std_bitset.h

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    while ( __byte_ptr < __end_ptr ) {
      __result += _Bit_count<true>::_S_bit_count[*__byte_ptr];
      __byte_ptr++;
    }
    return __result;
  }

  unsigned long _M_do_to_ulong() const {
    if (sizeof(_WordT) <= sizeof(unsigned long))
        return static_cast<unsigned long>(_M_w);
    else {
      const _WordT __mask = static_cast<_WordT>(static_cast<unsigned long>(-1));
      if (_M_w & ~__mask)
        __STL_THROW(overflow_error("bitset"));
      return static_cast<unsigned long>(_M_w);
    }
  }

  size_t _M_do_find_first(size_t __not_found) const;

  // find the next "on" bit that follows "prev"
  size_t _M_do_find_next(size_t __prev, size_t __not_found) const;

};

//
// Definitions of non-inline functions from the single-word version of
//  _Base_bitset.
//

template <class _WordT>
_Base_bitset<1, _WordT>::_Base_bitset(unsigned long __val)
{
  _M_do_reset();
  const size_t __n = min(sizeof(unsigned long)*CHAR_BIT,
                         __BITS_PER_WORDT(_WordT)*_Nw);
  for(size_t __i = 0; __i < __n; ++__i, __val >>= 1)
    if ( __val & 0x1 )
      _M_w |= _S_maskbit(__i);
}

template <class _WordT>
size_t _Base_bitset<1, _WordT>::_M_do_find_first(size_t __not_found) const
{
  _WordT __thisword = _M_w;

  if ( __thisword != static_cast<_WordT>(0) ) {
    // find byte within word
    for ( size_t __j = 0; __j < sizeof(_WordT); __j++ ) {
      unsigned char __this_byte
        = static_cast<unsigned char>(__thisword & (~(unsigned char)0));
      if ( __this_byte )
        return __j*CHAR_BIT + _First_one<true>::_S_first_one[__this_byte];

      __thisword >>= CHAR_BIT;
    }
  }
  // not found, so return a value that indicates failure.
  return __not_found;
}

template <class _WordT>
size_t
_Base_bitset<1, _WordT>::_M_do_find_next(size_t __prev,
                                         size_t __not_found ) const
{
  // make bound inclusive
  ++__prev;

  // check out of bounds
  if ( __prev >= __BITS_PER_WORDT(_WordT) )
    return __not_found;

    // search first (and only) word
  _WordT __thisword = _M_w;

  // mask off bits below bound
  __thisword &= (~static_cast<_WordT>(0)) << _S_whichbit(__prev);

  if ( __thisword != static_cast<_WordT>(0) ) {
    // find byte within word
    // get first byte into place
    __thisword >>= _S_whichbyte(__prev) * CHAR_BIT;
    for ( size_t __j = _S_whichbyte(__prev); __j < sizeof(_WordT); __j++ ) {
      unsigned char __this_byte
        = static_cast<unsigned char>(__thisword & (~(unsigned char)0));
      if ( __this_byte )
        return __j*CHAR_BIT + _First_one<true>::_S_first_one[__this_byte];

      __thisword >>= CHAR_BIT;
    }
  }

  // not found, so return a value that indicates failure.
  return __not_found;
} // end _M_do_find_next

//
// One last specialization: _M_do_to_ulong() and the constructor from
// unsigned long are very simple if the bitset consists of a single
// word of type unsigned long.
//

__STL_TEMPLATE_NULL
inline unsigned long
_Base_bitset<1, unsigned long>::_M_do_to_ulong() const { return _M_w; }

__STL_TEMPLATE_NULL
inline _Base_bitset<1, unsigned long>::_Base_bitset(unsigned long __val) {
  _M_w = __val;
}

#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */

// ------------------------------------------------------------
// Helper class to zero out the unused high-order bits in the highest word.

