Re: Speed of Random Numbers

[bauss via Digitalmars-d-learn]

On Saturday, 3 August 2019 at 17:47:46 UTC, Giovanni Di Maria 
wrote:
On Saturday, 3 August 2019 at 17:44:44 UTC, lithium iodate 
wrote:
On Saturday, 3 August 2019 at 16:35:34 UTC, Giovanni Di Maria 
wrote:
[...]

First off you could try to use a faster RNG engine than the 
default. The easiest way is to define a variable containing it 
and passing it to the functions each time.

auto rng = Xorshift(1234);
randomNumber = uniform!uint(rng);

This basic change approximately halved the 5 seconds your 
original example needs on my computer.
Another simple approach that I have tried is simply hashing 
the iterator using a fast hash function.
With xxHash32 I got the time down to 0.25 seconds. I also 
tried xxHash64 and FNV1a but they were not faster in my quick 
test.



Thank you very much Lithium Iodate
Now i will try it.
I let know you.
Thank you
Giovanni

If it doesn't matter if it's predictable or not then you could 
easily make your own simple random generator with would give 
"random" results.

Of course in general it's not usable:

import std.stdio;

class Random
{
    private:
    uint _seed;
    uint _interval;

    T abs(T)(T x)
    {
        T y = x > 0 ? T.max : cast(T)0;

        return (x ^ y) - y;
    }

    public:
    this()
    {
        import core.stdc.time;

        _seed = cast(uint)time(null);
        _interval = (_seed - 20559);
    }

    T next(T)(T max)
    {
        auto value = cast(T)(abs(_interval) % T.max);

        _interval -= (_interval / 10) * _seed;

        return value;
    }
}

void main()
{
    auto random = new Random;

    foreach (_; 0 .. 1000)
    {
        auto result = random.next!ubyte(255);
        writeln(result);
    }
}