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impl Arbitrary for SystemTime #178

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impl Arbitrary for SystemTime #178

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42 changes: 41 additions & 1 deletion src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ use core::num::{NonZeroU128, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZ
use core::ops::{Range, RangeBounds, RangeFrom, RangeInclusive, RangeTo, RangeToInclusive};
use core::str;
use core::time::Duration;
use std::borrow::{Cow, ToOwned};
use std::borrow::Cow;
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, LinkedList, VecDeque};
use std::ffi::{CString, OsString};
use std::hash::BuildHasher;
Expand All @@ -53,6 +53,7 @@ use std::path::PathBuf;
use std::rc::Rc;
use std::sync::atomic::{AtomicBool, AtomicIsize, AtomicUsize};
use std::sync::{Arc, Mutex};
use std::time::SystemTime;

/// Generate arbitrary structured values from raw, unstructured data.
///
Expand Down Expand Up @@ -1272,6 +1273,32 @@ impl<'a> Arbitrary<'a> for SocketAddr {
}
}

impl<'a> Arbitrary<'a> for SystemTime {
fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
// Create a SystemTime by adding or subtracting a duration from epoch.
// The result is not guaranteed to fit. Keep trying until a good SystemTime if found.
loop {
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@nagisa nagisa Mar 16, 2024 •
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It seems unfortunate to have behaviour of this that is dependent on the target, but since the underlying type requires it, why not #[cfg] or even just check at runtime if the platform considers UNIX_EPOCH the minimal representable timestamp and just not bother with the add/sub on those targets?

Would save on the input bytes and such.

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The docs for SystemTime don't guarantee anything regarding what fits in a SystemTime. So I would not know what to check for. Maybe UNIX_EPOCH is the minimum. Maybe the year 0 is the minimum.

Also, the maximum is interesting. It is not guaranteed that SystemTime::UNIX_EPOCH + Duration::MAX will fit. Therefore, it needs to be checked.

I could determine the minimum and maximum by trail and error, save this result and use that to make sure we never generate a value that doesn't fit. But that isn't great code as well.

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@nagisa nagisa Mar 19, 2024 •
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UNIX_EPOCH.checked_sub(Duration::from_nanos(1)).is_none() implies that UNIX_EPOCH is the min value representable. This could be checked once and saved in a static or somesuch.

Unfortunately I've no good ideas for the maximum, so yeah, you would need the loop regardless. Feel free to ignore the comment until others chime in.

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As I understand, the minimum and maximum are platform-specific. I see three possible solutions:

  1. Try to generate a value, check whether in range, if not, loop (current patch)
  2. Try to find min and max, save those values, then generate a value in between (thus moving the loop to the process of finding the min and max)
  3. Work with upstream to define the min and max as a constant (Duration has a min/max, so why shouldn't SystemTime)

let add: bool = u.arbitrary()?;
let duration: Duration = u.arbitrary()?;
if add {
if let Some(system_time) = SystemTime::UNIX_EPOCH.checked_add(duration) {
return Ok(system_time);
}
} else if let Some(system_time) = SystemTime::UNIX_EPOCH.checked_sub(duration) {
return Ok(system_time);
}
}
}

fn size_hint(depth: usize) -> (usize, Option<usize>) {
size_hint::and_all(&[
bool::size_hint(depth),
Duration::size_hint(depth),
(0, None),
])
}
}

#[cfg(test)]
mod test {
use super::*;
Expand Down Expand Up @@ -1587,6 +1614,19 @@ mod test {
);
assert_eq!((1, None), <(u8, Vec<u8>) as Arbitrary>::size_hint(0));
}

#[test]
fn size_hint_for_system_time() {
let system_time = SystemTime::size_hint(0);

// SystemTime::size_hint as minimum of bool + Duration
assert_eq!(
system_time.0,
size_hint::and(bool::size_hint(0), Duration::size_hint(0)).0
);
// SystemTime::size_hint has no maximum
assert_eq!(system_time.1, None);
}
}

/// Multiple conflicting arbitrary attributes are used on the same field:
Expand Down