素数日

[ホーム](https://redmine.noppi.jp)

# [[素数日]]

```cl

;; -*- coding: utf-8; -*-

(defconstant +start-day+ 20000101)

(defconstant +end-day+ 20991231)

(defconstant +day-elements+ (1+ +end-day+))

(defparameter *calendar-number*

  (make-array +day-elements+ :element-type 'boolean :initial-element t))

(defun eratosthenes ()

  (setf (svref *calendar-number* 0) nil)

  (setf (svref *calendar-number* 1) nil)

  (do ((n1 2 (1+ n1)))

      ((<= +end-day+ (* n1 n1)))

    (when (svref *calendar-number* n1)

      (do ((n2 (* n1 2) (+ n2 n1)))

          ((< +end-day+ n2))

        (setf (svref *calendar-number* n2) nil)))))

(defun primes ()

  (eratosthenes)

  (do ((n +start-day+ (1+ n))

       (result nil))

      ((< +end-day+ n) (reverse result))

    (when (svref *calendar-number* n)

      (setf result (cons n result)))))

(defun leap-year-p (year)

  ;; year % 4 == 0 && year % 100 != 0 || year % 400 == 0

  (or (and (zerop (mod year 4))

           (not (zerop (mod year 100))))

      (zerop (mod year 400))))

(defun dayp (n)

  (let ((yyyy (floor n 10000))

        (mm (floor (mod n 10000) 100))

        (dd (mod n 100)))

    (case mm

      ((1 3 5 7 8 10 12)

       (<= 1 dd 31))

      ((4 6 9 11)

       (<= 1 dd 30))

      (2

       (if (leap-year-p yyyy)

           (<= 1 dd 29)

           (<= 1 dd 28))))))

(defun prime-days ()

  (remove-if-not #'dayp (primes)))

(defun prime-days-2026 ()

  (remove-if-not (lambda (yyyymmdd)

                   (= 2026 (floor yyyymmdd 10000)))

                 (prime-days)))

```