Can I Optimize this code with better array manipulation?

Question

I have a byte array of a certain size. I would like to grab it's last four bytes and put them next to each other to form a four byte hexadecimal value, in a variable. This way I can compare a four-byte CRC-32 value with my variable and check whether the CRC values are the same. Now this is how I am doing this at the moment:

static unsigned long whole_four = 0; // This variable holds the last four bytes of the  received bytes which are the CRC bytes

//Note that "MAX_INPUT" is the size of my array and "data" is just a pointer I have used as an argument to a function to point to the elements a buffer array.

whole_four = ((unsigned long)(*(data+(MAX_INPUT-4)))<< 24)  | ((unsigned long)(*(data+(MAX_INPUT-3)))<< 16) | ((unsigned long)(*(data+(MAX_INPUT-2)))<< 8) | ((unsigned long)(*(data+(MAX_INPUT-1))));

So as you can see I am shifting and "or"ing the the last four elements of my array to make a four byte variable. Now, my question is: Is there any faster way to achieve this and reduce the amount of processing power required? I also like to mention that I am compiling this code on an Arduino Uno. Any help or hints is much appreciated.

Answer 1

If endian-ness isn't a issue (as Yves says if the two CRC's to be compared are obtained the same way), you could try with:

memcpy(&whole_four, data + MAX_INPUT - 4, sizeof(whole_four));

A modern compiler will inline this call to something well adapted to your situation considering data type alignment (a char array doesn't necessarily have the correct alignment) / processor.

PS memcpy will interpret the bytes of data with the host system byte order (little endian), while your method interprets the bytes from data in big-endian order.

So memcpy will only work if you can obtain the other term of comparison in the same format.

Answer 2

*(unsigned long*)(data + MAX_INPUT - 4) will fetch the four bytes in a single go. Depending on indianness, this can give you two different results. But if the two CRC's to be compared are obtained the same way, it does not matter.