module GamesDice
  # basic die that rolls 1..N, typically with equal weighting for each value
  #  d = Die.new(6)
  #  d.roll # => Integer in range 1..6
  #  d.result # => same Integer value as returned by d.roll
  class Die
    # sides is e.g. 6 for traditional cubic die, or 20 for icosahedron.
    # It can take non-traditional values, such as 7, but must be at least 1.
    # prng is an object that has a rand(x) method. If provided, it will be called as
    # prng.rand(sides), and is expected to return an integer in range 0...sides
    def initialize( sides, prng=nil )
      @sides = Integer(sides)
      raise ArgumentError, "sides value #{sides} is too low, it must be 1 or greater" if @sides < 1
      raise ArgumentError, "prng does not support the rand() method" if prng && ! prng.respond_to?(:rand)
      @prng = prng
      @result = nil
    end

    # number of sides as set by #new
    attr_reader :sides

    # integer result of last call to #roll, nil if no call made yet
    attr_reader :result

    # minimum possible value
    def min
      1
    end

    # maximum possible value
    def max
      @sides
    end

    # returns a hash of value (Integer) => probability (Float) pairs
    def probabilities
      return @probabilities if @probabilities
      density = 1.0/@sides
      @probabilities = (1..@sides).inject({}) { |h,x| h[x] = density; h }
    end

    # returns mean expected value as a Float
    def expected_result
      0.5 * (1 + @sides)
    end

    # returns probability than a roll will produce a number greater than target integer
    def probability_gt target
      probability_ge( Integer(target) + 1 )
    end

    # returns probability than a roll will produce a number greater than or equal to target integer
    def probability_ge target
      target = Integer(target)
      return 1.0 if target <= 1
      return 0.0 if target > @sides
      return 1.0 * (1.0 + @sides - target )/@sides
    end

    # returns probability than a roll will produce a number less than or equal to target integer
    def probability_le target
      target = Integer(target)
      return 1.0 if target >= @sides
      return 0.0 if target < 1
      return 1.0 * target/@sides
    end

    # returns probability than a roll will produce a number less than target integer
    def probability_lt target
      probability_le( Integer(target) - 1 )
    end

    # generates Integer between #min and #max, using rand()
    def roll
      if @prng
        @result = @prng.rand(@sides) + 1
      else
        @result = rand(@sides) + 1
      end
    end

    # always nil, available for compatibility with ComplexDie
    def rerolls
      nil
    end

    # always nil, available for compatibility with ComplexDie
    def maps
      nil
    end
  end # class Die
end # module GamesDice