Calculate the correct week number for weeks starting at any date. #4857

http://en.wikipedia.org/wiki/ISO_week

Contributed by Holger Just

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3789 e93f8b46-1217-0410-a6f0-8f06a7374b81

diff --git a/public/javascripts/calendar/calendar.js b/public/javascripts/calendar/calendar.js
index 9088e0e897681b380f9046a954a1b0ba3d9d672c..3c8a5912e1dbbc4f03e3e74efe8eb66d5979db1b 100644 (file)
--- a/public/javascripts/calendar/calendar.js
+++ b/public/javascripts/calendar/calendar.js
@@ -1694,15 +1694,27 @@ Date.prototype.getDayOfYear = function() {
        return Math.floor(time / Date.DAY);
 };
 
-/** Returns the number of the week in year, as defined in ISO 8601. */
+/** Returns the number of the week in year, as defined in ISO 8601.
+    This function is only correct if `this` is the first day of the week. */
 Date.prototype.getWeekNumber = function() {
-       var d = new Date(this.getFullYear(), this.getMonth(), this.getDate(), 0, 0, 0);
-       var DoW = d.getDay();
-       d.setDate(d.getDate() - (DoW + 6) % 7 + 3); // Nearest Thu
-       var ms = d.valueOf(); // GMT
-       d.setMonth(0);
-       d.setDate(4); // Thu in Week 1
-       return Math.round((ms - d.valueOf()) / (7 * 864e5)) + 1;
+       var d =  new Date(this.getFullYear(), this.getMonth(), this.getDate());
+       var days = 1000*60*60*24; // one day in milliseconds
+       
+       // get the thursday of the current week
+       var this_thursday = new Date(
+               d.valueOf() // selected date
+               - (d.getDay() % 7)*days   // previous sunday
+               + 4*days                  // + 4 days
+       ).valueOf();
+       
+       // the thursday in the first week of the year
+       var first_thursday = new Date(
+               new Date(this.getFullYear(), 0, 4).valueOf() // January 4 is in the first week by definition
+               - (d.getDay() % 7)*days   // previous sunday
+               + 4*days                  // + 4 days
+       ).valueOf();
+       
+       return Math.round((this_thursday - first_thursday) / (7*days)) + 1;
 };
 
 /** Checks date and time equality */