How to store CFBitVector (or any CFType) in Core Data using CFData/NSData?

Question

I have limited experience working with the Core Foundation types & collections, so apologies if this is obvious.

I'm using the CFBitVector type to store some bit sequences, and I need to store it in a binary data format (so that it can be added to a Core Data store). The most sensible thing seems to be to store this in a CFData type, which can be toll-free bridged with an NSData and added to the store, but I am unsure of how to do this.

Can anybody help me out with a simple example of storing CFTypes in CF/NSData?

---

Edit:

Is this even the right approach? Should I try converting the CFBitVector into a series of ints which can then be stored in the data model? Or perhaps a transformable attribute?

Answer 1

The way I ended up doing this was to roll my own attribute transformer in order to convert a CFBitVectorRef into an NSData instance. The benefit of this is that I can really cram the bit array tightly into a block of binary data, as in my case I really need to keep the storage size to a minimum.

Below is the implementation of my CFBitVectorTransformer class. It essentially reads each bit and packs them into unsigned chars ("segments" in the code below), which are then appended to a mutable NSData buffer. The code would work with types larger than unsigned chars, however I wanted the smallest chunks possible in order to really minimise the size of the resulting data.

#define kBitsPerByte    8


@implementation CFBitVectorTransformer


+ (Class)transformedValueClass
{
    return [NSData class];
}

+ (BOOL)allowsReverseTransformation
{
    return YES;
}

/* CFBitVectorRef -> NSData */
- (id)transformedValue:(id)value
{
    if (!value) return nil;
    if ([value isKindOfClass:[NSData class]]) return value;

    /* Prepare the bit vector. */
    CFBitVectorRef bitVector = (__bridge CFBitVectorRef)value;
    CFIndex bitVectorCount = CFBitVectorGetCount(bitVector);

    /* Prepare the data buffer. */
    NSMutableData *bitData = [NSMutableData data];
    unsigned char bitVectorSegment = 0;
    NSUInteger bytesPerSegment = sizeof(char);
    NSUInteger bitsPerSegment = bytesPerSegment * kBitsPerByte;

    for (CFIndex bitIndex = 0; bitIndex < bitVectorCount; bitIndex++) {
        /* Shift the bit into the segment the appropriate number of places. */
        CFBit bit = CFBitVectorGetBitAtIndex(bitVector, bitIndex);
        int segmentShift = bitIndex % bitsPerSegment;
        bitVectorSegment |= bit << segmentShift;

        /* If this is the last bit we can squeeze into the segment, or it's the final bit, append the segment to the data buffer. */
        if (segmentShift == bitsPerSegment - 1 || bitIndex == bitVectorCount - 1) {
            [bitData appendBytes:&bitVectorSegment length:bytesPerSegment];
            bitVectorSegment = 0;
        }
    }

    return [NSData dataWithData:bitData];
}

/* NSData -> CFBitVectorRef */
- (id)reverseTransformedValue:(id)value
{
    if (!value) return NULL;
    if (![value isKindOfClass:[NSData class]]) return NULL;

    /* Prepare the data buffer. */
    NSData *bitData = (NSData *)value;
    char *bitVectorSegments = (char *)[bitData bytes];
    NSUInteger bitDataLength = [bitData length];

    /* Prepare the bit vector. */
    CFIndex bitVectorCapacity = bitDataLength * kBitsPerByte;
    CFMutableBitVectorRef bitVector = CFBitVectorCreateMutable(kCFAllocatorDefault, bitVectorCapacity);
    CFBitVectorSetCount(bitVector, bitVectorCapacity);

    for (NSUInteger byteIndex = 0; byteIndex < bitDataLength; byteIndex++) {
        unsigned char bitVectorSegment = bitVectorSegments[byteIndex];

        /* Store each bit of this byte in the bit vector. */
        for (NSUInteger bitIndex = 0; bitIndex < kBitsPerByte; bitIndex++) {
            CFBit bit = bitVectorSegment & 1 << bitIndex;
            CFIndex bitVectorBitIndex = (byteIndex * kBitsPerByte) + bitIndex;
            CFBitVectorSetBitAtIndex(bitVector, bitVectorBitIndex, bit);
        }
    }

    return (__bridge_transfer id)bitVector;
}


@end

This nicely abstracts the conversion of data, allowing you to just set the CFBitVectorRef as an attribute in the data model, and should be plenty fast enough for most purposes.

I hope this helps somebody else in a similar situation.