Implement RFC #2169 (Euclidean modulo). #49389

fanzier · Mar 26, 2018

Tracking issue: #49048

rust-highfive · Mar 26, 2018

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varkor · Mar 26, 2018

varkor requested changes Mar 26, 2018

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This is looking good! I've left a few very minor comments, but apart from that and fixing the tidy errors, I can't see any problems!

varkor · Mar 26, 2018

src/libcore/num/mod.rs

@@ -1318,6 +1460,40 @@ $EndFeature, "
            }
        }

+
+        doc_comment! {
+            concat!("Calculates the modulo of Euclidean divsion `self.mod_euc(rhs)`.


Nit: I would call this "the remainder" or "the modulus", as "modulo" is the operation, rather than the result.

Changing to "remainder" since "modulus" is another word for "divisor".

varkor · Mar 26, 2018

src/libcore/num/mod.rs

+
+
+        doc_comment! {
+            concat!("Calculates the modulo `self mod rhs` by Euclidean division.


varkor · Mar 26, 2018

src/libcore/num/mod.rs

+        doc_comment! {
+            concat!("Calculates the quotient of Euclidean division of `self` by `rhs`.
+
+This computes the integer n such that `self = n * rhs + self.mod_euc(rhs)`.


Nit: backticks around n for consistency (and in f32/f64).

varkor · Mar 26, 2018

src/libcore/num/mod.rs

+            #[unstable(feature = "euclidean_division", issue = "49048")]
+            #[inline]
+            pub fn wrapping_div_euc(self, rhs: Self) -> Self {
+                self.div_euc(rhs)


This can be the standard self / rhs to avoid the extra branch (like with wrapping_mod_euc).

Good catch, thanks.

varkor · Mar 27, 2018

varkor approved these changes Mar 27, 2018

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Looks good to me 👍

fanzier · Mar 27, 2018

I was just wondering whether the f32/f64 code should live in libstd or in libcore. Since it doesn't depend on anything special, I think it should be in core, not std. On the other hand, there is a lot of stuff in libstd/f32.rs that I would have guessed should be in libcore/num/f32.rs. What is the policy regarding this?

varkor · Mar 28, 2018

As far as I'm aware, the libstd methods are limited to those that rely on platform libraries / intrinsics (e.g. most of the mathematical methods). Pure-Rust methods tend to reside in libcore (which also makes them available for no_std). I wouldn't be surprised if the situation isn't entirely consistent at the moment.

~~I think libcore was the correct choice here.~~

Edit: just realised the floating-point methods are not in libcore right now. See later comment.

fanzier · Mar 28, 2018

@varkor That makes sense. So I should move the float methods to libcore?

Edit: Apparently, to move it there, I would have to add the div_euc and mod_euc methods to the num::Float trait, which seems like overkill. Let's keep it as is?

varkor · Mar 28, 2018

I got a little confused before, sorry! I think it would be better to move the methods to libcore and delegate to them in libstd, yes (see, for example, the relatively recent to_bits, which does the same). You will have to extend the Float trait, but this should also allow the methods to be used on no_std, which is useful and expected. (It's not like the Float trait is being kept deliberately minimal as far as I can tell.)

fanzier · Mar 28, 2018

Fair enough, I'll move it into libcore. But since div_euc uses f32::trunc, should I move trunc from libstd into num::Float as well, or use intrinsics::truncf32 directly instead?

varkor · Mar 28, 2018

But since div_euc uses f32::trunc, should I move trunc from libstd into num::Float as well, or use intrinsics::truncf32 directly instead?

Ah, I didn't spot that. In that case, let's just leave it in libstd for now. If anyone needs it in libcore later on, it can be moved then (unless anyone else has some strong opinions now?). I think the integer methods are the most important ones, anyway.

shepmaster · Apr 7, 2018

Ping from triage, @KodrAus ! Will you have time to review soon?

KodrAus · Apr 12, 2018

KodrAus reviewed Apr 12, 2018

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Sorry for the late review! This is looking good, thanks @fanzier. I've just left a few nitpicky comments about the docs. Once we're happy with those I think we can push this through.

KodrAus · Apr 12, 2018

src/libcore/num/mod.rs

+            concat!("Wrapping Euclidean division. Computes `self.div_euc(rhs)`,
+wrapping around at the boundary of the type.
+
+The only case where such wrapping can occur is when one divides `MIN / -1` on a signed type (where


The wording here feels a little wonky. How about:

Wrapping will occur in MIN / -1 on a signed type (where MIN is the negative minimal value for the type). This is equivalent to -MIN, a positive value that is too large to represent in the type. In this case, this method returns MIN itself.

I almost copy-pasted that documentation from wrapping_div. Should I update the documentation there as well or should that be a separate PR?

KodrAus · Apr 12, 2018

src/libcore/num/mod.rs

+            concat!("Wrapping Euclidean modulo. Computes `self.mod_euc(rhs)`, wrapping around at the
+boundary of the type.
+
+Such wrap-around never actually occurs mathematically; implementation artifacts make `x % y`


Same as above. How about:

Wrapping will occur in MIN % -1 on a signed type (where MIN is the negative minimal value for the type). In this case, this method return 0.

KodrAus · Apr 12, 2018

src/libcore/num/mod.rs

+            concat!("Calculates the quotient of Euclidean division `self.div_euc(rhs)`.
+
+Returns a tuple of the divisor along with a boolean indicating whether an arithmetic overflow would
+occur. If an overflow would occur then self is returned.


We should put self in backticks here.

KodrAus · Apr 13, 2018

Thanks @fanzier this looks good to me! I'll leave a note on the tracking issue about libcore vs libstd.

@bors r+

bors · Apr 13, 2018

📌 Commit ca4e458 has been approved by KodrAus

bors · Apr 13, 2018

⌛️ Testing commit ca4e458 with merge f9f9050...

bors · Apr 13, 2018

☀️ Test successful - status-appveyor, status-travis
Approved by: KodrAus
Pushing f9f9050 to master...

Implement RFC #2169 (Euclidean division).
Tracking issue: #49048

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Address nits and tidy errors.

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Fix #![feature]s.

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Fix doctest (typo).

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Tracking issue for RFC 2169: Euclidean Modulo #49048
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Auto merge of #49389 - fanzier:euclidean-division, r=KodrAus
Implement RFC #2169 (Euclidean modulo). Tracking issue: #49048

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Add inherent methods in libcore for [T], [u8], str, f32, and f64 #49896

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Tracking issue for integer methods for Wrapping #32463
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libcore and f32::{abs, mod_euc} #54876

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Implement RFC #2169 (Euclidean modulo). #49389

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