pe041.lisp

(in-package :clamp)

;; https://github.com/smithzvk/cl-primality
(use-package :cl-primality)

(def digits (n)
  "Returns a list of the digits of N."
  (ret result '()
    (while (> n 0)
      (push (mod n 10) result)
      (zap #'floor n 10))))

(def permutations (xs)
  "Returns a list of all of the permutations of XS."
  (if (null xs)
      '(())
      (cart #'cons xs (permutations (rem it xs :count 1)))))

(def pandigitals (d)
  "Returns a list of all of the pandigital numbers with D digits."
  (map #'list->number (permutations (range 1 d))))

(def list->number (xs)
  "Takes a list of digits and returns the number they represent."
  (ret result 0
    (each d xs
      (zap #'* result 10)
      (++ result d))))

(def solve ()
  "Solves PE problem 41."
  (downfrom n 9 1 ; the largest pandigital has 9 digits
    (each pan (sort #'> (pandigitals n))
      (when (primep pan)
        (return-from solve pan)))))