Hacker News Reader

Everyone knows about bit-fiddling functions, what the poster was referring to is using this trick to not have to store the capacity for a dynamic vector. Which is genuinely very clever and rare, I don’t think I’ve ever seen it done quite like this either. I think that’s probably because it’s not really that good of an idea, if the vector can shrink, you really should store the capacity.

View on HN

I don't think it's that rare. I've been using the technique for years, and I've seen it done in other work. Bagwell's VList[1] for example uses the equivalent of `bit_ceil` to determine the size of each block without having to store it - and there are earlier works based on the same trick. RAOTS, which is referenced by the VList, mentions using the technique, but itself uses a slightly more complex trick where we can calculate the size of a block based on the approx square root of the length.

You can use the trick if the array can shrink as long as you always shrink the allocation when length goes below the next power of 2 not greater than len (which may make use of stdc_bit_floor).

[1]:https://cl-pdx.com/static/techlists.pdf

Reply on news.ycombinator.com