class Similarity

# File lib/similarity.rb, line 2
def self.calculate_distance(source, comparison)
    raise "This method cannot compare to NilClass" if comparison.nil?
    n = source.length
    m = comparison.length
    max = n/2

    # Returns either word's length if one of them is empty
    return m if 0 == n
    return n if 0 == m

    # Checks if both words are equal and returns 0
    return 0 if source.eql? comparison

    d = (0..m).to_a
    x = nil
    source.each_char.with_index do |char1, i|
        e = i+1
        comparison.each_char.with_index do |char2, j|
            cost = (char1 == char2) ? 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