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@ericnormand
Created March 22, 2021 14:58
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419 PurleyFunctional.tv Newsletter

String difference

You are given two strings, a and b. We consider each letter to be unique, meaning duplicates are significant. Write a function that returns the count of letters in b which do not occur in a.

Examples

(strdiff "abc" "") ;=> {} ;; no characters in b don't occur in a
(strdiff "abc" "abc") ;=> {} ;; ditto
(strdiff "" "abc") ;=> {\a 1 \b 1 \c 1}
(strdiff "axx" "abcc") ;=> {\b 1 \c 2}
(strdiff "xxxx" "xxxxxx") ;=> {\a 2} ;; two x's in b that don't occur in a

Thanks to this site for the challenge idea where it is considered Hard in Python. The problem has been modified from the original.

Please submit your solutions as comments on this gist.

To subscribe: https://purelyfunctional.tv/newsletter/

@s3dse
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s3dse commented Apr 9, 2021

(defn strdiff [a b]
  (loop [freq-b (frequencies b)
         acc {}]
    (cond
      (or (= a b) (empty? freq-b)) acc
      (empty? a) freq-b
      :else
      (let [[k vb] (first freq-b)
            va ((frequencies a) k 0)
            diff (- vb va)]
        (if (pos-int? diff)
          (recur (rest freq-b) (assoc acc k diff))
          (recur (rest freq-b) acc))))))

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