def levenshtein_distance str1, str2
    s = str1
    t = str2
    n = s.length
    m = t.length
    max = n/2

    return m if (0 == n)
    return n if (0 == m)
    return n if (n - m).abs > max

    d = (0..m).to_a
    x = nil

    n.times do |i|
      e = i+1

      m.times do |j|
        cost = (s[i] == t[j]) ? 0 : 1
        x = [
             d[j+1] + 1, # insertion
             e + 1,      # deletion
             d[j] + cost # substitution
            ].min
        d[j] = e
        e = x
      end

      d[m] = x
    end

    return x
  end
This code is based directly on the Text gem implementation
Returns a value representing the "cost" of transforming str1 into str2