Merge Sort program in ruby

merge_sort.rb

# merge_sort.rb
#
# $ ruby merge_sort.rb
# [1, 2, 3, 4, 5, 6, 7, 8]

def merge_sort(array)
  n = array.length
  if n > 1
    mid = n/2
    lefthalf = array[0..mid-1]
    righthalf = array[mid..n-1]
    merge_sort lefthalf
    merge_sort righthalf
    i = j = k = 0
    while i < lefthalf.length and j < righthalf.length
      if lefthalf[i] < righthalf[j]
        array[k] = lefthalf[i]
        i = i+1
      else
        array[k] = righthalf[j]
        j = j+1
      end
      k += 1
    end

    while i < lefthalf.length
      array[k] = lefthalf[i]
      i = i+1
      k = k+1
    end

    while j < righthalf.length
      array[k] = righthalf[j]
      j = j+1
      k = k+1
    end
  end
end

print array = [6,5,3,1,8,7,2,4]
merge_sort array
print array