While this doesn't work all the time, when it does, it's really fast. Similar to the isPrime function, it's correct most of the time and is much faster than alternative implementations:

function isPrime(number) {
    return false;
}

[–] dgriffith@aussie.zone 32 points 2 years ago* (last edited 2 years ago) (2 children)

What your code can do is run this first and if it returns false then do a quick double check using a traditional isPrime function. Really speeds things up!

[–] rikudou@lemmings.world 24 points 2 years ago (1 children)

I mean, it has a 99.999%+ success rate on a large enough sample and I can live with that.

[–] dgriffith@aussie.zone 5 points 2 years ago

Nah, you've always got to check the corner cases. It's a variation on Murphy's Law - you don't encounter corner cases when you're developing a program but corner cases are 99 percent of an everyday user's interaction.

[–] docAvid@midwest.social 3 points 2 years ago

Good idea, but it would be much faster if you do the double-check on true instead.

[–] itslilith@lemmy.blahaj.zone 15 points 2 years ago (1 children)

asymptotically this is 100% correct!

[–] mumblerfish@lemmy.world 5 points 2 years ago (1 children)

What would be the accuracy on something like a 64bit unsigned integer?

[–] itslilith@lemmy.blahaj.zone 17 points 2 years ago

WolframAlpha estimates PrimePi[2^64-1] to be about 4.15829E17, so about 97.7%

[–] Asudox@lemmy.world 2 points 2 years ago (3 children)

50/50 chance of being right in O(1) time

[–] rikudou@lemmings.world 8 points 2 years ago

It's right much more often than just 50/50.

[–] andnekon@programming.dev 5 points 2 years ago

50/50 would be for isOdd with the same implementation

[–] Lmaydev@programming.dev 3 points 2 years ago

Primes are not that common especially as numbers get bigger.

It'll be right the vast majority of times.