mutual_recursion/test/mutual_recursion_test.rb

# frozen_string_literal: true

require 'minitest/autorun'
require 'minitest/pride'

require_relative '../lib/mutual_recursion'

class InventoryTest < Minitest::Test
  include MutualRecursion

  def one(x, y = 0)
    return terminal_value(y) if x.negative?

    tail_call { two(x, y + 1) }
  end

  def two(x, y = 0)
    tail_call { one(x - 1, y) }
  end

  def direct(x, y = 0)
    return terminal_value(y) if x.negative?

    tail_call { direct(x - 1, y + 1) }
  end

  def lambda_returning
    terminal_value(-> { 24 })
  end

  def proc_returning(x, y = 0)
    return terminal_value(proc { "|#{y}|" }) if x.negative?

    tail_call { proc_returning(x - 1, y + 1) }
  end

  def inauthentic_tail_call
    tail_call do
      Class.new do
        attr_reader :value, :block

        def initialize
          @value = 99
          @block = proc { MutualRecursion.terminal_value(100) }
        end
      end.new
    end
  end

  def test_terminal_value
    tail = terminal_value(42)
    assert_equal(42, tail.invoke)
  end

  def test_single_recursion
    tail = tail_call { terminal_value(42) }
    assert_equal(42, tail.invoke)
  end

  def test_several_recursions
    tail = tail_call { tail_call { tail_call { terminal_value(42) } } }
    assert_equal(42, tail.invoke)
  end

  def test_mutual_tail_recursion
    tail_one = one(50_000)
    tail_two = two(50_000)
    assert_equal(50_001, tail_one.invoke)
    assert_equal(50_000, tail_two.invoke)
  end

  def test_direct_tail_recursion
    tail = direct(50_000)
    assert_equal(50_001, tail.invoke)
  end

  def test_recursive_function_can_return_lambda
    tail = lambda_returning
    assert_kind_of(Proc, tail.invoke)
  end

  def test_recursive_function_can_return_proc
    tail = proc_returning(20)
    assert_kind_of(Proc, tail.invoke)
  end

  def test_inauthentic_tail_call_detected
    tail = inauthentic_tail_call
    assert_raises(MissingTailCallError) { tail.invoke }
  end

  def test_module_functions
    tail = MutualRecursion.tail_call { MutualRecursion.terminal_value(42) }
    assert_equal(42, tail.invoke)
  end
end