Pseudo-random number generator

ysuh · June 2016

I need a random number generator that does not need to be true random number. Any idea on implementing a (pseudo-)random number generator?

ilya_baran · June 2016

Here's a simple linear congruential PRNG I wrote some time back (and how to use it) -- more sophisticated stuff would need more work...

function lcprng(seed is number) returns function
{
    const a = 1103515245;
    const c = 12345;
    const m = 2^31;
    var state = new box(seed);
    return function() {
        state[] = (a * state[] + c) % m;
        return state[];
    };
}
annotation { "Feature Type Name" : "My Feature" }
export const myFeature = defineFeature(function(context is Context, id is Id, definition is map)
    precondition
    {
        // Define the parameters of the feature type
    }
    {
        var x = lcprng(1);
        println(x());
        println(x());
        println(x());
        println(x());
        println(x());
    });

ysuh · June 2016

Brilliant! Thanks for quick answer. However, I want a new "random" number whenever a new feature instance is created. Since the initial seed value is fixed to 1, in this case, I think it will always generate the same number.

ilya_baran · June 2016

You can attach the rng to the context using setVariable (and read it in later features using getVariable) or you can turn the feature id passed in into a seed (that's a little bit of work -- you probably need to call splitIntoCharacters and then do individual character-to-number lookups in a map -- post here if you need help with this.)

ysuh · June 2016

Thanks a lot. The idea of using the feature id should work. It will generate the same number for each feature when it is regenerated, which is good.

ulrich_petri · August 2016

I'm using this function to generate a seed from the id:

const chrMap = {
    'A':  0, 'B':  1, 'C':  2, 'D':  3, 'E':  4, 'F':  5, 'G': 6, 
    'H':  7, 'I':  8, 'J':  9, 'K': 10, 'L': 11, 'M': 12, 'N': 13, 
    'O': 14, 'P': 15, 'Q': 16, 'R': 17, 'S': 18, 'T': 19, 'U': 20, 
    'V': 21, 'W': 22, 'X': 23, 'Y': 24, 'Z': 25,
    'a': 26, 'b': 27, 'c': 28, 'd': 29, 'e': 30, 'f': 31, 'g': 32, 
    'h': 33, 'i': 34, 'j': 35, 'k': 36, 'l': 37, 'm': 38, 'n': 39, 
    'o': 40, 'p': 41, 'q': 42, 'r': 43, 's': 44, 't': 45, 'u': 46, 
    'v': 47, 'w': 48, 'x': 49, 'y': 50, 'z': 51,
    '_': 99, '-': 98
};

function idToNum(input is string) returns number {
    var out is string = "";
    for (var char in splitIntoCharacters(input)) {<br>        var res = match(char, REGEX_NUMBER);
        if (res.hasMatch) {
            out = out ~ toString(res.captures[0]);
        } else {
            out = out ~ toString(chrMap[char]);
        }
    }
    return stringToNumber(out) % 100000;
}

// Usage with the rng:

var rng = lcprng(idToNum(id[0]));

greg_frost204 · May 2020

I realise this is a very old post, but ulrich_perti's code snippet seems to be incomplete. Any chance you could post the whole function?

_anton · May 2020

@greg_frost204

I imagine you could do something like this (source):

const chrMap = {
    'A':  0, 'B':  1, 'C':  2, 'D':  3, 'E':  4, 'F':  5, 'G': 6, 
    'H':  7, 'I':  8, 'J':  9, 'K': 10, 'L': 11, 'M': 12, 'N': 13, 
    'O': 14, 'P': 15, 'Q': 16, 'R': 17, 'S': 18, 'T': 19, 'U': 20, 
    'V': 21, 'W': 22, 'X': 23, 'Y': 24, 'Z': 25,
    'a': 26, 'b': 27, 'c': 28, 'd': 29, 'e': 30, 'f': 31, 'g': 32, 
    'h': 33, 'i': 34, 'j': 35, 'k': 36, 'l': 37, 'm': 38, 'n': 39, 
    'o': 40, 'p': 41, 'q': 42, 'r': 43, 's': 44, 't': 45, 'u': 46, 
    'v': 47, 'w': 48, 'x': 49, 'y': 50, 'z': 51,
    '0': 52, '1': 53, '2': 54, '3': 55, '4': 56, '5': 57, '6': 58,
    '7': 59, '8': 60, '9': 61,
    '_': 62, '-': 63
};

// hashes a string into the [0, 2^32-1] range (picked arbitrarily)
function hash(input is string) returns number
{
    const mod = 2^32;
    var result = 5381;
    for (var char in splitIntoCharacters(input))
    {
        result = (result * 33 + chrMap[char]) % mod;
    }
    return result;
}

EvanReese · August 2020

@ilya_baran
How would I approach limiting the domain of the values to 0 to 1 (or any arbitrary value range)?

EvanReese · August 2020

If I knew the theoretical max and min of the RNG then maybe this would work. Is this how you'd go about it?

export function remap(value, rngMin, rngMax, targetMin, targetMax)

{

return (value-rngMin)*(targetMax-targetMin)/(rngMax-rngMin)+ targetMin;

}

ilya_baran · August 2020

The remap function looks right. If you look at the code, the output is taken modulo 2^31, so the prng range is integers between 0 and 2^31 - 1, inclusive.

EvanReese · August 2020

Thanks for explaining, Ilya. I think I stopped studying math before I learned what modulo is

I'll try this out!

EvanReese · August 2020

hey! it works! thanks again

Pseudo-random number generator

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