repeating random number sequences.

[monarch_dodra]

On Tuesday, 27 August 2013 at 12:59:19 UTC, Jason den Dulk wrote:
> Hi
>
> This code
>
>   foreach (j; 0..5)
>     writeln(rndGen().take(5));
>   writeln(uniform(0, 1024));
>   foreach (j; 0..5)
>     writeln(rndGen().take(5));
>
> produces this output
>
>   [3410716173, 2484862302, 280352965, 1820347603, 850366086]
>   [3410716173, 2484862302, 280352965, 1820347603, 850366086]
>   [3410716173, 2484862302, 280352965, 1820347603, 850366086]
>   [3410716173, 2484862302, 280352965, 1820347603, 850366086]
>   [3410716173, 2484862302, 280352965, 1820347603, 850366086]
>   813
>   [2484862302, 280352965, 1820347603, 850366086, 604192828]
>   [2484862302, 280352965, 1820347603, 850366086, 604192828]
>   [2484862302, 280352965, 1820347603, 850366086, 604192828]
>   [2484862302, 280352965, 1820347603, 850366086, 604192828]
>   [2484862302, 280352965, 1820347603, 850366086, 604192828]
>
> Is this correct behaviour? It appears a little inconsistant to 
> me.
>
> Regards
> Jason

No. It is incorrect behavior. It is a *very*  big problem we know 
about, and are trying to fix. The problem (arguably) is so big, 
it warrants making a brand new random2 module.

Long story short: The current PRNG's a value types. This means 
that when you pass them around (by value) you are duplicating 
them. The fix (for us) is to make them all  reference types, so 
that passign them around doesn't duplicate them.

In the meantime, workarounds include: Generating a new PRGN every 
time you need a new sequence, or creating your own wrapper around 
rndGen();

struct GoodPrng
{
     enum empty = false;
     void popFront()
     {
         rndGen().popFront();
     }
     auto front() @property
     {
         return rndGen().front;
     }
}
GoodPrng goodPrng()
{
     return GoodPrng();
}

This should work:
void main()
{
     foreach (j; 0..5)
         writeln(GoodPrng().take(5));
     writeln(uniform(0, 1024));
     foreach (j; 0..5)
         writeln(GoodPrng().take(5));
}

[47855835, 916983782, 2006604655, 2074198403, 772414269]
[2220086136, 3393309461, 3644080841, 1053550911, 2595436893]
[3782843356, 2688374946, 3181159978, 628742771, 1672837671]
[192566424, 3205182805, 521392827, 1528745543, 2713259487]
[1188847012, 820882915, 1616362385, 837154982, 553045938]
400
[2350166972, 951558946, 1940218749, 1245693761, 3154088887]
[4184834298, 2262977512, 3554532516, 3345988025, 1171565042]
[3698692802, 538668063, 4044473111, 666762521, 865383943]
[3357865623, 3653338316, 276404459, 1764534280, 1999822962]
[1118055308, 3030179246, 3422085781, 4056620356, 201644357]

We apologize for the inconvenience.