Engine.hpp

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/*
Copyright 2010-2011, D. E. Shaw Research.
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#ifndef __Engine_dot_hpp_
#define __Engine_dot_hpp_
 
#include "../features/compilerfeatures.h"
#include "../array.h"
#include <limits>
#include <stdexcept>
#include <sstream>
#include <algorithm>
#include <vector>
#if R123_USE_CXX11_TYPE_TRAITS
#include <type_traits>
#endif
 
namespace r123{
template<typename CBRNG>
struct Engine {
    typedef CBRNG cbrng_type;
    typedef typename CBRNG::ctr_type ctr_type;
    typedef typename CBRNG::key_type key_type;
    typedef typename CBRNG::ukey_type ukey_type;
    typedef typename ctr_type::value_type result_type;
 
protected:
    cbrng_type b;
    key_type key;
    ctr_type c;
    ctr_type v;
 
    void fix_invariant(){
        if( v.back() != 0 ) {
            result_type vv = v.back();
            v = b(c, key);
            v.back() = vv;
        }
    }        
public:
    explicit Engine() : b(), c() {
        ukey_type x = {{}};
        v.back() = 0;
        key = x;
    }
    explicit Engine(result_type r) : b(), c() {
        ukey_type x = {{typename ukey_type::value_type(r)}};
        v.back() = 0;
        key = x;
    }
    // 26.5.3 says that the SeedSeq templates shouldn't particpate in
    // overload resolution unless the type qualifies as a SeedSeq.
    // How that is determined is unspecified, except that "as a
    // minimum a type shall not qualify as a SeedSeq if it is
    // implicitly convertible to a result_type."  
    //
    // First, we make sure that even the non-const copy constructor
    // works as expected.  In addition, if we've got C++11
    // type_traits, we use enable_if and is_convertible to implement
    // the convertible-to-result_type restriction.  Otherwise, the
    // template is unconditional and will match in some surpirsing
    // and undesirable situations.
    Engine(Engine& e) : b(e.b), key(e.key), c(e.c){
        v.back() = e.v.back();
        fix_invariant();
    }
    Engine(const Engine& e) : b(e.b), key(e.key), c(e.c){
        v.back() = e.v.back();
        fix_invariant();
    }
#if __cplusplus >= 201103L
    Engine& operator=(const Engine&) = default;
    Engine& operator=(Engine&&) = default;
#endif
 
    template <typename SeedSeq>
    explicit Engine(SeedSeq &s
#if R123_USE_CXX11_TYPE_TRAITS
                    , typename std::enable_if<!std::is_convertible<SeedSeq, result_type>::value>::type* =0
#endif
                    )
        : b(), c() {
        ukey_type ukey = ukey_type::seed(s);
        key = ukey;
        v.back() = 0;
    }
    void seed(result_type r){
        *this = Engine(r);
    }
    template <typename SeedSeq>
    void seed(SeedSeq &s
#if R123_USE_CXX11_TYPE_TRAITS
                    , typename std::enable_if<!std::is_convertible<SeedSeq, result_type>::value>::type* =0
#endif
              ){ 
        *this = Engine(s);
    }
    void seed(){
        *this = Engine();
    }
    friend bool operator==(const Engine& lhs, const Engine& rhs){
        return lhs.c==rhs.c && lhs.v.back() == rhs.v.back() && lhs.key == rhs.key;
    }
    friend bool operator!=(const Engine& lhs, const Engine& rhs){
        return lhs.c!=rhs.c || lhs.v.back()!=rhs.v.back() || lhs.key!=rhs.key;
    }
 
    friend std::ostream& operator<<(std::ostream& os, const Engine& be){
        return os << be.c << " " << be.key << " " << be.v.back();
    }
 
    friend std::istream& operator>>(std::istream& is, Engine& be){
        is >> be.c >> be.key >> be.v.back();
        be.fix_invariant();
        return is;
    }
 
    // The <random> shipped with MacOS Xcode 4.5.2 imposes a
    // non-standard requirement that URNGs also have static data
    // members: _Min and _Max.  Later versions of libc++ impose the
    // requirement only when constexpr isn't supported.  Although the
    // Xcode 4.5.2 requirement is clearly non-standard, it is unlikely
    // to be fixed and it is very easy work around.  We certainly
    // don't want to go to great lengths to accommodate every buggy
    // library we come across, but in this particular case, the effort
    // is low and the benefit is high, so it's worth doing.  Thanks to
    // Yan Zhou for pointing this out to us.  See similar code in
    // ../MicroURNG.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; }
 
    result_type operator()(){
        if( c.size() == 1 )     // short-circuit the scalar case.  Compilers aren't mind-readers.
            return b(c.incr(), key)[0];
        result_type& elem = v.back();
        if( elem == 0 ){
            v = b(c.incr(), key);
            result_type ret = v.back();
            elem = c.size()-1;
            return ret;
        }
        return v[--elem];
    }
 
    void discard(R123_ULONG_LONG skip){
        // don't forget:  elem counts down
        size_t nelem = c.size();
        size_t sub = skip % nelem;
        result_type& elem  = v.back();
        skip /= nelem;
        if (elem < sub) {
            elem += nelem;
            skip++;
        }
        elem -= sub;
        c.incr(skip);
        fix_invariant();
    }
         
    //--------------------------
    // Some bonus methods, not required for a Random Number
    // Engine
 
    // Constructors and seed() method for ukey_type seem useful
    // We need const and non-const to supersede the SeedSeq template.
    explicit Engine(const ukey_type &uk) : key(uk), c(){ v.back() = 0; }
    explicit Engine(ukey_type &uk) : key(uk), c(){  v.back() = 0; }
    void seed(const ukey_type& uk){
        *this = Engine(uk);
    }        
    void seed(ukey_type& uk){
        *this = Engine(uk);
    }        
 
#if R123_USE_CXX11_TYPE_TRAITS
    template <typename DUMMY=void>
    explicit Engine(const key_type& k,
                    typename std::enable_if<!std::is_same<ukey_type, key_type>::value, DUMMY>::type* = 0)
        : key(k), c(){ v.back() = 0; }
 
    template <typename DUMMY=void>
    void seed(const key_type& k,
              typename std::enable_if<!std::is_same<ukey_type, key_type>::value, DUMMY>::type* = 0){
        *this = Engine(k);
    }
#endif
 
    // Forward the e(counter) to the CBRNG we are templated
    // on, using the current value of the key.
    ctr_type operator()(const ctr_type& c) const{
        return b(c, key);
    }
 
    key_type getkey() const{
        return key;
    }
 
    // N.B.  setkey(k) is different from seed(k) because seed(k) zeros
    // the counter (per the C++11 requirements for an Engine), whereas
    // setkey does not.
    void setkey(const key_type& k){
        key = k;
        fix_invariant();
    }
 
    // Maybe the caller want's to know the details of
    // the internal state, e.g., so it can call a different
    // bijection with the same counter.
    std::pair<ctr_type, result_type> getcounter() const {
        return std::make_pair(c, v.back());
    }
 
    // And the inverse.
    void setcounter(const ctr_type& _c, result_type _elem){
        static const size_t nelem = c.size();
        if( _elem >= nelem )
            throw std::range_error("Engine::setcounter called  with elem out of range");
        c = _c;
        v.back() = _elem;
        fix_invariant();
    }
 
    void setcounter(const std::pair<ctr_type, result_type>& ce){
        setcounter(ce.first, ce.second);
    }
};
} // namespace r123
 
#endif