aoc2020 day25

aoc2020-day25.lisp

;;;; +----------------------------------------------------------------+
;;;; | Advent of Code 2020                                            |
;;;; +----------------------------------------------------------------+

(defpackage #:snippets/aoc2020/day25
  (:use #:cl)
  (:import-from
   #:ironclad
   #:expt-mod)
  (:export
   #:day25))

(in-package #:snippets/aoc2020/day25)

(defun day25 (input)
  (destructuring-bind (a b) input
    (let* ((m 20201227)
           (d2 (dotimes (d m)
                 (let ((v (expt-mod 7 d m)))
                   (when (= v b)
                     (return d))))))
      (list (expt-mod a d2 m)))))