gem 'handicapper', github: 'le-doude/handicapper'

 previous_rounds = # if you have some previous handicap differentials load them here in an Array
 calculator = Handicapper::Calculator(differentials: previous_rounds, gender: :male)
 
 # This is the settings for Ko'olau golf course from the gold/tournament tees.
 round_settings = Handicapper::RoundSettings.new(
  78.2, # course rating
  153, # slope rating
  [5,4,4,3,4,4,5,3,4,4,5,4,3,4,4,5,3,4] # par for each holes in order of play
 )
 # Your round has to be in the same order as the par scores above
 my_round = [9, 6, 7, 4, 4, 3, 8, 7, 7, 6, 6, 4, 3, 4, 5, 8, 4, 7] # I wish
 
 # calculate returns the updated handicap differential after considering the submitted round
 handicap_differential1 = calculator.add_round(round_settings: round_settings, scores: my_round)
 # or you can use
 handicap_differential2 = calculator.add_round(course_rating: 78.2, slope: 153, pars: [5,4,4,3,4,4,5,3,4,4,5,4,3,4,4,5,3,4], scores: my_round) 

 # if you do not have the hole by hole score you can use the total. The result might not be the same as with the official USGA calculations though if you did not adjust your total
 handicap_differential3 = calculator.add_round(round_settings: round_settings, adjusted_score: 132)
 # which is equivalent to
 handicap_differential4 = calculator.add_round(course_rating: 78.2, slope: 153, adjusted_score: 132) 
 # you can use current handicap if you have no new round but want it calculated
 handicap_index = calculator.current_handicap