Test.

compare.rb

#encoding: utf-8

class String
  def make_ngram(n)
    ary = self.split(//)
    ngram_ary = []
    p = ary.length
    for i in 0..(p - n) do
      ngram_ary << ary[i...(i + n)]
    end
    ngram_ary
  end
end

def compare(str_a, str_b)
  a = str_a.make_ngram(3)
  b = str_b.make_ngram(3)
  count = 0
  
  a.each do |elem|
    for i in 0..(b.length) do
      if elem == b[i]
        count += 1
        b.delete_at i
        break
      end
    end
  end
  
  num = count.to_f
  den = a.length > b.length ? a.length : b.length
  
  puts num / den
end

compare(ARGV[0], ARGV[1])