before = "one two three four"
after  = "one six three four five"


word_count_before = before.scan(/w+/).size
word_count_after  = after.scan(/w+/).size

if  word_count_before > word_count_after #if the string got smaller we still want a positive number when comparing the two
  bigger  = word_count_before
  smaller = word_count_after
else
  bigger  = word_count_after
  smaller = word_count_before
end

word_difference  =  bigger - smaller

before = "one two three four"
after  = "one six three four five"

before, after = [before, after].map(&:split)
common = [before, after].reduce &:&

before_not_after = before - common
after_not_before = after - common

before, after = [before, after].map(&:split)
# after execution of the line below, before array will contain result
after.each { |e| (i = before.index(e)) && before.delete_at(i) }

one   = "een redelijk lange tekst om na te gaan of dit programma het verschil kan maken tussen soortgelijke teksten door rekening te houden met combinaties van woorden"
two   = "een redelijk lange tekst met bijna dezelfde woorden als de vorige om na te gaan of dit programma het verschil kan maken tussen soortgelijke teksten door rekening te houden met combinaties van woorden"
three = "een totaal andere tekst, ik maak hem lang genoeg om representabel te zijn en zet er enkele woorden bij die in de eerste tekst ook voorkomen"

class String
    def similarities_with text, lookafter_count=2, lookbefore_count=2
        r = [self.split, text.split].each.inject([]) do |r, a|
            r << a.each_with_index.inject([]) do |m, (element, index)|
              m << a[index-lookbefore_count..index+lookafter_count]
            end
        end
    (r.first & r.last).reject(&:empty?).count
    end
end

one.similarities_with one # 24
one.similarities_with two # 20
one.similarities_with three # 0
"one two three four".similarities_with("one six three four five", 0, 0) # 3
"one two three four".similarities_with("one six three four five", 1, 1) # 0

# and now the difference
one.similarities_with(one) - one.similarities_with(two) # 4