on Sun Oct 02 2016, Hooman Mehr <swift-users-AT-swift.org> wrote: >> On Oct 2, 2016, at 5:23 PM, Dave Abrahams via swift-users >> <swift-users@swift.org> wrote: >> Presumably you mean: >> >> let r2 = r±0.0005 // turn a Rational into a Double with no more than >> // .0005 rounding error >> >> ? That is supercute! >> > > It is actually the other way around: It returns a rational given a > double so that the maximum difference of the created rational with the > input double is the specified tolerance.
Oh, nifty. How about the other direction? >> Have you thought about what this would look like after we land >> https://github.com/apple/swift/pull/3796 (see >> https://github.com/apple/swift-evolution/blob/master/proposals/0104-improved-integers.md) >> ? >> > > That is the next thing I want to look at, the next chance I get. Thank you! > For now, it isn’t much more than a quick hack on a slow Friday… > >> You can also find a working prototype of that code at >> > https://github.com/apple/swift/blob/b7622b41756e33bdf3f3415320ffec52aec73281/test/Prototypes/Integers.swift.gyb >> It would be great to know that the protocols we've designed actually >> work out for Rational numbers. With these protocols, can you make your >> Rational generic on the underlying integer type? >> >> There's a BigInt implementation based on these protocols here: >> https://github.com/natecook1000/swift/commit/45c52a75dcc15024649a5e093a5da4ee031595c2 >> >> It would be really cool to see it work with a generic version of your >> Rational type. > > Interesting idea. A BigInt type can really lift Rational to new > levels. Interesting case study to see how well the new protocols work. Quite so! [FWIW, I rather wish that BigInt was using two's complement representation, as the protocols were designed to make that work smoothly] >> It's interesting to see that one wants a different algorithm for GCD >> when using BigInts: >> https://en.wikipedia.org/wiki/Binary_GCD_algorithm#cite_note-11 >> >> I wonder how that should be dispatched… > > Moreover, there is something I ran into after posting: On today’s CPU > architectures with fast integer divide, simple binary CGD is slower > than basic Euler’s algorithm using remainders. Binary GCD needs the > equivalent of __builtin_ctz to run faster. The new integer protocols expose that as a "leadingZeros" property. > It would be nice if compiler could detect the shift loops and replace > the whole loop with x86 BSFL instruction. Talk to the llvm people :-). You'll want to use masking shifts under the new integer protocols. > I need to put some thought on whether there is an efficient means of > switching the algorithm based on the actual size (or nature) of the > given Int type, otherwise we may have to specialize, which is not good > from a generics standpoint. It depends exactly what you mean by that term :-) -- -Dave _______________________________________________ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
