;; Numerical.lsp
;;
;; A collection of functions for factoring, primality testing and a
;; function to construct powersets
;;
;; Author: John DeSanto, john@johndesanto.com
;;
;;
;; function: powerset
;; Return the power set of a list
(define (powerset x)
(if (empty? x)
(list '())
(let ( (element (first x))
(p (powerset (rest x))))
(append (map (fn (subset) (cons element subset)) p) p) )))
;; function: factor-group
;; A different factoring algorithm uses "factor" but groups the results
;; e.g. (factor-group 45) --> ((3 2) (5 1))
(define (factor-group num)
(let (factorlist (factor num) factorlist-grouped '())
(dolist (y (unique factorlist))
(push (append (list y) (count (list y) factorlist)) factorlist-grouped -1))
factorlist-grouped))
;; function: isprime?
;; This is the slow but sure method to determining if a number is prime.
(define (isprime? n)
(if (divides? 2 n)
nil
(= 1 (length (factor n)))))
;; function: divides?
;; Sometimes written "small | large"
(define (divides? small large)
(= 0 (mod large small)))
;; function: nextprime
;;
(define (nextprime n)
(do-until (isprime? n)
(inc n))
n)
;; function: nextprime2
;; Marginally faster. Doesn't test even numbers, but the extra conditional
;; slows it down.
(define (nextprime2 n)
(inc n)
(if (divides? 2 n)
(nextprime2 n)
(if (isprime? n) n (nextprime2 (inc n)))))
;; function: isprime?-v2 (incomplete)
;;
;; Another primality checker. This may be faster if the function is sent a list of known primes.
;; For very large primes, perhaps even those larger than can be represented as integers, it is
;; common practice to test the prime by dividing by the first 10000 or so primes. This routine
;; could also be improved by replacing "factor" with an algorithm that is optimized for larger
;; numbers.
(define (isprime?-v2 n known_primes)
(if (find 0 (map mod (series n 1 (length known_primes)) known_primes))
nil
(= 1 (length (factor n)))))

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