How to write tests?

README.md

1. Identify what are diff. use cases we need to test, and define 'test' method for each. For the above example, we need,

• breaking_fact
  • test for 0
  • test for 1
  • test for any other number (say 3)
  • test for big number, which should break and throw exception.
• fact
  • test for 0
  • test for 1
  • test for any other number
  • test for big number.

2. Use assertions to test the outcome, with appropriate message. All cases should cover all possible cases by which we can call the method, i.e all positive/negative cases.
3. Test it using command 'ruby factorial_test.rb'

factorial.rb

class Factorial
  
  #
  # This method calculates factorial using recursion, it breaks after \
  # some limit (say for number more than 8000) So we can not use it for \
  # calculating factorial for big numbers.
  #
  def self.breaking_fact(n=1)
    return 1 if n == 0
    n * breaking_fact(n-1)
  end

  #
  # Calculates factorial without looping, and using simple while loop. 
  # This works like a charm.
  #
  def self.fact(n=1)
    return 1 if n == 0
    count = n
    total = 1
    while count > 1 do 
      total *= count
      count -= 1
    end
    total
  end

end

factorial_test.rb

require 'minitest/autorun'
require_relative 'factorial'

class FactorialTest < Minitest::Test
  
  def test_breaking_fact_factorial_of_3 
    assert_equal Factorial.breaking_fact(3), (3 * 2 * 1), "breaking_fact(3) should be 6"
  end

  def test_breaking_fact_factorial_of_1 
    refute_equal Factorial.breaking_fact(1), (1 * 0), "breaking_fact(1) should not be 0"
  end

  def test_breaking_fact_factorial_of_0 
    refute_equal Factorial.breaking_fact(0), (0 * -1), "breaking_fact(0) should not be 0"
  end

  def test_breaking_fact_factorial_of_8999
    err = assert_raises SystemStackError  do  
      Factorial.breaking_fact(8999)
    end
    assert_match /stack level too deep/, err.message
  end
  
  def test_fact_factorial_of_3 
    assert_equal Factorial.fact(3), (3 * 2 * 1), "fact(3) should be 6"
  end

  def test_fact_factorial_of_1 
    refute_equal Factorial.fact(1), (1 * 0), "fact(1) should not be 0"
  end

  def test_fact_factorial_of_0 
    refute_equal Factorial.fact(0), (0 * -1), "fact(0) should not be 0"
  end
  
  def test_fact_factorial_of_8999
    assert Factorial.fact(8999).is_a?(Numeric), "should be a number"
  end

end

Thanks @zenspider for suggesting the improvements.

1. assert !Factorial.fact(8999).nil?, "should be a number” doesn’t test that it is a number. Just not nil.
2. no no no. Use assert_equal and refute_equal to get good error messages.