#ifdef __STL_CLASS_PARTIAL_SPECIALIZATION

template <class _WordT, size_t _Extrabits> struct _Sanitize {
  static void _M_do_sanitize(_WordT& __val)
    { __val &= ~((~static_cast<_WordT>(0)) << _Extrabits); }
};

template <class _WordT> struct _Sanitize<_WordT, 0> {
  static void _M_do_sanitize(_WordT) {}
};

#else /* __STL_CLASS_PARTIAL_SPECIALIZATION */
 
template <class _WordT, size_t _Extrabits> struct _Sanitize {
  static void _M_do_sanitize(_WordT& __val) {
    if (_Extrabits != 0)
      __val &= ~((~static_cast<_WordT>(0)) << _Extrabits);
  }
};
 
#endif /* __STL_CLASS_PARTIAL_SPECIALIZATION */

// ------------------------------------------------------------
// Class bitset.
//   _Nb may be any nonzero number of type size_t.
//   Type _WordT may be any unsigned integral type.

template<size_t _Nb, class _WordT = unsigned long>
class bitset : private _Base_bitset<__BITSET_WORDS(_Nb,_WordT), _WordT>
{
private:
  typedef _Base_bitset<__BITSET_WORDS(_Nb,_WordT), _WordT> _Base;

  // Import base's protected interface.  Necessary because of new template
  // name resolution rules.

#ifdef __STL_HAS_NAMESPACES
  using _Base::_S_whichword;
  using _Base::_S_whichbyte;
  using _Base::_S_whichbit;
  using _Base::_S_maskbit;
  using _Base::_M_getword;
  using _Base::_M_hiword;
  using _Base::_M_do_and;
  using _Base::_M_do_or;
  using _Base::_M_do_xor;
  using _Base::_M_do_left_shift;
  using _Base::_M_do_right_shift;
  using _Base::_M_do_flip;
  using _Base::_M_do_set;
  using _Base::_M_do_reset;
  using _Base::_M_is_equal;
  using _Base::_M_is_any;
  using _Base::_M_do_count;
  using _Base::_M_do_to_ulong;
  using _Base::_M_do_find_first;
  using _Base::_M_do_find_next;
#endif /* __STL_HAS_NAMESPACES */

private:
  void _M_do_sanitize() {
    _Sanitize<_WordT,_Nb%__BITS_PER_WORDT(_WordT) >
      ::_M_do_sanitize(_M_hiword());
  }

public:

  // bit reference:
  class reference;
  friend class reference;
  
  class reference {
    friend class bitset;

    _WordT *_M_wp;
    size_t _M_bpos;

    // left undefined
    reference();

  public:
    reference( bitset& __b, size_t __pos ) {
      _M_wp = &__b._M_getword(__pos);
      _M_bpos = _S_whichbit(__pos);
    }

    ~reference() {}

    // for b[i] = __x;
    reference& operator=(bool __x) {
      if ( __x )
        *_M_wp |= _S_maskbit(_M_bpos);
      else
        *_M_wp &= ~_S_maskbit(_M_bpos);

      return *this;
    }

    // for b[i] = b[__j];
    reference& operator=(const reference& __j) {
      if ( (*(__j._M_wp) & _S_maskbit(__j._M_bpos)) )
        *_M_wp |= _S_maskbit(_M_bpos);
      else
        *_M_wp &= ~_S_maskbit(_M_bpos);

      return *this;
    }

    // flips the bit
    bool operator~() const { return (*(_M_wp) & _S_maskbit(_M_bpos)) == 0; }

    // for __x = b[i];
    operator bool() const { return (*(_M_wp) & _S_maskbit(_M_bpos)) != 0; }

    // for b[i].flip();
    reference& flip() {
      *_M_wp ^= _S_maskbit(_M_bpos);
      return *this;
    }
  };

  // 23.3.5.1 constructors:
  bitset() {}
  bitset(unsigned long __val) :
    _Base_bitset<__BITSET_WORDS(_Nb,_WordT), _WordT>(__val) {}

#ifdef __STL_MEMBER_TEMPLATES
  template<class _CharT, class _Traits, class _Alloc>
  explicit bitset(const basic_string<_CharT,_Traits,_Alloc>& __s,
                  size_t __pos = 0)
    : _Base()
  {
    if (__pos > __s.size())
      __STL_THROW(out_of_range("bitset"));
    _M_copy_from_string(__s, __pos, 
		        basic_string<_CharT,_Traits,_Alloc>::npos);
  }

  template<class _CharT, class _Traits, class _Alloc>
    bitset(const basic_string<_CharT, _Traits, _Alloc>& __s,
	   size_t __pos,
	   size_t __n)
    : _Base() 
    {
      if (__pos > __s.size()) 
	__STL_THROW(out_of_range("bitset"));
      _M_copy_from_string(__s, __pos, __n);
    }
#else 
  explicit bitset(const basic_string<char>& __s,
		  size_t __pos = 0,
		  size_t __n = basic_string<char>::npos) 
    : _Base() 
    {
      if (__pos > __s.size()) 
        __STL_THROW(out_of_range("bitset"));
      _M_copy_from_string(__s, __pos, __n);
    }

