Circular comparison in Ruby

rock_paper_scissors.rb

class RockPaperScissors

  ITEMS = %W(A P B N)

  def self.compare(item, other_item)
    new(item).compare other_item
  end


  def initialize(item)

    # input validations?

    @item = item
  end

  def compare(other_item)

    # input validations?

    indexes_subtraction = ITEMS.index(@item) - ITEMS.index(other_item)

    case indexes_subtraction
    when 1, -1
      - indexes_subtraction
    else
      indexes_subtraction <=> 0
    end

  end

end

require 'test/unit'
include MiniTest::Assertions

assert_equal RockPaperScissors.compare('A', 'A'), 0
assert_equal RockPaperScissors.compare('P', 'P'), 0
assert_equal RockPaperScissors.compare('B', 'B'), 0
assert_equal RockPaperScissors.compare('N', 'N'), 0
assert_equal RockPaperScissors.compare('A', 'P'), 1
assert_equal RockPaperScissors.compare('P', 'A'), -1
assert_equal RockPaperScissors.compare('P', 'B'), 1
assert_equal RockPaperScissors.compare('B', 'P'), -1
assert_equal RockPaperScissors.compare('B', 'N'), 1
assert_equal RockPaperScissors.compare('N', 'B'), -1
assert_equal RockPaperScissors.compare('N', 'A'), 1
assert_equal RockPaperScissors.compare('A', 'N'), -1