Is there a practical limit to the size of bit masks?

问题

There's a common way to store multiple values in one variable, by using a bitmask. For example, if a user has read, write and execute privileges on an item, that can be converted to a single number by saying read = 4 (2^2), write = 2 (2^1), execute = 1 (2^0) and then add them together to get 7.

I use this technique in several web applications, where I'd usually store the variable into a field and give it a type of MEDIUMINT or whatever, depending on the number of different values.

What I'm interested in, is whether or not there is a practical limit to the number of values you can store like this? For example, if the number was over 64, you couldn't use (64 bit) integers any more. If this was the case, what would you use? How would it affect your program logic (ie: could you still use bitwise comparisons)?

I know that once you start getting really large sets of values, a different method would be the optimal solution, but I'm interested in the boundaries of this method.

回答1:

Off the top of my head, I'd write a set_bit and get_bit function that could take an array of bytes and a bit offset in the array, and use some bit-twiddling to set/get the appropriate bit in the array. Something like this (in C, but hopefully you get the idea):

// sets the n-th bit in |bytes|. num_bytes is the number of bytes in the array
// result is 0 on success, non-zero on failure (offset out-of-bounds)
int set_bit(char* bytes, unsigned long num_bytes, unsigned long offset)
{
  // make sure offset is valid
  if(offset < 0 || offset > (num_bytes<<3)-1) { return -1; }

  //set the right bit
  bytes[offset >> 3] |= (1 << (offset & 0x7));

  return 0; //success 
}

//gets the n-th bit in |bytes|. num_bytes is the number of bytes in the array
// returns (-1) on error, 0 if bit is "off", positive number if "on"
int get_bit(char* bytes, unsigned long num_bytes, unsigned long offset)
{
  // make sure offset is valid
  if(offset < 0 || offset > (num_bytes<<3)-1) { return -1; }

  //get the right bit
  return (bytes[offset >> 3] & (1 << (offset & 0x7));
}

回答2:

I've used bit masks in filesystem code where the bit mask is many times bigger than a machine word. think of it like an "array of booleans";

(journalling masks in flash memory if you want to know)

many compilers know how to do this for you. Adda bit of OO code to have types that operate senibly and then your code starts looking like it's intent, not some bit-banging.

My 2 cents.

回答3:

With a 64-bit integer, you can store values up to 2^64-1, 64 is only 2^6. So yes, there is a limit, but if you need more than 64-its worth of flags, I'd be very interested to know what they were all doing :)

How many states so you need to potentially think about? If you have 64 potential states, the number of combinations they can exist in is the full size of a 64-bit integer.

If you need to worry about 128 flags, then a pair of bit vectors would suffice (2^64 * 2).

Addition: in Programming Pearls, there is an extended discussion of using a bit array of length 10^7, implemented in integers (for holding used 800 numbers) - it's very fast, and very appropriate for the task described in that chapter.

回答4:

Some languages ( I believe perl does, not sure ) permit bitwise arithmetic on strings. Giving you a much greater effective range. ( (strlen * 8bit chars ) combinations )

However, I wouldn't use a single value for superimposition of more than one /type/ of data. The basic r/w/x triplet of 3-bit ints would probably be the upper "practical" limit, not for space efficiency reasons, but for practical development reasons.

( Php uses this system to control its error-messages, and I have already found that its a bit over-the-top when you have to define values where php's constants are not resident and you have to generate the integer by hand, and to be honest, if chmod didn't support the 'ugo+rwx' style syntax I'd never want to use it because i can never remember the magic numbers )

The instant you have to crack open a constants table to debug code you know you've gone too far.

回答5:

Old thread, but it's worth mentioning that there are cases requiring bloated bit masks, e.g., molecular fingerprints, which are often generated as 1024-bit arrays which we have packed in 32 bigint fields (SQL Server not supporting UInt32). Bit wise operations work fine - until your table starts to grow and you realize the sluggishness of separate function calls. The binary data type would work, were it not for T-SQL's ban on bitwise operators having two binary operands.

回答6:

For example .NET uses array of integers as an internal storage for their BitArray class. Practically there's no other way around.

That being said, in SQL you will need more than one column (or use the BLOBS) to store all the states.

回答7:

You tagged this question SQL, so I think you need to consult with the documentation for your database to find the size of an integer. Then subtract one bit for the sign, just to be safe.

Edit: Your comment says you're using MySQL. The documentation for MySQL 5.0 Numeric Types states that the maximum size of a NUMERIC is 64 or 65 digits. That's 212 bits for 64 digits.

Remember that your language of choice has to be able to work with those digits, so you may be limited to a 64-bit integer anyway.

来源：https://stackoverflow.com/questions/177054/is-there-a-practical-limit-to-the-size-of-bit-masks

标签

sql

bit-manipulation

bitmask