Luke Boswell (May 03 2023 at 00:35): Luke Boswell (May 03 2023 at 00:35):We were discussing the internal representation of time, and I suggested we change from U128 to I64 for the internal representation as this was what @Hannes is using for the DateTime lib. I've started looking at updating roc-lang/basic-cli to make this change for Utc.roc and have remembered why I used U128. The Rust library provides nanoseconds with U128 e.g. core::time::Duration::as_nanos(&self) -> u128 and the platform currently measures time at that level of precision. Is this an issue that needs to be resolved? I assume we can always just use Num.toI64 : Int * -> I64 as required. when converting to a DateTime.
Richard Feldman (May 03 2023 at 00:37):interesting, I wonder why they chose that :face_with_raised_eyebrow:
Luke Boswell (May 03 2023 at 00:40):Duration has historically lacked a way to get the actual number of nanoseconds it contained as a normal Rust type because u64 was of insufficient range, and f64 of insufficient precision. The u128 type solves both issues, so I propose adding an as_nanos function to expose the capability.
Richard Feldman (May 03 2023 at 00:41):looking at https://man7.org/linux/man-pages/man2/clock_gettime.2.html and https://stackoverflow.com/a/471287 it appears that if you get the time in Linux, the answer is specified as:
i32 or i64 (it's up to the particular C implementation which to choose; if they choose i32 they are susceptible to the Year 2038 problem)i32 Hannes (May 03 2023 at 00:44):That's how I've defined duration in my datetime library:
Duration : { seconds : I64, nanoseconds : U32 }
copied from Rust's chrono crate.
Richard Feldman (May 03 2023 at 00:45):so I guess if you're storing seconds as i64 and nanoseconds as i32 then if you want to combine those into one single nanoseconds value, I think you need another 30 bits (1 nanosecond is 1e9 seconds, 2^30 is a little over 1e9)
Richard Feldman (May 03 2023 at 00:46):if nanoseconds is u32 and seconds is i64 then I think putting them together you should be able to just barely fit them both into a single i128 number of nanoseconds unless my math is wrong :sweat_smile:
Hannes (May 03 2023 at 01:29):The maximum value that can be stored is Num.maxI64 seconds + 999_999_999 nanoseconds = 2^63 seconds, which I think is more than Num.maxI128 nanoseconds = (2^127 − 1) nanoseconds
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Hannes (May 03 2023 at 01:32):The way chrono handles this is if you try to convert a duration to nanoseconds that is longer than i64::MAX nanoseconds it returns None
Brendan Hansknecht (May 03 2023 at 01:32):No, it fits: 2^63*10^9+999,999,999 = 9...e+27 and 2^127 = 1...e+38
Brendan Hansknecht (May 03 2023 at 01:33):your result has a negative amount of days because the nanoseconds being subtracted are bigger than the amount of seconds.
Brendan Hansknecht (May 03 2023 at 01:35):They fit with tons of extra space, right? You need only 93 bits. So you have 34 bits to spare.
Brendan Hansknecht (May 03 2023 at 01:36):2^93-(2^63*10^9+999,999,999) = 6.80148277428e+26. So everything fits in the 93 bits if I understand correctly.
Hannes (May 03 2023 at 01:37):Ah, yes, you're correct. I got the right result but interpreted the negative sign wrong!
Last updated: Jul 06 2025 at 12:14 UTC