bit_field.h

/*---------------------------------------------------------
Copyright (c) 2015 Jeff Preshing
 
This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any damages
arising from the use of this software.
 
Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:
 
1. The origin of this software must not be misrepresented; you must not
   claim that you wrote the original software. If you use this software
   in a product, an acknowledgement in the product documentation would be
   appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be
   misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.
---------------------------------------------------------*/
 
#ifndef __CPP11OM_BITFIELD_H__
#define __CPP11OM_BITFIELD_H__
 
#include <cassert>
namespace util {
template <typename T, int Offset, int Bits> struct BitFieldMember {
    T value;
 
    static_assert(Offset + Bits <= (int)sizeof(T) * 8, "Member exceeds bitfield boundaries");
    static_assert(Bits < (int)sizeof(T) * 8, "Can't fill entire bitfield with one member");
 
    static constexpr T Maximum = (T(1) << Bits) - 1;
    static constexpr T Mask = Maximum << Offset;
    T maximum() const { return Maximum; }
    T one() const { return T(1) << Offset; }
 
    operator T() const { return (value >> Offset) & Maximum; }
 
    BitFieldMember& operator=(T v) {
        assert(v <= Maximum); // v must fit inside the bitfield member
        value = (value & ~Mask) | (v << Offset);
        return *this;
    }
 
    BitFieldMember& operator+=(T v) {
        assert(T(*this) + v <= Maximum); // result must fit inside the bitfield member
        value += v << Offset;
        return *this;
    }
 
    BitFieldMember& operator-=(T v) {
        assert(T(*this) >= v); // result must not underflow
        value -= v << Offset;
        return *this;
    }
 
    BitFieldMember& operator++() { return *this += 1; }
    BitFieldMember operator++(int) { // postfix form
        BitFieldMember tmp(*this);
        operator++();
        return tmp;
    }
    BitFieldMember& operator--() { return *this -= 1; }
    BitFieldMember operator--(int) { // postfix form
        BitFieldMember tmp(*this);
        operator--();
        return tmp;
    }
};

template <typename T, int BaseOffset, int BitsPerItem, int NumItems> class BitFieldArray {
public:
    T value;
 
    static_assert(BaseOffset + BitsPerItem * NumItems <= (int)sizeof(T) * 8, "Array exceeds bitfield boundaries");
    static_assert(BitsPerItem < (int)sizeof(T) * 8, "Can't fill entire bitfield with one array element");
 
    static const T Maximum = (T(1) << BitsPerItem) - 1;
    T maximum() const { return Maximum; }
    int numItems() const { return NumItems; }
 
    class Element {
    private:
        T& value;
        int offset;
 
    public:
        Element(T& value, int offset)
        : value(value)
        , offset(offset) {}
        T mask() const { return Maximum << offset; }
 
        operator T() const { return (value >> offset) & Maximum; }
 
        Element& operator=(T v) {
            assert(v <= Maximum); // v must fit inside the bitfield member
            value = (value & ~mask()) | (v << offset);
            return *this;
        }
 
        Element& operator+=(T v) {
            assert(T(*this) + v <= Maximum); // result must fit inside the bitfield member
            value += v << offset;
            return *this;
        }
 
        Element& operator-=(T v) {
            assert(T(*this) >= v); // result must not underflow
            value -= v << offset;
            return *this;
        }
 
        Element& operator++() { return *this += 1; }
        Element operator++(int) { // postfix form
            Element tmp(*this);
            operator++();
            return tmp;
        }
        Element& operator--() { return *this -= 1; }
        Element operator--(int) { // postfix form
            Element tmp(*this);
            operator--();
            return tmp;
        }
    };
 
    Element operator[](int i) {
        assert(i >= 0 && i < NumItems); // array index must be in range
        return Element(value, BaseOffset + BitsPerItem * i);
    }
 
    const Element operator[](int i) const {
        assert(i >= 0 && i < NumItems); // array index must be in range
        return Element(value, BaseOffset + BitsPerItem * i);
    }
};
} // namespace util

#define BEGIN_BF_DECL(typeName, T)                                                                                                         \
    union typeName {                                                                                                                       \
        struct {                                                                                                                           \
            T val;                                                                                                                         \
        } backing;                                                                                                                         \
        typeName(T v = 0) { backing.val = v; }                                                                                             \
        typeName& operator=(T v) {                                                                                                         \
            backing.val = v;                                                                                                               \
            return *this;                                                                                                                  \
        }                                                                                                                                  \
        operator T&() { return backing.val; }                                                                                              \
        operator T() const { return backing.val; }                                                                                         \
        T operator()() const { return backing.val; }                                                                                       \
        using StorageType = T;
 
#define BF_FIELD(memberName, offset, bits) util::BitFieldMember<StorageType, offset, bits> memberName;
 
#define BF_ARRAY(memberName, offset, bits, numItems) util::BitFieldArray<StorageType, offset, bits, numItems> memberName;
 
#define END_BF_DECL() }
#endif // __CPP11OM_BITFIELD_H__