How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

(defun is-leap-year (year)
  (or (and (zerop (mod year 4))
           (not (zerop (mod year 100))))
      (zerop (mod year 400))))

(defun days-in-month (month year)
  (case month
    ((1 3 5 7 8 10 12) 31)
    ((4 6 9 11) 30)
    (2 (if (is-leap-year year)
           29
           28))))

(defun euler-19 ()
  (let ((day-of-week 2) ; 1 Jan 1901 was a Tuesday
        (sundays 0))
    (loop :for year :from 1901 :to 2000 :do
      (loop :for month :from 1 :to 12 :do
        (when (zerop day-of-week) ; 0 represents Sunday
          (incf sundays))
        (setf day-of-week
              (mod
               (+ day-of-week
                  (days-in-month month year))
               7))))
    sundays))

(print (euler-19))

CL-USER> (euler-19)
171
CL-USER> 

— Me@2024-01-30 01:46:11 PM

.

.

2024.01.30 Tuesday (c) All rights reserved by ACHK