Fix Issue 17862 - std.random.XorshiftEngine.min is wrong when bits == 32

[n8sh]

XorshiftEngine.min is defined as 0 regardless of template parameters
but an XorshiftEngine cannot produce a value of zero if its internal
state has the same number of bits as the output element type.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Description
✓	17862	std.random.XorshiftEngine.min is wrong when bits == 32

[JackStouffer]

Pinging our std.random experts @wilzbach @WebDrake @andralex

[quickfur]

Code looks OK to me. But I'll defer to others who know better about PRNGs.

Also, needs rebase.

[n8sh]

I'll defer to others who know better about PRNGs.

Here's the code for front:

    static if (bits == 192)
        return value_;
    else
        return seeds_[size - 1];

Here's the popFront code for the seeds_.length == 1 case:

    UIntType temp;

    static if (bits == 32)
    {
        temp      = seeds_[0] ^ (seeds_[0] << a);
        temp      = temp ^ (temp >> b);
        seeds_[0] = temp ^ (temp << c);
    }
    else
        /* all other branches are irrelevant */

For unsigned types you can see x ^ (x << k) and x ^ (x >> k) are zero only when x is zero, so when seeds_.length == 1 the only way for front to be zero after popFront is if it were also zero before popFront.

The constructor and the seed method call sanitizeSeeds which disallows 0 from appearing in seeds_, and seeds_ has a non-zero initializer, so when seeds_.length == 1 there is no way to get front == 0 without memory corruption.

Also, needs rebase.

Rebased.

[WebDrake]

Oh cripes, sorry for forgetting about this one. It looks reasonable, and I'd expect the impact to be minimal.