r11722: add day-of-week

diff --git a/ChangeLog b/ChangeLog
index a0186a4d2db909412a1429eefb51604e93d62201..2bd645b887939ef4df07d84594c1c3ece55def40 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,6 @@
+01 Jun 2007  Kevin Rosenberg <kevin@rosenberg.net>
+       * datetime.lisp: Add day-of-week
+
 07 Jan 2007  Kevin Rosenberg <kevin@rosenberg.net>
        * Version 1.94
        * signals.lisp: Conditionalize Lispworks support to :unix *features*

diff --git a/datetime.lisp b/datetime.lisp
index 0346661cff309be2d75acf6480ccced646ab91bd..8357da030b95b5e028d80711e072e6fb83cfdc30 100644 (file)
--- a/datetime.lisp
+++ b/datetime.lisp
@@ -32,7 +32,7 @@
                   "July" "August" "September" "October" "November"
                   "December")
                 (1- mn))
-           (format nil "~A" dy) 
+           (format nil "~A" dy)
             (format nil "~A" yr)
            (format nil "~2,'0D:~2,'0D:~2,'0D" hr min sec))))
 
@@ -108,6 +108,20 @@
               mincol colinc minpad padchar
               (subseq monthstring 0 truncate)))))
 
+(defconstant* +zellers-adj+ #(0 3 2 5 0 3 5 1 4 6 2 4))
+
+(defun day-of-week (year month day)
+  "Day of week calculation using Zeller's Congruence.
+Input: The year y, month m (1 ≤ m ≤ 12) and day d (1 ≤ d ≤ 31).
+Output: n - the day of the week (Sunday = 0, Saturday = 6)."
+
+  (when (< month 3)
+    (decf year))
+  (mod
+   (+ year (floor year 4) (- (floor year 100)) (floor year 400)
+      (aref +zellers-adj+ (1- month)) day)
+   7))
+
 ;;;; Daylight Saving Time calculations
 
 ;; Daylight Saving Time begins for most of the United States at 2