Bit output

[kkk]

Hello,

I'm programming a set of functions for bit output. This will be buffered, suppose I have two pointers:

unsigned char *buffer, *buffer_start;

The pointer buffer points to the current byte and buffer_start always points to the beginning of the dynamically allocated buffer.

My question is:

Should I introduce an integer variable buf_count that keeps the number of bytes ready in the buffer or should I calculate the number of bytes in the buffer by this formula: buffer - buffer_start? I'll need to know the number of bytes in the buffer in order to compare it with the constant BUFFER_SIZE and know when the buffer is full and ready to be written to a file.

My main concern here is speed.

[tabstop]

It would seem that you are checking every time, so either you do the subtraction buffer - buffer_start each time, or you have a separate variable and you add 1 each time. I don't know that there's going to be much difference between adding and subtracting in terms of time taken.

(In olden times, comparisons against zero were supposed to be faster than comparisons against a number, so using a "countdown" approach might, emphasize might, in theory, emphasize theory, save a minuscule, emphasize minuscule, amount of time.)

[itCbitC]

How do you plan on updating buf_count as it's dynamic; by incrementing it?

[kkk]

How do you plan on updating buf_count as it's dynamic; by incrementing it?

Yes, incrementing it by one each time I increment the buffer pointer.

[itCbitC]

Yes, incrementing it by one each time I increment the buffer pointer.

It's hard to say since whatever you do can always be optimized by the compiler.
buffer - buffer_start results need to be cast to the proper type before comparing to BUFFER_SIZE.
buffer_count already is of the proper type but needs more steps for incrementing it each time.

[iMalc]

C++ containers like vector typically subtract the two pointers to get the size.
Since they're designed for efficiency, it's probably a good choice.

buffer - buffer_start results need to be cast to the proper type before comparing to BUFFER_SIZE.

No it doesn't. subtracting two pointers gives a ptrdiff_t which is a basic signed integer type.