drand48 is a small LCG and I think it also needs to be seeded before you use it with srand48; otherwise it’ll give the same sequence each time.
Saagar Jha > On May 24, 2017, at 08:15, Edward Connell <ewconn...@gmail.com> wrote: > > Thank you for all the feedback and options. In further investigation, I also > ran across the family of xx48 rngs, which appear to be on both platforms. > They claim to be uniformly distributed. > > drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48, seed48, srand48 > > http://pubs.opengroup.org/onlinepubs/007908775/xsh/drand48.html > <http://pubs.opengroup.org/onlinepubs/007908775/xsh/drand48.html> > > Any reason not to use one of these instead? > > Thanks, Ed > > On Tue, May 23, 2017 at 3:18 AM, Jens Persson <j...@bitcycle.com > <mailto:j...@bitcycle.com>> wrote: > Here's a stripped down verison of my implementation of the Xoroshiro128+ PRNG: > https://gist.github.com/anonymous/527602968812f853d6147aea8c66d660 > <https://gist.github.com/anonymous/527602968812f853d6147aea8c66d660> > > /Jens > > > On Mon, May 22, 2017 at 11:21 PM, Jens Persson <j...@bitcycle.com > <mailto:j...@bitcycle.com>> wrote: > Sorry for the premature send ... > Here is the site: http://xoroshiro.di.unimi.it <http://xoroshiro.di.unimi.it/> > There is also a section there about "generating uniform doubles in unit > interval" which is worth reading. > And here's how to get uniform floating point values in the range [0, 1) from > various (supposedly) random bit patterns: > extension Double { > init(unitRange v: UInt64) { > let shifts: UInt64 = 63 - UInt64(Double.significandBitCount) > self = Double(v >> shifts) * (.ulpOfOne/2) > } > init(unitRange v: UInt32) { > self = (Double(v) + 0.5) / (Double(UInt32.max) + 1.0) > } > init(unitRange v: UInt16) { > self = (Double(v) + 0.5) / (Double(UInt16.max) + 1.0) > } > init(unitRange v: UInt8) { > self = (Double(v) + 0.5) / (Double(UInt8.max) + 1.0) > } > } > extension Float { > init(unitRange v: UInt64) { > let shifts: UInt64 = 63 - UInt64(Float.significandBitCount) > self = Float(v >> shifts) * (.ulpOfOne/2) > } > init(unitRange v: UInt32) { > let shifts: UInt32 = 31 - UInt32(Float.significandBitCount) > self = Float(v >> shifts) * (.ulpOfOne/2) > } > init(unitRange v: UInt16) { > let a = Float(v) + 0.5 > let b = Float(UInt16.max) + 1.0 > self = a / b > } > init(unitRange v: UInt8) { > let a = Float(v) + 0.5 > let b = Float(UInt8.max) + 1.0 > self = a / b > } > } > > You will get a very fast and good quality prng using xoroshiro, converting to > unit range floating point and then back to uniform range int if you want to, > much much faster than arc4random. > > /Jens > > > > > On Mon, May 22, 2017 at 11:17 PM, Jens Persson <j...@bitcycle.com > <mailto:j...@bitcycle.com>> wrote: > Check out the generators (especially xoroshiro) on this site: > > On Mon, May 22, 2017 at 6:54 PM, Saagar Jha via swift-users > <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: > > Saagar Jha > >> On May 22, 2017, at 08:44, Edward Connell via swift-users >> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: >> >> Any ideas when Foundation on Linux will support arc4random_uniform? This is >> kind of an important function. >> There doesn't seem to be any decent substitute without requiring the >> installation of libbsd-dev, which turns out to be messy. Currently I am >> doing this, but glibc random with mod does not produce good quality numbers, >> due to modulo bias. > > Modulo bias is easy to deal with, though, if you force random to produce a > range that is a multiple of the range that you’re trying to produce: > > guard range > 0 else { return 0 } > var random: Int > repeat { > random = Int(random()) > } while(random > LONG_MAX / range * range) > return random % range > >> >> Has anyone come up with a better solution to get a true uniform distribution >> that isn't super messy? >> >> import Foundation >> >> #if os(Linux) >> import Glibc >> #endif >> >> >> public func random_uniform(range: Int) -> Int { >> guard range > 0 else { return 0 } >> #if os(Linux) >> return Int(random()) % range >> #else >> return Int(arc4random_uniform(UInt32(range))) >> #endif >> } >> >> >> Thanks, Ed >> _______________________________________________ >> swift-users mailing list >> swift-users@swift.org <mailto:swift-users@swift.org> >> https://lists.swift.org/mailman/listinfo/swift-users >> <https://lists.swift.org/mailman/listinfo/swift-users> > > > _______________________________________________ > swift-users mailing list > swift-users@swift.org <mailto:swift-users@swift.org> > https://lists.swift.org/mailman/listinfo/swift-users > <https://lists.swift.org/mailman/listinfo/swift-users> > > > > >
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