#endif /* __STL_MEMBER_TEMPLATES */

  // 23.3.5.2 bitset operations:
  bitset<_Nb,_WordT>& operator&=(const bitset<_Nb,_WordT>& __rhs) {
    _M_do_and(__rhs);
    return *this;
  }

  bitset<_Nb,_WordT>& operator|=(const bitset<_Nb,_WordT>& __rhs) {
    _M_do_or(__rhs);
    return *this;
  }

  bitset<_Nb,_WordT>& operator^=(const bitset<_Nb,_WordT>& __rhs) {
    _M_do_xor(__rhs);
    return *this;
  }

  bitset<_Nb,_WordT>& operator<<=(size_t __pos) {
    _M_do_left_shift(__pos);
    _M_do_sanitize();
    return *this;
  }

  bitset<_Nb,_WordT>& operator>>=(size_t __pos) {
    _M_do_right_shift(__pos);
    _M_do_sanitize();
    return *this;
  }

  //
  // Extension:
  // Versions of single-bit set, reset, flip, test with no range checking.
  //

  bitset<_Nb,_WordT>& _Unchecked_set(size_t __pos) {
    _M_getword(__pos) |= _S_maskbit(__pos);
    return *this;
  }

  bitset<_Nb,_WordT>& _Unchecked_set(size_t __pos, int __val) {
    if (__val)
      _M_getword(__pos) |= _S_maskbit(__pos);
    else
      _M_getword(__pos) &= ~_S_maskbit(__pos);

    return *this;
  }

  bitset<_Nb,_WordT>& _Unchecked_reset(size_t __pos) {
    _M_getword(__pos) &= ~_S_maskbit(__pos);
    return *this;
  }

  bitset<_Nb,_WordT>& _Unchecked_flip(size_t __pos) {
    _M_getword(__pos) ^= _S_maskbit(__pos);
    return *this;
  }

  bool _Unchecked_test(size_t __pos) const {
    return (_M_getword(__pos) & _S_maskbit(__pos)) != static_cast<_WordT>(0);
  }

  // Set, reset, and flip.

  bitset<_Nb,_WordT>& set() {
    _M_do_set();
    _M_do_sanitize();
    return *this;
  }

  bitset<_Nb,_WordT>& set(size_t __pos) {
    if (__pos >= _Nb)
      __STL_THROW(out_of_range("bitset"));

    return _Unchecked_set(__pos);
  }

  bitset<_Nb,_WordT>& set(size_t __pos, int __val) {
    if (__pos >= _Nb)
      __STL_THROW(out_of_range("bitset"));

    return _Unchecked_set(__pos, __val);
  }

  bitset<_Nb,_WordT>& reset() {
    _M_do_reset();
    return *this;
  }

  bitset<_Nb,_WordT>& reset(size_t __pos) {
    if (__pos >= _Nb)
      __STL_THROW(out_of_range("bitset"));

    return _Unchecked_reset(__pos);
  }

  bitset<_Nb,_WordT>& flip() {
    _M_do_flip();
    _M_do_sanitize();
    return *this;
  }

  bitset<_Nb,_WordT>& flip(size_t __pos) {
    if (__pos >= _Nb)
      __STL_THROW(out_of_range("bitset"));

    return _Unchecked_flip(__pos);
  }

  bitset<_Nb,_WordT> operator~() const {
    return bitset<_Nb,_WordT>(*this).flip();
  }

  // element access:
  //for b[i];
  reference operator[](size_t __pos) { return reference(*this,__pos); }
  bool operator[](size_t __pos) const { return _Unchecked_test(__pos); }

  unsigned long to_ulong() const { return _M_do_to_ulong(); }

#if  defined(__STL_MEMBER_TEMPLATES) && \
     defined(__STL_EXPLICIT_FUNCTION_TMPL_ARGS)

  template <class _CharT, class _Traits, class _Alloc>
  basic_string<_CharT, _Traits, _Alloc> to_string() const {
    basic_string<_CharT, _Traits, _Alloc> __result;
    _M_copy_to_string(__result);