compare.rb

a = ['a', 'c', 'd', '']
b = ['b', 'c', 'd', '']
compare(a,a) # 0
compare(a,b) # -1
compare(b,a) # 1

a = ['a', 'c', 'd', '']
b = ['c', 'd', '']
compare(a,b) # -1

a = ['c', 'd', '']
b = ['a','c', 'd', '']
compare(a,b) # -1

def compare_equal(a,b)
  return 0 if a == [''] && b == ['']
  a_head  = a[0]
  a_tail  = a[1..-1]
  b_head  = b[0]
  b_tail =  b[1..-1]
  result  = compare_equal(a_tail, b_tail)
  return result if result != 0
  if result == 0
    a_head <=> b_head
  end
end

def compare(a,b)
  length = [a.length, b.length].min
  a_tail_start = a.length - length
  b_tail_start = b.length - length
  a_tail = a[a_tail_start..-1]
  b_tail = b[b_tail_start..-1]
  result = compare_equal(a_tail, b_tail)
  # a and b are in the same domain level
  return result if result != 0
  # a is sub domain of the b
  return -1 if a_tail_start != 0 || b_tail_start != 0
  # a and b are exactly the same
  return 0
end