Implement Roots for BigInt and BigUint #56

mancabizjak · 2018-07-11T10:11:34Z

Supersedes #51 .

Since there is now a Roots trait with sqrt, cbrt and nth_root methods in the num-integer crate, this PR implements it for BigInt and BigUint types. I also added inherent methods on both types to allow the users access to all these functions without having to import Roots.

PS: nth_root currently uses num_traits::pow. Should we perhaps wait for #54 to get merged, and then replace the call to use the new pow::Pow implementation on BigUint?

This commit implements num-integer::Roots trait for BigInt and BigUint types, and also adds sqrt, cbrt, nth_root as inherent methods to allow access to them without importing Roots trait. For each type tests were added as submodules in the roots test module. Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

cuviper

Requested a few small changes.

The new Pow ought to be faster, since it can do ops by reference (i.e. less cloning), but we could just make that improvement in a followup.

cuviper · 2018-07-12T22:22:53Z

src/biguint.rs

+        }
+
+        let n = n as usize;
+        let n_min_1 = (n as usize) - 1;


Don't need to cast n as usize twice.

cuviper · 2018-07-12T22:27:34Z

src/biguint.rs

+
+    /// Returns the truncated principal square root of `self` --
+    /// see [Roots::sqrt](Roots::sqrt).
+    // struct.BigInt.html#trait.Roots


These links don't look right -- did you forget to complete this?

Eh, forgot to remove.

cuviper · 2018-07-12T22:31:21Z

src/biguint.rs

@@ -1026,6 +1026,52 @@ impl Integer for BigUint {
    }
 }

+impl Roots for BigUint {
+    fn nth_root(&self, n: u32) -> Self {


I think it would probably be valuable to have dedicated sqrt and cbrt implementations -- did you try it? Maybe benchmarks would prove me wrong, but we can also wait until later.

Here are the benchmark results:

test roots_cbrt ... bench: 48,670 ns/iter (+/- 1,322) test roots_cbrt_nth_root_3 ... bench: 52,285 ns/iter (+/- 1,541) test roots_sqrt ... bench: 53,752 ns/iter (+/- 4,954) test roots_sqrt_nth_root_2 ... bench: 57,622 ns/iter (+/- 1,585)

roots_cbrt and roots_sqrt test the optimized versions - I took sqrt from #51 and adjusted it for cbrt. The other two benchmarks just called nth_root(2) and nth_root(3) - of course, for the sake of benchmark, I didn't forward the calls to nth_root with n=2 and n=3 to the optimized sqrt and cbrt, but I'll definitely do that, since I see this is how it's done in impls for primitive types as well.

I would prefer to incorporate the dedicated implementations in this PR, although I'm struggling a little since I'm not particularly fond of having parts of nth_root, sqrt, cbrt replicated in all the methods. But I'm not too sure what (if anything) would be best to do about it 🙂

cuviper · 2018-07-12T22:34:22Z

tests/roots.rs

+    use std::str::FromStr;
+
+    fn check(x: i32, n: u32, expected: i32) {
+        let big_x: BigUint = FromPrimitive::from_i32(x).unwrap();


If you use an unsigned type instead of i32, then From makes this simpler -- BigUint::from(x).

cuviper · 2018-07-12T22:35:01Z

tests/roots.rs

+    use num_traits::FromPrimitive;
+
+    fn check(x: i32, n: u32, expected: i32) {
+        let big_x: BigInt = FromPrimitive::from_i32(x).unwrap();


Similarly, BigInt::from(x) should be fine.

cuviper · 2018-07-12T22:39:32Z

benches/bigint.rs

+    // hugely impacts the performance of nth_root due to exponentiation to
+    // the power of n-1. Using very large values for n is also not very realistic,
+    // and any n > x's bit size produces 1 as a result anyway.
+    let n: u8 = rng.gen();


This is from a seeded RNG, so what value do you get? I think you might as well just pick a reasonable n directly. Random x is still fine.

