Ruby code for code kata, solving missing numbers

missing_numbers.rb

require "test/unit"

NOT_A_NUMBER = 1
ALL_GOOD = 0

def update_previous_value_if_one_ahead(i, prev)
  if prev == nil
    prev = i.to_i
  else
    check_value = prev.to_i + 1
    if check_value == i
      prev = i.to_i
    end
  end
  prev.to_i
end

def find_missing_number(sequence)
  arr = sequence.split(' ')
  unless arr.length
    return ALL_GOOD
  end
  prev = nil


  arr.each do |i|
    # Check if not a number
    if i =~ /\D/
      return NOT_A_NUMBER
    end
    i = i.to_i
    # Wouldn't normally recommend this approach.
    if prev == nil and i == 2
      return 1
    end
    prev = update_previous_value_if_one_ahead(i, prev)

    if i != prev
          return prev + 1
    end
  end
  ALL_GOOD
end

class BLUR_TEST < Test::Unit::TestCase

  def test_will_return_one_if_not_a_number
    assert_equal(NOT_A_NUMBER, find_missing_number('a'))
  end

  def test_will_return_missing_number
    assert_equal(
        4,
        find_missing_number("1 2 3 5"),
        "It must work for missing middle terms")
  end

  def test_will_return_missing_number_two
    assert_equal(
        2,
        find_missing_number("1 3")
    )
  end

  def test_will_return_0_for_empty_set
    assert_equal(
        0,
        find_missing_number("")
    )
  end

  def test_will_return_missing_number_again
    assert_equal(
        1,
        find_missing_number("2 3 4 5")
    )
  end
  def test_will_return_1_on_invalid_sequence
    assert_equal(
        1,
        find_missing_number("2 1 4 3 a")
    )
  end

end