Re: worst case time using hash-tables in Lua

Subject: Re: worst case time using hash-tables in Lua
From: Andrea &lt;andrea.l.vitali@ ... &gt;
Date: Tue, 12 May 2020 08:35:26 -0700

yes I have read the thread thank you very much

has anyone thought about using crit-bit / patricia trees? they seem to give the same functionality of hash-tables, with guaranteed performance (no worst case), and the capability to support in-order scan of the tree; see: https://cr.yp.to/critbit.html

about the performance: worst case is proportional to the length of the key, not the number of elements in the tree; the key is checked bit-by-bit (or byte-by-byte); no expensive key comparisn (I am thinking about strcmp); there is no hash involved, so no risk of flooding.

In fact I discovered this when thinking about the security of the hash function used in Lua - it seems many other languages already adopted SipHash which gives the strong guarantee that no one can predict the hash of new data even if they have seen the hash of previous data; look at the wikipedia entry and here https://131002.net/siphash/ on the latter site there is a link to the presentation https://cr.yp.to/talks/2012.12.12/slides.pdf where crit-bit trees are mentioned as the preferred solution (rather than using hash-table with secure hash functions)

so did anyone try with crit-bit/patricia trees?

Andrea

On Tue, May 12, 2020 at 2:58 AM Philippe Verdy <verdyp@gmail.com> wrote:

As I replied in another message, it is absolutely NOT necessary to have ANY chaining pointer per node: collision lists can just add (resp. substrict) a constant prime to the current node index (and take the result module the hash table size), to find the next (resp.) previous node in the chain, until you find the node with the correct hash or a null node.

Andrea Vitali