Better testing through Google

[simont]

The other day I needed a lot of primes in a hurry, and I judged that it would be quicker just to sit down and write a simple Sieve of Eratosthenes program from memory than to faff about trying to google up one that somebody had already written and that wasn't in some unhelpful language.

So I wrote one; then I disposed of the obvious bugs by checking its output rigorously for numbers up to 10; then I ran it for numbers up to 100 and didn't see anything obviously wrong (though I didn't look that hard). Then I wondered whether there was any good way to be more confident of its correctness.

On a whim, I made it generate all the primes up to 2^32, and fed the output text file to md5sum (with Unix line endings). Then I pasted the resulting checksum into Google – and found a hit! Somebody else had generated the same set of primes, checksummed them in exactly the same way, and posted the MD5 on a web forum which was talking about prime-generating programs so that other people on the forum could use it as a test case. Just the confirmation I wanted.

The silly thing is that if I'd tried to google for things like ‘md5sum of primes up to 2^32’, it wouldn't have been remotely successful. But once you already know what you think the answer is (at least in cases where it's a mess of digits), googling for that will tell you whether anyone else agreed with you.

Comments

[pm215]

mnementh$ primes 1 > /tmp/zz9.primes
mnementh$ md5sum /tmp/zz9.primes 
037a526651ff4d6babb3b1a23bb83097  /tmp/zz9.primes

...right?

(That's the standard BSD primes(6). 2^32-1 is as high as it will go, so it's only just good enough for this job...)

[simont]

That's the one – turns up the same single Google hit as I got.

I only found out about primes(6) after I wrote mine. But I made sure mine did go higher than 2^32, even if I didn't ask it to on this occasion...

[fivemack.livejournal.com]

I tend to find this more infuriating than useful; to google for the result of a two-month computation that you thought was new and exciting and to get a hit does not fill me with the most ecstatic kind of well-fermented joy.

[simont]

If you want well-fermented joy, you should try an off-licence rather than Google!

But yes, I can imagine that would be less fun. I get a similar effect (though typically preceded by rather fewer months of work) when I think of a clever pun, google for it, and find I'm only the second or fifth or tenth person to think of it. (If everybody had thought of it before, that's less frustrating, for some reason; the pessimal thing is to be the second.) I suppose it's the difference between googling for something hoping it's new, and googling for it hoping it's not.

[tigerfort.livejournal.com]

(If everybody had thought of it before, that's less frustrating, for some reason; the pessimal thing is to be the second.)

I suspect that's because if you're the second, then you've just missed out on a clever idea, whereas if 10^8 other people have thought of it, then it's a human universal that comes to everyone.

[mooism.livejournal.com]

Ah, Google and the "If Wikimedia Made Rainbow Tables" sketch.

[gerald_duck]

Rapidly finding primes up to 2³² became a lot easier when computers started having more than 512Mbytes of RAM.