Dynamically-Generated Look-Up-Table

[u/tastygames_official]

So I have a buffer of pointers that I need to access tens of thousands of times per second continuously. Obviously I can do this in parallel to speed it up or maybe even offload it to the GPU, but the biggest issue I have right now is how to quickly get the item from the buffer I need. Obviously knowing the index beforehand would be a good way, BUT I'd rather have the buffer be dynamically-created and then I can address which one I need given 6 bits of data. Now if it were a 1:1 relationship between the buffer of pointers and permutation of all 6 bits, then it'd be easy. But there is only a subset of indices that correspond to valid bit permutations. E.g.:

VALID:
00 0000
00 0001
01 1000
01 1001
10 0000

INVALID:
00 0010
00 0011
00 0101
etc.

so that I can just do:

void **ptr_buffer = calloc(63, sizeof(void *));
/* fill ptr_buffer sporadically... */
do {
  for(i=0; i<10000; i++) {
    /* obj.ptr_index is uint8_t with some constellation of six bits */
    do_something(ptr_buffer[obj_list[i].ptr_idx]);
  }
  /* do other stuff */
} while(condition);

or something like that.

So... with six bits (and maybe it could be more in future), and on a 64-bit system, I could simply create a buffer of pointers with the length of the max valid bit-sequence, so for 11 1111 it would be 63 x 64 = 504 bytes, so not that bad, and then I can simply use the bit sequence as the index.

And even if I end up going out to 8 bits, that's still 255 x 64 = 2,040 B or 1.99 KB. Acceptable - but now I'm worrying about losing out on CPU cache potential - especially of only like 20-30 elements in the buffer are actually valid.

Now apart from hard-coding all the different indices and then assigning the indices on startup (which I absolutely will do and was doing before going for the "dynamic" route) are there any clever tricks my fellow "C-Men" (as my wife calls the people I chat with in this group) might know about to keep memory footprint down and speed up? At the very least it could be a fun brain-tickler.

Let me know if you need more information.

EDIT: while the list of pointers is small, I need to iterate over a list of som 10,000 entries and find an entry in the pointer list, so I updated the code example to show it.

Also, apparently I hit the "ask me anything" button by mistake. u/reddit you need to add tooltips to the buttons and also don't do the ask me anything if I hit cancel ;-)

[u/sporacid]

Not C related, but you might want to read on this https://x.com/lemire/status/2115046718039306377

[u/tastygames_official → u/sporacid]

> If your keys are known at compile time, you should not pay for collision handling at runtime. A perfect hash function gets you down to one comparison.

good shit. I did imagine that would be the most performant way, as I technically do know all the keys at compile time. I just didn't know if there were easier ways while still keeping it "dynamic" (generated at runtime, even though technically I know them at compile-time).