MicroURNG.hpp

Go to the documentation of this file.

/*
Copyright 2010-2011, D. E. Shaw Research.
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

* Redistributions of source code must retain the above copyright
  notice, this list of conditions, and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright
  notice, this list of conditions, and the following disclaimer in the
  documentation and/or other materials provided with the distribution.

* Neither the name of D. E. Shaw Research nor the names of its
  contributors may be used to endorse or promote products derived from
  this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#ifndef __MicroURNG_dot_hpp__
#define __MicroURNG_dot_hpp__

#include <stdexcept>
#include <limits>

namespace r123{
template<typename CBRNG>
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 int BITS = 32;
    typedef typename cbrng_type::ctr_type ctr_type;
    typedef typename cbrng_type::key_type key_type;
    typedef typename cbrng_type::ukey_type ukey_type;
    typedef typename ctr_type::value_type result_type;

    R123_STATIC_ASSERT( std::numeric_limits<result_type>::digits >= BITS, "The result_type must have at least 32 bits" );

    result_type operator()(){
        if(last_elem == 0){
            // jam n into the high bits of c
            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, ukey_type _uk) : b(_b), c0(_c0), k(_uk), n(0), last_elem(0) {
        chkhighbits();
    }
    MicroURNG(ctr_type _c0, ukey_type _uk) : b(), c0(_c0), k(_uk), n(0), last_elem(0) {
        chkhighbits();
    }

    // _Min and _Max work around a bug in the library shipped with MacOS Xcode 4.5.2.
    // See the commment in conventional/Engine.hpp.  
    const static result_type _Min = 0;
    const static result_type _Max = ~((result_type)0);

    static R123_CONSTEXPR result_type min R123_NO_MACRO_SUBST () { return _Min; }
    static R123_CONSTEXPR result_type max R123_NO_MACRO_SUBST () { return _Max; }
    // extra methods:
    const ctr_type& counter() const{ return c0; }
    void reset(ctr_type _c0, ukey_type _uk){
        c0 = _c0;
        chkhighbits();
        k = _uk;
        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 = ((uint64_t)std::numeric_limits<result_type>::max R123_NO_MACRO_SUBST ())>>BITS;
        if((r&mask) != r)
            throw std::runtime_error("MicroURNG: c0, does not have high bits clear");
    }
};
} // namespace r123
#endif