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Optimize Calendar.ISO microsecond-to-time conversion #15920

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Merged
josevalim merged 1 commit into from
Sep 20, 2026
+19 −7
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Optimize Calendar.ISO microsecond-to-time conversion #15920

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26 changes: 19 additions & 7 deletions lib/elixir/lib/calendar/iso.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -203,8 +203,12 @@ defmodule Calendar.ISO do
@seconds_per_day 24 * 60 * 60
@last_second_of_the_day @seconds_per_day - 1
@microseconds_per_second 1_000_000
@microseconds_per_minute @seconds_per_minute * @microseconds_per_second
@microseconds_per_hour @seconds_per_hour * @microseconds_per_second
@parts_per_day @seconds_per_day * @microseconds_per_second

@minutes_reciprocal div(1 <<< 42, @microseconds_per_minute >>> 8) + 1
@seconds_reciprocal div(1 <<< 40, @microseconds_per_second >>> 6) + 1
@minutes_from_seconds_reciprocal div(1 <<< 32, @seconds_per_minute) + 1
@hours_from_seconds_reciprocal div(1 <<< 32, @seconds_per_hour) + 1

Expand Down Expand Up @@ -890,17 +894,25 @@ defmodule Calendar.ISO do

def time_from_day_fraction({parts_in_day, parts_per_day}) do
total_microseconds = divide_by_parts_per_day(parts_in_day, parts_per_day)
{hours, microseconds_in_hour} = div_rem(total_microseconds, @microseconds_per_hour)

{hours, rest_microseconds1} =
div_rem(total_microseconds, @seconds_per_hour * @microseconds_per_second)

{minutes, rest_microseconds2} =
div_rem(rest_microseconds1, @seconds_per_minute * @microseconds_per_second)

{seconds, microseconds} = div_rem(rest_microseconds2, @microseconds_per_second)
{minutes, seconds, microseconds} = microseconds_in_hour_to_time(microseconds_in_hour)
{hours, minutes, seconds, {microseconds, 6}}
end

# Adapted from https://www.benjoffe.com/fast-time-of-day
@compile {:inline, microseconds_in_hour_to_time: 1}
defp microseconds_in_hour_to_time(microseconds_in_hour)
when microseconds_in_hour in 0..(@microseconds_per_hour - 1)//1 do
# Pre-shifts keep reciprocal products within small integers on 64-bit BEAM.
minutes = ((microseconds_in_hour >>> 8) * @minutes_reciprocal) >>> 42
seconds_in_hour = ((microseconds_in_hour >>> 6) * @seconds_reciprocal) >>> 40

microseconds = microseconds_in_hour - seconds_in_hour * @microseconds_per_second
seconds = band(seconds_in_hour + minutes * 4, 63)
{minutes, seconds, microseconds}
end

defp divide_by_parts_per_day(parts_in_day, @parts_per_day), do: parts_in_day

defp divide_by_parts_per_day(parts_in_day, parts_per_day),
Expand Down
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