This commit overrides default implementations of Roots::sqrt and Roots::cbrt for BigInt and BigUint with optimized ones. It also improves tests and resolves minor inconsistencies. Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

mancabizjak · 2018-07-13T11:05:16Z

Thanks for the review! I overrided sqrt and cbrt methods with versions slightly more performant than their default implementations with nth_root. Recurring checks for trivial cases are now a bit more compact.

In addition, after glancing at the test code for implementations of Roots on PrimInts, I felt that the test code was not thorough enough, so I improved it a little.

I also fixed some inconsistencies in comments and made panic messages a bit clearer.

cuviper

I understand your concern about duplicating similar code, but I think the performance gain is worth it, and it's not that much code.

cuviper · 2018-07-17T00:34:13Z

src/bigint.rs

+
+    /// Returns the truncated principal square root of `self` --
+    /// see [Roots::sqrt](Roots::sqrt).
+    // struct.BigInt.html#trait.Roots


Another stray link? And I think all of these links like [Roots::sqrt](Roots::sqrt) won't actually work, AFAICS.

When I run cargo doc --no-deps --open and navigate to the page of BigInt or BigUint structs, paths of the form Roots::sqrt are rendered as links to https://docs.rs/num-integer/0.1/num_integer/trait.Roots.html#method.sqrt - I'm a little confused, is this not what we want? I was merely following this RFC, but I could be missing something.

Ah, it seems that works on nightly, but not beta or stable rustc yet. We should make those explicit for now.

cuviper · 2018-07-17T00:41:16Z

src/biguint.rs

+        let n_min_1 = n - 1;
+
+        let bit_len = self.len() * big_digit::BITS;
+        let guess = BigUint::one() << (bit_len/n + 1);


The bit_len can be just self.bits(). That should also get a better initial guess when the most-significant word is less "full".

Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

cuviper · 2018-07-19T00:00:06Z

Thanks!

bors r+

56: Implement Roots for BigInt and BigUint r=cuviper a=mancabizjak Supersedes #51 . Since there is now a `Roots` trait with `sqrt`, `cbrt` and `nth_root` methods in the `num-integer` crate, this PR implements it for `BigInt` and `BigUint` types. I also added inherent methods on both types to allow the users access to all these functions without having to import `Roots`. PS: `nth_root` currently uses `num_traits::pow`. Should we perhaps wait for #54 to get merged, and then replace the call to use the new `pow::Pow` implementation on `BigUint`? Co-authored-by: Manca Bizjak <manca.bizjak@xlab.si>

bors · 2018-07-19T00:09:32Z

Build succeeded

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mancabizjak mentioned this pull request Jul 11, 2018

Implement sqrt for BigInt and BigUint #51

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mancabizjak force-pushed the impl-roots branch from ce04a0f to 1f25906 Compare July 11, 2018 10:15

cuviper requested changes Jul 12, 2018

View reviewed changes

Implement optimized sqrt, cbrt methods

2b473e9

This commit overrides default implementations of Roots::sqrt and Roots::cbrt for BigInt and BigUint with optimized ones. It also improves tests and resolves minor inconsistencies. Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

mancabizjak force-pushed the impl-roots branch from 990a520 to 2b473e9 Compare July 13, 2018 10:53

cuviper reviewed Jul 17, 2018

View reviewed changes

mancabizjak added 2 commits July 17, 2018 15:06

Remove stale doc path, use self.bits()

bbe7b18

Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

Update links to Roots trait methods

1d45ca9

Signed-off-by: Manca Bizjak <manca.bizjak@xlab.si>

cuviper approved these changes Jul 18, 2018

View reviewed changes

bors bot merged commit 1d45ca9 into rust-num:master Jul 19, 2018

mancabizjak deleted the impl-roots branch July 30, 2018 11:15

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Implement Roots for BigInt and BigUint #56

Implement Roots for BigInt and BigUint #56

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Implement Roots for BigInt and BigUint #56

Implement Roots for BigInt and BigUint #56

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