Optimize from_gregorian_seconds

Author: josevalim

lib/elixir/lib/calendar/iso.ex

@@ -1772,6 +1772,15 @@ defmodule Calendar.ISO do
     {days, day_fraction}
   end
 
+  @doc false
+  def gregorian_seconds_to_naive_datetime(seconds, microsecond) do
+    {days, seconds} = div_rem(seconds, @seconds_per_day)
+    {hours, seconds} = div_rem(seconds, @seconds_per_hour)
+    {minutes, seconds} = div_rem(seconds, @seconds_per_minute)
+    {year, month, day} = date_from_iso_days(days)
+    {year, month, day, hours, minutes, seconds, {microsecond, 6}}
+  end
+
   @doc false
   def iso_days_to_unit({days, {parts, ppd}}, unit) do
     day_microseconds = days * @parts_per_day

lib/elixir/lib/calendar/naive_datetime.ex

@@ -1058,10 +1058,16 @@ defmodule NaiveDateTime do
         calendar \\ Calendar.ISO
       )
       when is_integer(seconds) do
-    iso_days = Calendar.ISO.gregorian_seconds_to_iso_days(seconds, microsecond)
-
     {year, month, day, hour, minute, second, {microsecond, _}} =
-      calendar.naive_datetime_from_iso_days(iso_days)
+      case calendar do
+        Calendar.ISO ->
+          # Optimized version that skips unnecessary iso days conversion
+          Calendar.ISO.gregorian_seconds_to_naive_datetime(seconds, microsecond)
+
+        _ ->
+          iso_days = Calendar.ISO.gregorian_seconds_to_iso_days(seconds, microsecond)
+          calendar.naive_datetime_from_iso_days(iso_days)
+      end
 
     %NaiveDateTime{
       calendar: calendar,