merge_sort.rb

class Mergesort
  attr_reader :array_sorted

  def initialize
    @array_sorted = []
  end

  def init(array)
    sort(array)
  end

  private

  def sort(array)
    return array if array.length <= 1

    mid = (array.length / 2).round

    left  = array.take(mid)
    right = array.drop(mid)

    sorted_left = sort(left)
    sorted_right = sort(right)

    @array_sorted = merge(sorted_left, sorted_right)
  end
 
  def merge(left, right)
    return left if right.empty?
    return right if left.empty?

    small_number = if left.first <= right.first
      left.shift
    else
      right.shift
    end

    recursive = merge(left, right)

    [small_number].concat(recursive)
  end
end

# test
m = Mergesort.new
m.init([1,4,10,2,3,32,0])
p m.array_sorted