Add ISO week number calculation functions to the calendar module in stdlib

This new feature adds the missing week number function to the calendar module
of the stdlib application. The implementation conforms to the ISO 8601 standard.
The new feature has been implemented tested and documented.

1 files changed, 32 insertions, 0 deletions

diff --git a/lib/stdlib/doc/src/calendar.xml b/lib/stdlib/doc/src/calendar.xml
index 36f0c03162..f90d8308b6 100644
--- a/lib/stdlib/doc/src/calendar.xml
+++ b/lib/stdlib/doc/src/calendar.xml
@@ -63,6 +63,14 @@
       given as local time, they must be converted to universal time, in
       order to get the correct value of the elapsed time between epochs.
       Use of the function <c>time_difference/2</c> is discouraged.</p>
+    <p>There exists different definitions for the week of the year.
+      The calendar module contains a week of the year implementation
+      which conforms to the ISO 8601 standard. Since the week number for
+      a given date can fall on the previous, the current or on the next
+      year it is important to provide the information which year is it
+      together with the week number. The function <c>iso_week_number/0</c>
+      and <c>iso_week_number/1</c> returns a tuple of the year and the
+      week number.</p>
   </description>
 
   <section>
@@ -154,6 +162,30 @@ time() = {Hour, Minute, Second}
       </desc>
     </func>
     <func>
+      <name>iso_week_number() -> IsoWeekNumber</name>
+      <fsummary>Compute the iso week number for the actual date</fsummary>
+      <type>
+        <v>IsoWeekNumber = {int(), int()}</v>
+      </type>
+      <desc>
+        <p>This function returns the tuple {Year, WeekNum} representing
+          the iso week number for the actual date. For determining the
+          actual date, the function <c>local_time/0</c> is used.</p>
+      </desc>
+    </func>
+    <func>
+      <name>iso_week_number(Date) -> IsoWeekNumber</name>
+      <fsummary>Compute the iso week number for the given date</fsummary>
+      <type>
+        <v>Date = date()</v>
+        <v>IsoWeekNumber = {int(), int()}</v>
+      </type>
+      <desc>
+        <p>This function returns the tuple {Year, WeekNum} representing
+          the iso week number for the given date.</p>
+      </desc>
+    </func>
+    <func>
       <name>last_day_of_the_month(Year, Month) -> int()</name>
       <fsummary>Compute the number of days in a month</fsummary>
       <desc>