merge sort in ruby

mergesort.rb

#!/usr/bin/env ruby
# -*- coding:utf-8 -*-

def merge_sort(a)
  if a.length <= 1
    a
  else
    (b, c) = split(a)
    merge( merge_sort(b), merge_sort(c))
  end
end

def split (a)
  if a.length == 1
    [a, []]
  else
    half_index = a.length / 2;
    [a[0..(half_index - 1)], a[half_index..-1]]
  end
end

def merge (a, b)
  if a.empty?
    b
  elsif b.empty?
    a
  else
    a_first = a[0]
    b_first = b[0]

    if a_first <= b_first
      a.shift
      [a_first, *( merge(a, b) )]
    else
      b.shift
      [b_first, *( merge(a, b) )]
    end
  end
end


a = []
20.times do |x|
  a << (rand 100)
end

puts "before sort :"
p a
puts "after sort: "
p merge_sort( a )

## before sort :
## [61, 57, 30, 57, 79, 74, 0, 56, 21, 89, 8, 20, 76, 57, 28, 27, 92, 50, 67, 42]
## after sort:
## [0, 8, 20, 21, 27, 28, 30, 42, 50, 56, 57, 57, 57, 61, 67, 74, 76, 79, 89, 92]