# A collection of mutually exclusive odds for different outcomes.
# Includes methods for sports betting arbitrage. Due to the discrete nature
# of real-world monetary amounts, there may be small rounding errors.
class MutuallyExclusiveCollection
  # create a new collection with the given odds
  # @param [Array<FixedOdds>] mutually_exclusive_outcome_odds the odds for all the mutually exclusive outcomes
  def initialize mutually_exclusive_outcome_odds
    @mutually_exclusive_outcome_odds = mutually_exclusive_outcome_odds.sort
  end

  # the least likely of the odds to occur
  # @return [FixedOdds] the least likely odd
  def least_likely
    @mutually_exclusive_outcome_odds.first
  end

  # the most likely of the odds to occur
  # @return [FixedOdds] the most likely odd
  def most_likely
    @mutually_exclusive_outcome_odds.last
  end  

  # the odds in ascending order of probability
  # @return [Array<FixedOdds>] odds in ascending probability
  def in_ascending_probability
    @mutually_exclusive_outcome_odds
  end

  # the odds in descending order of probability
  # @return [Array<FixedOdds>] odds in descending probability
  def in_descending_probability
    @mutually_exclusive_outcome_odds.reverse
  end

  # tells if arbitrage is possible for a collection of odds
  # @return [Boolean] true if profit can be made regardless
  # of the outcome, false otherwise
  def arbitrageable?
    sum_inverse_outcome < 1
  end

  # the bookmaker's return rate
  # @return [Number] the bookmaker's return rate as a percentage
  def bookmakers_return_rate
    fs = fractions
    1 - fs.reduce(:*) / fs.reduce(:+) if fs.any?
  end

  # hash of the odds and what percentage of the total stake should go on each
  # @return [Hash<FixedOdds, Number>] hash of odds to percentages
  def percentages
    @mutually_exclusive_outcome_odds.each_with_object({}) {|odds, hash| hash[odds] = 1 / odds.fractional_odds / sum_inverse_outcome }
  end

  # hash of the odds and what stakes to put on each given a total stake
  # @param [Money] total_stake the money to distribute on the outcomes
  # @return [Hash<FixedOdds, Money>] hash of odds to stakes
  def stakes_for_total_stake total_stake
    @mutually_exclusive_outcome_odds.each_with_object({}) {|odds, hash| hash[odds] = total_stake / odds.fractional_odds / sum_inverse_outcome }
  end

  # hash of the odds and the stakes needed to make the specified profit
  # @param (see #stakes_for_total_stake)
  # @return (see #stakes_for_total_stake)
  def stakes_for_profit desired_profit
    stakes_for_total_stake stake_to_profit desired_profit
  end

  # the stake needed to win the desired profit
  # @param [Money] desired_profit the profit to gain from arbitrage
  # @return [Money] the stake necessary to realise the desired profit
  def stake_to_profit desired_profit
    desired_profit / profit_percentage
  end

  # the profit won given the total stake to distribute
  # @param (see #stakes_for_total_stake)
  # @return [Money] the profit that can be made from a total stake
  def profit_on_stake total_stake
    total_stake * profit_percentage
  end

  # profit percentage available on this arb
  # @return [Number] the percentage profit available on the arb
  def profit_percentage
    sum = sum_inverse_outcome
    (1 - sum) / sum
  end

  private
    def sum_inverse_outcome
      fractions.reduce(0) {|sum, n| sum + Rational(1, n) }
    end

    def fractions
      @mutually_exclusive_outcome_odds.collect &:fractional_odds
    end
end