Merge branch 'ih/calendar-iso-8601-erl' into dev

* ih/calendar-iso-8601-erl:
  Add ISO week number calculation functions to the calendar module in stdlib

OTP-9087

3 files changed, 103 insertions, 4 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>
diff --git a/lib/stdlib/src/calendar.erl b/lib/stdlib/src/calendar.erl
index ddc0666f77..57b7c28dee 100644
--- a/lib/stdlib/src/calendar.erl
+++ b/lib/stdlib/src/calendar.erl
@@ -28,6 +28,8 @@
 	 gregorian_days_to_date/1,
 	 gregorian_seconds_to_datetime/1,
 	 is_leap_year/1,
+	 iso_week_number/0,
+	 iso_week_number/1,
 	 last_day_of_the_month/2,
 	 local_time/0, 
 	 local_time_to_universal_time/1, 
@@ -70,6 +72,7 @@
 -type second()   :: 0..59.
 -type daynum()   :: 1..7.
 -type ldom()     :: 28 | 29 | 30 | 31. % last day of month
+-type weeknum()  :: 1..53.
 
 -type t_now()    :: {non_neg_integer(),non_neg_integer(),non_neg_integer()}.
 
@@ -77,6 +80,7 @@
 -type t_time()         :: {hour(),minute(),second()}.
 -type t_datetime()     :: {t_date(),t_time()}.
 -type t_datetime1970() :: {{year1970(),month(),day()},t_time()}.
+-type t_yearweeknum()  :: {year(),weeknum()}.
 
 %%----------------------------------------------------------------------
 
@@ -172,6 +176,42 @@ is_leap_year1(Year) when Year rem 400 =:= 0 ->
 is_leap_year1(_) -> false.
 
 
+%%
+%% Calculates the iso week number for the current date.
+%%
+-spec iso_week_number() -> t_yearweeknum().
+iso_week_number() ->
+    {Date, _} = local_time(),
+    iso_week_number(Date).
+
+
+%%
+%% Calculates the iso week number for the given date.
+%%
+-spec iso_week_number(t_date()) -> t_yearweeknum().
+iso_week_number({Year, Month, Day}) ->
+    D = date_to_gregorian_days({Year, Month, Day}),
+    W01_1_Year = gregorian_days_of_iso_w01_1(Year),
+    W01_1_NextYear = gregorian_days_of_iso_w01_1(Year + 1),
+    if W01_1_Year =< D andalso D < W01_1_NextYear ->
+	    % Current Year Week 01..52(,53)
+	    {Year, (D - W01_1_Year) div 7 + 1};
+	D < W01_1_Year ->
+	    % Previous Year 52 or 53
+	    PWN = case day_of_the_week(Year - 1, 1, 1) of
+		4 -> 53;
+		_ -> case day_of_the_week(Year - 1, 12, 31) of
+			4 -> 53;
+			_ -> 52
+		     end
+		end,
+	    {Year - 1, PWN};
+	W01_1_NextYear =< D ->
+	    % Next Year, Week 01
+	    {Year + 1, 1}
+    end.
+
+
 %% last_day_of_the_month(Year, Month)
 %%
 %% Returns the number of days in a month.
@@ -377,6 +417,19 @@ dty(Y, D1, D2) when D1 < D2 ->
 dty(Y, _D1, D2) ->
     {Y, D2}.
 
+%%
+%% The Gregorian days of the iso week 01 day 1 for a given year.
+%%
+-spec gregorian_days_of_iso_w01_1(year()) -> non_neg_integer().
+gregorian_days_of_iso_w01_1(Year) ->
+    D0101 = date_to_gregorian_days(Year, 1, 1),
+    DOW = day_of_the_week(Year, 1, 1),
+    if DOW =< 4 ->
+	D0101 - DOW + 1;
+    true ->
+	D0101 + 7 - DOW + 1
+    end.
+
 %% year_day_to_date(Year, DayOfYear)  = {Month,  DayOfMonth}
 %%
 %% Note: 1 is the first day of the month. 
diff --git a/lib/stdlib/test/calendar_SUITE.erl b/lib/stdlib/test/calendar_SUITE.erl
index 81b0299118..8192d035ca 100644
--- a/lib/stdlib/test/calendar_SUITE.erl
+++ b/lib/stdlib/test/calendar_SUITE.erl
@@ -28,7 +28,8 @@
 	 day_of_the_week_calibrate/1,
 	 leap_years/1,
 	 last_day_of_the_month/1,
-	 local_time_to_universal_time_dst/1]).
+	 local_time_to_universal_time_dst/1,
+	 iso_week_number/1]).
 
 -define(START_YEAR, 1947).			
 -define(END_YEAR, 2012).
@@ -38,7 +39,7 @@ suite() -> [{ct_hooks,[ts_install_cth]}].
 all() -> 
     [gregorian_days, gregorian_seconds, day_of_the_week,
      day_of_the_week_calibrate, leap_years,
-     last_day_of_the_month, local_time_to_universal_time_dst].
+     last_day_of_the_month, local_time_to_universal_time_dst, iso_week_number].
 
 groups() -> 
     [].
@@ -55,7 +56,6 @@ init_per_group(_GroupName, Config) ->
 end_per_group(_GroupName, Config) ->
     Config.
 
-
 gregorian_days(doc) ->
     "Tests that date_to_gregorian_days and gregorian_days_to_date "
     "are each others inverses from ?START_YEAR-01-01 up to ?END_YEAR-01-01. "
@@ -170,6 +170,15 @@ local_time_to_universal_time_dst_x(Config) when is_list(Config) ->
 	    {comment,"Bug in mktime() in this OS"}
     end.
 
+iso_week_number(doc) ->
+	"Test the iso week number calculation for all three possibilities."
+	" When the date falls on the last week of the previous year,"
+	" when the date falls on a week within the given year and finally,"
+	" when the date falls on the first week of the next year.";
+iso_week_number(suite) ->
+	[];
+iso_week_number(Config) when is_list(Config) ->
+	?line check_iso_week_number().
 
 %%
 %% LOCAL FUNCTIONS
@@ -259,7 +268,12 @@ check_last_day_of_the_month({SYr, SMon}, {EYr, EMon}) when SYr < EYr ->
 check_last_day_of_the_month(_, _) ->
     ok.
 
-
+%% check_iso_week_number
+%%
+check_iso_week_number() ->
+    ?line {2004, 53} = calendar:iso_week_number({2005, 1, 1}),
+    ?line {2007, 1} = calendar:iso_week_number({2007, 1, 1}),
+    ?line {2009, 1} = calendar:iso_week_number({2008, 12, 29}).