potter kata

harry.rb

class Memo
  def initialize; @memo = {}; end
  def ized(key, &block); (k = @memo[key]) ? k : @memo[key] = block.call; end
end

class Harry
  NSLOTS = 5
  DISCOUNTS = [0, 0, 0.05, 0.10, 0.20, 0.25]
  UNIT_PRICE = 8

  def self.compute_masks
    def self.mask_to_array(mask); (0...NSLOTS).map { |i| 1 & (mask >> i) }.reverse; end
    (1..(2**NSLOTS)-1).map { |m| mask_to_array(m) }
  end

  MASKS = compute_masks

  def sum(a); a.inject(0) {|acc, i| acc+i}; end
  def initialize; @memo = Memo.new; end

  def price(bs)
    @memo.ized(bs.sort) do
      sum(bs).zero? ? 0 : MASKS.select { |s| valid_picks(bs, s) }.map { |s| discounted(sum(s)) + price(pick_books(bs, s)) }.min
    end
  end

  def valid_picks(books, s); pick_books(books, s).all? { |i| i >= 0 }; end
  def pick_books(books, selection); books.zip(selection).map { |orig_count, i| orig_count - i }; end
  def discounted(uniq_count); (1-DISCOUNTS[uniq_count]) * UNIT_PRICE * uniq_count; end
end