Get all unsorted combinations for a certain number items of array

array_combination.rb

def combine(arr, size)
  return [] if size > arr.size || size <= 0
  return [arr] if size == arr.size
  return arr.map{|item| [item]} if size == 1
  last = arr[-1]
  left = arr[0..-2]
  a = combine(left, size - 1).map{|item| item + [last]}
  b = combine(left, size)
  return a + b
end

def combine_count(arr, size)
  x = arr.size
  y = size
  a = 1
  b = 1
  size.times do
    a *= x
    x -= 1
    b *= y
    y -= 1
  end
  return a / b
end

def combine_test(arr, size)
  combine(arr, size).size == combine_count(arr, size)
end