Querying a bitmap distinguishing 0-values

I'm writing a piece of software that will process huge amounts of data. For memory efficiency, some of the fields in the records being processed are stored in a single bitmap.

Using #defines I declared what both the po开发者_如何学JAVAsition and the size of each field in the bitmap is.

Now I need a fast way to check against predefined values. My first attempt was this:

bool checkBit(unsigned short int a_bitMap, unsigned short int a_bitValue, unsigned short int a_bitPosition)
{
    // Values are always bit-0 based, so we need to shift here
    unsigned short int _value = (a_bitValue << a_bitPosition);
    return ((a_bitMap & _value) == _value);
}

Here, a_bitMap is the bitmap for the record being processed holding data for all fields, and a_bitValue is the (defined) coded value being checked (which would, in case of flags, always be 1).

This works and is fast, but this way I cannot use the value '0' as a coded value (so 3 bit give me only 7 options, not 8) so I came up with this solution:

bool checkBit(unsigned short int a_bitMap, unsigned short int a_bitValue, unsigned short int a_bitPosition, unsigned short int a_bitSize)
{
    static const unsigned short int bitmapSize = 8 * sizeof(a_bitMap);
    unsigned short int _shift = bitmapSize - (a_bitPosition + a_bitSize);
    a_bitMap = (a_bitMap << _shift);
    a_bitMap = (a_bitMap >> (a_bitPosition + _shift));
    return (a_bitMap == a_bitValue);
}

This works exactly the way I want... but it slows down the process dramatically. The number of records that need to be processed goes into the billions, so these extra shifts have a huge impact. That's also why I'd like to avoid std::bitset, since billions of object instantiations gives too much overhead as well.

How can I do this while still being able to distinguish 0-values?

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You probably want something like this:

bool checkBit(unsigned short int a_bitMap, unsigned short int a_bitPosition)
{
    unsigned short int _value = (1 << a_bitPosition);
    return (a_bitMap & _value) != 0;
}

This checks for the a_bitPosition's bit set/unset in a_bitMap and returns true if that specific bit is 1 and false otherwise.

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As a solution I'd define and pass in a mask rather than position and size so if you wanted a 2 bit data from position 3 and 4 of an 8 bit map you'd define

#define mask = 0x18;//binary 00011000

then in check bit you just need to:

return (map & mask) == value;