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
  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(50_000)
    assert_equal(50_001, tail.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
    assert_kind_of(Proc, tail.invoke)
  end

  def test_non_tail_call_detected
    tail = bad_return
    assert_raises(TypeError) { tail.invoke }
  end
end

class TailCallDecoy
  attr_reader :value, :block

  def initialize
    @value = 99
    @block = proc { 25 }
  end
end

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

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

def two(x, y)
  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
  terminal_value(proc { 24 })
end

def bad_return
  tail_call { TailCallDecoy.new }
end