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feat: add comparison function for Duration #14472

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feat: add comparison function for Duration #14472

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49 changes: 49 additions & 0 deletions lib/elixir/lib/calendar/duration.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -299,6 +299,55 @@ defmodule Duration do
}
end

@doc """
Compares two durations.

Returns `:eq` if the durations are equal, `:lt` if the first duration is less than the second, and `:gt` if the first
duration is greater than the second.

## Examples

iex> Duration.compare(Duration.new!(second: 10), Duration.new!(second: 20))
:lt
iex> Duration.compare(Duration.new!(second: 20), Duration.new!(second: 10))
:gt
iex> Duration.compare(Duration.new!(second: 10), Duration.new!(second: 10))
:eq
"""
@doc since: "1.19.0"
@spec compare(t, t) :: :eq | :lt | :gt
def compare(%Duration{} = d1, %Duration{} = d2) do
d1_us = to_microseconds(d1)
d2_us = to_microseconds(d2)

cond do
d1_us == d2_us -> :eq
d1_us < d2_us -> :lt
d1_us > d2_us -> :gt
end
end

@second_in_us 1_000_000 # 1 second in microseconds
@minute_in_us @second_in_us * 60 # 1 minute = 60 seconds
@hour_in_us @minute_in_us * 60 # 1 hour = 60 minutes
@day_in_us @hour_in_us * 24 # 1 day = 24 hours
@week_in_us @day_in_us * 7 # 1 week = 7 days
@month_in_us @day_in_us * 30 # 1 month = 30 days
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I don't think we can compare durations without knowing the date context since months are not always 30 days long.

As an example:

duration_a = Duration.new(month: 1)
duration_b = Duration.new(day: 30)

Duration.compare(duration_a, duration_b) # => :eq
Date.shift(~D[2020-02-01], duration_a)  #=> ~D[2020-03-01]
Date.shift(~D[2020-02-01], duration_b)  #=> ~D[2020-03-02]

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And for DateTime.shift it might even depend on the timezone if there's DST changes.

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@yordis yordis May 6, 2025 •
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@maennchen yeah I was wondering if year must be kept outside the comparing; the Kernel.to_timeout/1 already raise an exception based on month and year ... It would be the same with days 😭

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@maennchen maennchen May 6, 2025 •
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We could have something like this for convenience:

def compare(relative_to) do
  fn a, b ->
    # calc based on relative_to
  end
end

But essentially all you would be doing is:

[Naive]Date[Time].compare(
  [Naive]Date[Time].shift(relative_to, a),
  [Naive]Date[Time].shift(relative_to, b)
)

I'm wondering if the better course it to just add some docs pointing users to compare shifted dates instead of comparing durations directly and explain why.

@year_in_us @day_in_us * 365 # 1 year = 365 days

def to_microseconds(duration) do
{microsecond, _precision} = duration.microsecond

duration.year * @year_in_us +
duration.month * @month_in_us +
duration.week * @week_in_us +
duration.day * @day_in_us +
duration.hour * @hour_in_us +
duration.minute * @minute_in_us +
duration.second * @second_in_us +
microsecond
end

@doc """
Parses an [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601#Durations) formatted duration string to a `Duration` struct.

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