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 36 points 9 months ago* (last edited 9 months ago) (3 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 25 points 9 months 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 6 points 9 months 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 5 points 9 months ago (1 children)

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

[–] xmunk@sh.itjust.works 1 points 9 months ago

This is a power(ful) idea.

Are my stats/programmers in the house?

[–] fibojoly@sh.itjust.works 4 points 9 months ago

Better. Return true if the number is in a stored list of known primes, otherwise return false right away. But then, start a separate thread with an actual verification algorithm. When the verification is done, if it was actually a prime number, you just crash the program with a WasActuallyPrime exception.

[–] itslilith@lemmy.blahaj.zone 16 points 9 months ago (1 children)

asymptotically this is 100% correct!

[–] mumblerfish@lemmy.world 5 points 9 months ago (1 children)

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

[–] itslilith@lemmy.blahaj.zone 17 points 9 months ago

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

[–] Asudox@lemmy.world 2 points 9 months ago (3 children)

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

[–] rikudou@lemmings.world 8 points 9 months ago

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

[–] andnekon@programming.dev 5 points 9 months ago

50/50 would be for isOdd with the same implementation

[–] Lmaydev@programming.dev 3 points 9 months ago

Primes are not that common especially as numbers get bigger.

It'll be right the vast majority of times.