[WIP] fast multiplication #67
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| pub fn mul_wide(&self, rhs: &Self) -> (Self, Self) { |
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Please keep this function const fn.
You'll need to use loops instead of iterators, but otherwise it shouldn't be too difficult.
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Yes I switched to using while loops, but we can't use &mut T in const contexts and split_at, split_at_mut are not const fns. I'll have to redesign the impl if mul_wide is to be a const fn.
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Aah yeah, the use of split_at/split_at_mut is problematic for const fn.
The closest thing that's stable with const fn is probably subslice patterns, but I'm not aware of anything which would provide a proper analogue for split_at/split_at_mut.
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One way to move this forward would be to keep the current schoolbook multiplication method for const fn mul_wide, and then use the fast implementation in the Mul traits.
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Apparently there's a konst crate which implements split_at_mut as a const fn.
IIUC, for it to be constant-time it needs rust version 1.64 at minimum, and it's using some unsafe code.
However, it seems like this crate's MSRV is 1.65, so perhaps that'd work?
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Would prefer not to add additional dependencies. Curious how it works though... I thought all mut references were still unstable in const fn
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Regarding &mut T in const fns it seems like this is blocking that: rust-lang/rust#57349, although I may be wrong, I feel out of my depth here.
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Yeah, I'm confused how the implementation in the konst crate could work on stable. It uses from_raw_parts_mut which is marked const: unstable, and the function doesn't compile on 1.70.0: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=79b84c558fb0ada4fa9e6980a797aa93
error[E0658]: mutable references are not allowed in constant functions
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Closing in favor of #402 which implements Karatsuba, which seems more appropriate for our applications.
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Fixes #66
This is a draft PR to add Toom-Cook multiplication with 3 evaluation points to improve the complexity of multiplication.