Random123/MicroURNG.hpp

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/*
Copyright 2010-2011, D. E. Shaw Research.
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#ifndef __MicroURNG_dot_hpp__
#define __MicroURNG_dot_hpp__

#include <stdexcept>
#include <limits>

namespace r123{
template<typename CBRNG, unsigned int _BITS>
class MicroURNG{
    // According to C++0x, a URNG requires only a result_type,
    // operator()(), min() and max() methods.  Everything else
    // (ctr_type, key_type, reset() method, etc.) is "value added"
    // for the benefit of users that "know" that they're dealing with
    // a MicroURNG.
public:
    typedef CBRNG cbrng_type;
    static const unsigned BITS = _BITS;
    typedef typename cbrng_type::ctr_type ctr_type;
    typedef typename cbrng_type::key_type key_type;
    typedef typename ctr_type::value_type result_type;

    result_type operator()(){
        if(last_elem == 0){
            // jam n into the high bits of c
            if( n >= (((R123_ULONG_LONG)1)<<BITS) )
                throw std::runtime_error("Incremented high bits of MicroURNG's counter too many times");
            const size_t W = std::numeric_limits<result_type>::digits;
            ctr_type c = c0;
            c[c0.size()-1] |= n<<(W-BITS);
            rdata = b(c,k);
            n++;
            last_elem = rdata.size();
        }
        return rdata[--last_elem];
    }
    MicroURNG(cbrng_type _b, ctr_type _c0, key_type _k) : b(_b), c0(_c0), k(_k), n(0), last_elem(0) {
        chkhighbits();
    }
    MicroURNG(ctr_type _c0, key_type _k) : b(), c0(_c0), k(_k), n(0), last_elem(0) {
        chkhighbits();
    }
    result_type min() const{
        return std::numeric_limits<result_type>::min();
    }
    result_type max() const{
        return std::numeric_limits<result_type>::max();
    }
    // extra methods:
    const ctr_type& counter() const{ return c0; }
    const key_type& key() const{ return k; }
    void reset(ctr_type _c0, key_type _k){
        c0 = _c0;
        chkhighbits();
        k = _k;
        n = 0;
        last_elem = 0;
    }

private:
    cbrng_type b;
    ctr_type c0;
    key_type k;
    R123_ULONG_LONG n;
    size_t last_elem;
    ctr_type rdata;
    void chkhighbits(){
        result_type r = c0[c0.size()-1];
        result_type mask = std::numeric_limits<result_type>::max()>>BITS;
        if((r&mask) != r)
            throw std::runtime_error("MicroURNG: c0, does not have high bits clear");
    }
};
} // namespace r123
#endif