assert_differences code example

assert_differences_code.rb

# Drop into test_helper.rb or similar
def assert_differences(expression_array, by:, message: nil, &block)
  expressions = Array(expression_array)
  change_by = Array(by).map(&:to_i)

  if expressions.size != change_by.size
    raise ArgumentError, 'Each expression must have a corresponding value to change by'
  end

  exps = expressions.map.with_index do |e, i|
    callable = e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
    {
      callable: callable,
      before: callable.call,
      expected_change: change_by[i]
    }
  end

  retval = yield

  expressions.zip(exps).each do |code, e|
    difference = e[:expected_change]
    error      = "#{code.inspect} didn't change by #{difference}"
    error      = "#{message}.\n#{error}" if message
    assert_equal(e[:before] + difference, e[:callable].call, error)
  end

  retval
end