feed.
   */
  function shouldTrigger(address rateFeedID) public returns (bool triggerBreaker) {
    (uint256 currentMedian, ) = sortedOracles.medianRate(rateFeedID);

    uint256 previousRatesEMA = medianRatesEMA[rateFeedID];
    if (previousRatesEMA == 0) {
      // Previous recorded EMA will be 0 the first time this rate feed is checked.
      medianRatesEMA[rateFeedID] = currentMedian;
      return false;
    }

    FixidityLib.Fraction memory smoothingFactor = FixidityLib.wrap(getSmoothingFactor(rateFeedID));
    medianRatesEMA[rateFeedID] = FixidityLib
      .wrap(currentMedian)
      .multiply(smoothingFactor)
      .add(FixidityLib.wrap(previousRatesEMA).multiply(FixidityLib.fixed1().subtract(smoothingFactor)))
      .unwrap();

    return exceedsThreshold(previousRatesEMA, currentMedian, rateFeedID);
  }
}