How do I set the bits in a byte?

Question

I need to store a series of 1s and 0s of arbitrary length.

I had planned to use ints, but then it occurred to me that really all I need is a bitstream.

NSMutableData seems like just the thing. Except all I see anyone talking about is how to set bytes on it, or store jpegs or strings in it. I need to get way more granular than that.

Given a series of 1s and 0s such as: 110010101011110110, how do I make it into an NSData object--and how do I get it out?

NSData's appendBytes:length: and mutableBytes are all at the byte level, and I need to start lower. Storing those 1s and 0s as bytes doesn't make sense, when the bytes themselves are made of sets of 1s and 0s. I'm having trouble finding anything telling me how to set bits.

Here's some faux code:

NSString *sequence = @"01001010000010"; //(or int sequence, or whatever)
for (...){//iterate through whatever it is--this isn't what I need help with
     if ([sequence intOrCharOrWhateverAtIndex: index] == 0) {
          //do something to set a bit -- this is what I need help with
     } else {
          //set the bit the other way -- again, this is what I need help with 
     }
}
NSData *data = [NSData something]; //wrap it up and save it -- help here too

Answer 1

Do you literally have 1s and 0s? Like... ASCII numerals? I would use NSString to store that. If by 1s and 0s you mean a bunch of bits, then just divide the number of bits by 8 to get the number of bytes and make an NSData of the bytes.

(Editing to add untested code to convert a bitstream to a buffer)

//Assuming the presence of an array of 1s and 0s stored as some numeric type, called bits, and the number of bits in the array stored in a variable called bitsLength
NSMutableData *buffer = [NSMutableData data];
for (int i = 0; i < bitsLength; i += 8) {
    char byte = 0;
    for (int bit = 0; bit < 8 && i + bit < bitsLength; bit++) {
        if (bits[i + bit] > 0) {
            byte += (1 << bit);
        }
    }
    [buffer appendBytes:&byte length:1];
}