* @param a the current value
* @param b the target value
* @param start the start of the domain
* @param end the end of the domain
* @return the shortest direction (the sign) and distance (the magnitude)
* @param a the current value
* @param b the target value
* @param start the start of the domain
* @param end the end of the domain
* @return the shortest direction (the sign) and distance (the magnitude)