High performance function for creating hex encoded string from NSData

hexEncodedStringFromData.m

NSString *hexEncodedStringFromData(NSData *data)
{
    static const char hexChars[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

    // Store the length, otherwise it would be re-evalued on each loop iteration
    NSUInteger length = data.length;

    // malloc result data NULL terminated string
    char *resultData = malloc(length * 2 + 1);
    const unsigned char *sourceData = (const unsigned char *)data.bytes;

    // at least in my tests, performing the multiplication twice like this is always faster than 
    // storing & reusing it or using a separate counter that you increment by 2
    for (NSUInteger index = 0; index < length; index++) {
        // shift off 4 lowers bits
        resultData[index * 2] = hexChars[sourceData[index] >> 4];
        // mask off upper 4 bits
        resultData[index * 2 + 1] = hexChars[sourceData[index] & 15];
    }
    // NULL termination
    resultData[length * 2] = 0;

    // convert result to NSString
    NSString *result = [NSString stringWithCString:resultData encoding:NSASCIIStringEncoding];
    free(resultData);

    return result;
}