Add Computus in Emacs Lisp

contents/computus/code/elisp/gauss-easter.el

@@ -0,0 +1,37 @@
+(require 'cl-lib)
+(require 'subr-x)
+
+(defun computus (year &optional servois)
+  "Calculates the day of Easter for a given YEAR.
+Also computes its SERVOIS number when non-nil."
+  (let* ((a (mod year 19))                  ; year's position on the 19-year metonic cycle
+         (k (floor year 100))               ; century index
+         (p (floor (+ 13 (* 8 k)) 25))      ; shift of metonic cycle, add a day offset every 300 years
+         (q (floor k 4))                    ; correction for non-observed leap days
+         (m (mod (+ 15 (- p) k (- q)) 30))  ; correction to starting point of calculation each century
+         (d (mod (+ (* 19 a) m) 30))        ; number of days from March 21st until the full moon
+         (n (mod (+ 4 k (- q)) 7))          ; century-based offset in weekly calculation
+         (b (mod year 4))                   ; correction for leap days
+         (c (mod year 7))                   ; also a correction for leap days
+                                            ; days from d to next Sunday
+         (e (mod (+ (* 2 b) (* 4 c) (* 6 d) n) 7)))
+    ; historical corrections for April 26th and 25th
+    (when (or (and (= d 29) (= e 6))
+              (and (= d 28) (= e 6) (> a 10)))
+      (setq e -1))
+    (cl-values
+     ; determination of the correct month for Easter
+     (if (> (+ 22 d e) 31)
+         (format "April %s" (+ d e -9))
+       (format "March %s" (+ 22 d e)))
+     ; optionally return a value for the Servois table
+     (if servois (mod (+ 21 d) 31)))))
+
+(princ (format "%s"
+               (string-join
+                '("The following are the dates of the Pascal full moon (using Servois\n"
+                  "notation) and the date of Easter for 2020-2030 AD:\n"
+                  "Year    Servois number    Easter\n"))))
+(dolist (year (number-sequence 2020 2030))
+  (cl-multiple-value-bind (easter servois) (computus year t)
+    (princ (format "%-8s%-18s%-s\n" year servois easter))))

contents/computus/computus.md

@@ -328,6 +328,8 @@ For now, we have the code outputting a tuple of $$d$$ and $$e$$, so users can us
 [import, lang:"javascript"](code/javascript/gauss_easter.js)
 {% sample lang="typescript" %}
 [import, lang:"typescript"](code/typescript/gauss_easter.ts)
+{% sample lang="elisp" %}
+[import, lang:"elisp"](code/elisp/gauss-easter.el)
 {% endmethod %}