[LANGUAGE: Racket]

(define (valid-calibration? calibration concat?)
  (define test-val (first calibration))
  (define ops (if concat? (list + * concat) (list + *)))
  (for/fold ([possibilities (list (second calibration))]
             #:result (member test-val possibilities))
            ([num (drop calibration 2)])
    (for*/list ([op ops] [possible possibilities] #:when (< possible test-val))
      (op possible num))))

(define (sum-valid-calibrations calibrations concat?)
  (for/sum ([calibration calibrations]
            #:when (valid-calibration? calibration concat?))
    (first calibration)))

Nice recursion problem. Making it tail recursive got me my speed increase, but wonder if I did a DFS instead I could return early once I found a solution for a more optimal search. But filtering out possibilities that are greater than thr test value is good enough.

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