Somewhat better unxorshift

unxorshift.c

uint64_t unxorshift(uint64_t xs, uint32_t shamt)
{
    // NB shamt >= 1 required; shamt=0 is not reversible for obvious reasons :)
    // I am still convinced there's a much tighter way to code this, but this one
    // is less terrible.
    uint64_t x = xs;
    for (uint32_t k = 32; k >= 1; k >>= 1)
    {
        uint32_t l = k * shamt;
        if (l < 64)
            x ^= x >> l;
    }

    return x;
}