Question

I won’t go into details, but I’m attempting to implement an algorithm similar to the Boyer-Moore-Horspool algorithm, only using hex color values instead of characters (i.e., there is a much greater range).

Following the example on Wikipedia, I originally had this:

size_t jump_table[0xFFFFFF + 1];
memset(jump_table, default_value, sizeof(jump_table);

However, 0xFFFFFF is obviously a huge number and this quickly causes C to seg-fault (but not stack-overflow, disappointingly).

Basically, what I need is an efficient associative array mapping integers to integers. I was considering using a hash table, but having a malloc’d struct for each entry just seems overkill to me (I also do not need hashes generated, as each key is a unique integer and there can be no duplicate entries).

Does anyone have any alternatives to suggest? Am I being overly pragmatic about this?

Update

For those interested, I ended up using a hash table via the uthash library.

Answer 1

0xffffff is rather too large to put on the stack on most systems, but you absolutely can malloc a buffer of that size (at least on current computers; not so much on a smartphone). Whether or not you should do it for this task is a separate issue.

Edit: Based on the comment, if you expect the common case to have a relatively small number of entries other than the “this color doesn’t appear in the input” skip value, you should probably just go ahead and use a hash map (obviously only storing values that actually appear in the input).

(ignore earlier discussion of other data structures, which was based on an incorrect recollection of the algorithm under discussion — you want to use a hash table)