mutual_recursion/lib/mutual_recursion.rb

# frozen_string_literal: true

module MutualRecursion
  class TailCall
    attr_reader :value, :block

    def initialize(value = nil, &block)
      @value = value
      @block = block
    end

    ##
    # Invoke this tail call. Only the returned value from the initial call to
    # the recursive function should be invoked.
    #
    # @return [Object] the terminal_value of this tail call

    def invoke
      self.then do |tail|
        while tail.block
          tail = tail.block.call
          raise MissingTailCallError unless tail.is_a?(TailCall)
        end

        tail.value
      end
    end
  end

  class MissingTailCallError < StandardError
    def initialize(msg = 'expected a tail call')
      super
    end
  end

  module_function

  ##
  # Make a direct or indirect recursive call in tail position.
  #
  # @yieldreturn [MutualRecursion::TailCall] a tail call
  # @return [MutualRecursion::TailCall] a non terminal tail call

  def tail_call
    TailCall.new { yield }
  end

  ##
  # Indicates that the recursion has ended with the provided value.
  #
  # @param [Object] value the terminal value
  # @return [MutualRecursion::TailCall] a terminal tail call that will return the given value

  def terminal_value(value)
    TailCall.new(value)
  end
end