MicroURNG.hpp

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

#include <stdexcept>
#include <limits>

namespace random_iterator_r123 {
  template <typename CBRNG>
  class MicroURNG {
    // According to C++11, 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;

    RANDOM_ITERATOR_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 RANDOM_ITERATOR_R123_CONSTEXPR result_type min
    RANDOM_ITERATOR_R123_NO_MACRO_SUBST() {
      return _Min;
    }
    static RANDOM_ITERATOR_R123_CONSTEXPR result_type max
    RANDOM_ITERATOR_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;
    RANDOM_ITERATOR_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
                              RANDOM_ITERATOR_R123_NO_MACRO_SUBST()) >>
                         BITS;
      if ((r & mask) != r)
        throw std::runtime_error("MicroURNG: c0, does not have high bits clear");
    }
  };
} // namespace random_iterator_r123
#endif