Merge branch 'FNickRU/stdlib/optimize_calendar/PR-2121/OTP-15572' into maint

* FNickRU/stdlib/optimize_calendar/PR-2121/OTP-15572:
  Optimize calendar:gregorian_days_to_date/1

2 files changed, 41 insertions, 12 deletions

diff --git a/lib/stdlib/src/calendar.erl b/lib/stdlib/src/calendar.erl
index bb5d450cd6..3a083d9fda 100644
--- a/lib/stdlib/src/calendar.erl
+++ b/lib/stdlib/src/calendar.erl
@@ -529,24 +529,41 @@ valid_date({Y, M, D}) ->
 
 %% day_to_year(DayOfEpoch) = {Year, DayOfYear}
 %%
-%% The idea here is to first guess a year, and then adjust. Although
-%% the implementation is recursive, at most 1 or 2 recursive steps
+%% The idea here is to first set the upper and lower bounds for a year,
+%% and then adjust a range by interpolation search. Although complexity
+%% of the algorithm is log(log(n)), at most 1 or 2 recursive steps
 %% are taken.
-%% If DayOfEpoch is very large, we need far more than 1 or 2 iterations,
-%% since we just subtract a yearful of days at a time until we're there.
 %%
 -spec day_to_year(non_neg_integer()) -> {year(), day_of_year()}.
 day_to_year(DayOfEpoch) when DayOfEpoch >= 0 ->
-    Y0 = DayOfEpoch div ?DAYS_PER_YEAR,
-    {Y1, D1} = dty(Y0, DayOfEpoch, dy(Y0)),
+    YMax = DayOfEpoch div ?DAYS_PER_YEAR,
+    YMin = DayOfEpoch div ?DAYS_PER_LEAP_YEAR,
+    {Y1, D1} = dty(YMin, YMax, DayOfEpoch, dy(YMin), dy(YMax)),
     {Y1, DayOfEpoch - D1}.
 
--spec dty(year(), non_neg_integer(), non_neg_integer()) ->
+-spec dty(year(), year(), non_neg_integer(), non_neg_integer(),
+    non_neg_integer()) ->
 		{year(), non_neg_integer()}.
-dty(Y, D1, D2) when D1 < D2 -> 
-    dty(Y-1, D1, dy(Y-1));
-dty(Y, _D1, D2) ->
-    {Y, D2}.
+dty(Min, Max, _D1, DMin, _DMax) when Min == Max ->
+    {Min, DMin};
+dty(Min, Max, D1, DMin, DMax) ->
+    Diff = Max - Min,
+    Mid = Min + (Diff * (D1 - DMin)) div (DMax - DMin),
+    MidLength =
+        case is_leap_year(Mid) of
+            true -> ?DAYS_PER_LEAP_YEAR;
+            false -> ?DAYS_PER_YEAR
+        end,
+    case dy(Mid) of
+        D2 when D1 < D2 ->
+            NewMax = Mid - 1,
+            dty(Min, NewMax, D1, DMin, dy(NewMax));
+        D2 when D1 - D2 >= MidLength ->
+            NewMin = Mid + 1,
+            dty(NewMin, Max, D1, dy(NewMin), DMax);
+        D2 ->
+            {Mid, D2}
+    end.
 
 %%
 %% The Gregorian days of the iso week 01 day 1 for a given year.
diff --git a/lib/stdlib/test/calendar_SUITE.erl b/lib/stdlib/test/calendar_SUITE.erl
index df62c0921d..c6d9dbca4a 100644
--- a/lib/stdlib/test/calendar_SUITE.erl
+++ b/lib/stdlib/test/calendar_SUITE.erl
@@ -24,6 +24,7 @@
 -export([all/0, suite/0,groups/0,init_per_suite/1, end_per_suite/1, 
 	 init_per_group/2,end_per_group/2, 
 	 gregorian_days/1,
+	 big_gregorian_days/1,
 	 gregorian_seconds/1,
 	 day_of_the_week/1,
 	 day_of_the_week_calibrate/1,
@@ -36,13 +37,16 @@
 -define(START_YEAR, 1947).			
 -define(END_YEAR, 2012).
 
+-define(BIG_START_YEAR, 20000000).
+-define(BIG_END_YEAR, 20000020).
+
 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,
-     iso_week_number, system_time, rfc3339].
+     iso_week_number, system_time, rfc3339, big_gregorian_days].
 
 groups() -> 
     [].
@@ -67,6 +71,14 @@ gregorian_days(Config) when is_list(Config) ->
     MaxDays = calendar:date_to_gregorian_days({?END_YEAR, 1, 1}),
     check_gregorian_days(Days, MaxDays).
 
+%% Tests that date_to_gregorian_days and gregorian_days_to_date
+%% are each others inverses from ?BIG_START_YEAR-01-01 up to ?BIG_END_YEAR-01-01.
+%% At the same time valid_date is tested.
+big_gregorian_days(Config) when is_list(Config) ->
+    Days = calendar:date_to_gregorian_days({?BIG_START_YEAR, 1, 1}),
+    MaxDays = calendar:date_to_gregorian_days({?BIG_END_YEAR, 1, 1}),
+    check_gregorian_days(Days, MaxDays).
+
 %% Tests that datetime_to_gregorian_seconds and
 %% gregorian_seconds_to_date are each others inverses for a sampled
 %% number of seconds from ?START_YEAR-01-01 up to ?END_YEAR-01-01: We check