Re: worst case time using hash-tables in Lua

Absolutely agree - this is why I am using C with no dynamic allocation at all - and other people is using Ada/Spark when they need more flexibility and correctness proofs

Incidentally, you have touched the other topic which I find interesting: garbage collection - who knows if there is a way to make it automatic as it is done in Rust?

If I may ask, has anyone had a deep look at the read-only tables in eLua? they seem to be a nice enhancement (especially because if a table is read-only, the expensive look up for one of their methods is not really needed, it could be done when generating the bytecode once and for all, with a performance boost and eliminating the need to cache methods in local variables)

If I understand correctly Lua 5.4 will have "const" but this refers to the variables and their bindings not to the values they are bound to, so table cannot be made 'constant' / read-only (I know that a table can be made read-only by redefining the newindex metamethod)

Andrea

On Tue, May 12, 2020 at 12:30 PM Francisco Olarte <folarte@peoplecall.com> wrote:

Andrea:

On Tue, May 12, 2020 at 8:56 PM Andrea <andrea.l.vitali@gmail.com> wrote:
> and yes, your are right: environments such as Avionics/Automotive/Aerospace may require strict guarantees on worst case - and this is precisely why I am investigating this
> can Lua be made safer for these environments? (I mean in a seamless way: dropping in a replacement for the "table" type implementation)

A replacement hash table may aid you. But if you have hard realtime
requirements of the type "do this in that many time units or people
die", you may also want to consider if a garbage collection scheme is
right for you. Times have changed, by I still remember the times where
I was not allowed to use dynamic memory allocation after control lopp
start ( because it is very hard to prove upper bounds for allocation
times under memory pressure, or even allocation success, so I
allocated fro the worst case on boot, before things got rolling ).

Also, dropping in a replacement hash table may be harder than it
looks, I have not looked too much, but I suspect its implementation
interacts with the garbage collector, I mean, the gc needs to know how
to collect from a table, and the weak reference stuff is also going to
need consideration.

For this kind of things it might be easier for you to implement other
data structures just in lua and use them where they are needed. In
fact, IMO, I would not use an interpreted language like lua for life
supporting things, I would go to a compiled language with a lot of
type checking and use a restricted set. Not for speed, but I've been
used interpreted languages for decaded, and I've had a program running
for more than a decade suddenly aborting due to an unexpected code
path I did not catch in tests on a typo which would not have even
compiled in C/C++/Java.

Francisco Olarte.
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