mergesort.rb

#!/usr/bin/env ruby

# a1 : [ int, int, ... ]
# a2 : [ int, int, ... ]
def merge(a1, a2 = nil)
  return a1 if a2.nil?

  r = []
  loop do
    case
    when a1.count < 1 && a2.count < 1 then break
    when a1.count < 1                 then r << a2.shift
    when a2.count < 1                 then r << a1.shift
    when a1[0] <= a2[0]               then r << a1.shift
    when a1[0] >  a2[0]               then r << a2.shift
    end
  end
  return r
end

def msort(arr)
  len = arr.count
  return arr[0] if len < 2

  mid = len / 2
  return merge(
    msort(arr[0...mid]),
    msort(arr[mid...len])
  )
end

input = ARGV.map{|v| [ v.to_i ] } # Convert to integer, non-numeric value => 0
p msort(input)