#Rolled a small descrete rng, plz rate

38 messages · Page 1 of 1 (latest)

night bay
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I rolled a small random number generator that should be able to handle weighted probabilities as a test to compare the code size of hand rolling to C++'s discrete rng, plz rate it and suggest how it can be made better, been a while since I did anything in C

https://godbolt.org/z/cr6sE9M8d

Compiler Explorer - C

int rand_discrete(int* distribution, size_t dist_size) {
// Error whatever you want here
if(dist_size == 0)
return -1;

int prob_total = distribution[0];
for(size_t i = 1; i < dist_size; ++i)
    prob_total += distribution[i];

int random_variable = rand() % prob_total;

double probability = (double)prob_t...
night bay
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@native root

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Hippity hoppity please review

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I want to be yelled at

thorn wren
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It might be helpful to plot this rng output on a graph or generate a large image of random grayscale colors to try and find patterns. rand() is usually an LCG which is fast but very bad at avoiding patterns, so it'd be a good test to see if your adaptation mitigates those patterns

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Oh wait this is simply adapting rand to a custom curve you "draw" with equally spaced buckets having different probabilities

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It seems fine from a code perspective, using modulo will skew the results slightly but it's difficult to fix and you're already using rand which is bad enough

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But after the last loop, is that even reachable? Maybe it would make sense to raise an assertion error there

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If it is reachable, maybe just return the index of the last bucket

night bay
thorn wren
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Rand is ...not great quality

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Especially the lower bits (which are what you get with modulo) have extremely predictable patterns

night bay
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Is there a better rng for C then?

thorn wren
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Not in the stdlib

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arc4random is good from bsd, libsodium has rng so good it's usable for crypto, pcg is good and easy to copy paste as it's 1 short function

night bay
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I'll check those out, I started this as a classmate was showing the C++ discrete randoms and mentioning how he didn't know you could that

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Which led me to making this

thorn wren
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Apparently arc4random is also crypto secure

night bay
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Hence why it's kinda trash

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But now I gotta make this good

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I'll keep these in mind and fix it up

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Danke

native root
native root
night bay
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Wouldn't an endless loop of cryptographic (or even non cryptographic) hashes be sufficient to break that kind of uniformity?

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Ie hash the stored value then save the hash as the new value

native root
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Sure

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Cryptographically secure random number generators are generally based on block ciphers

night bay
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Then why are LCGs used?

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Is the speed diff that important?

native root
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LCGs aren't even fast 😅

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(Faster than a csprng probably, but very slow compared to better non-crypto prngs)

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It's just that's what was used in the 80's when C was a thing so obviously it must still be used now because we can't move on from mistakes of the past

night bay
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Based

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Danke

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Kinda wanna make this an actual library now