Rough implementation of the Sørensen index of two strings

first.rb

def sørensen_index(string_a, string_b)
  matches_a = get_bigrams string_a.dup
  matches_b = get_bigrams string_b.dup
  similarities = matches_a & matches_b
  sum_bigrams = matches_a.count + matches_b.count
  2 * similarities.count / sum_bigrams.to_f
end

def get_bigrams(str)
  bigrams = []
  while str.length > 1 do
    bigrams << str[0..1] 
    str[0] = ''
  end
  bigrams
end

second.rb

class SorensenIndex
  def initialize(*strings)
    @bigram_sets = strings.map {|s| BigramSet.new s}
  end

  def similarities
    @bigram_sets.first & @bigram_sets.last
  end

  def total
    @bigram_sets.first.count + @bigram_sets.last.count
  end

  def calculate
    2 * similarities.count / total.to_f
  end
end

class BigramSet
  attr_reader :bigrams

  def initialize(string)
    get_bigrams string.dup
  end

  def &(alt_set)
    @bigrams & alt_set.bigrams
  end

  def count
    @bigrams.count
  end

  private
    def get_bigrams(str)
      @bigrams = []
      while str.length > 1 do
        @bigrams << str[0..1] 
        str[0] = ''
      end
    end
end

third.rb

class String
  include Enumerable
  alias_method :each, :each_char
  def bigrams
    self.each_cons(2).to_a
  end
end

class SorensenIndex
  def initialize(*strings)
    @bigram_sets = strings.map {|s| s.bigrams}
  end

  def calculate
    2 * similarities.count / total.to_f
  end

  class << self
    def calculate(*strings)
      SorensenIndex.new(strings).calculate
    end
  end

  private
    def similarities
      @bigram_sets.first & @bigram_sets.last
    end

    def total
      @bigram_sets.first.count + @bigram_sets.last.count
    end
end