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It sounds like you're trying to reimplement something Lua already does ;) - I hope this is not the case.

spir wrote:
-1- Is the object identity internally implemented as its actual memory address?

-2- Is there a way to get this value (like python's id() func)
Unfortunately no. Would be nice to have a topointer in lua - you can make your own via the C api very easily. The odd occasion has popped up that I've wanted one..

-3- I read somewhere that table key hashes (*) are obtained from key adresses (or identities, as opposed to hashing data itself). Is this correct? If yes, what kind of hash formula/algorithm is then used?
Yes, Lua keys are based on table/userdata/coroutine identities, not the values they contain.

-4- If the above is true, then why isn't this the common way, since it seems to have only advantages:
I cannot imagine why any language would do it any other way.

-5- The simplest way to hash integers is certainly to get the modulo N, where N is the number of indexes (and of "buckets" in case of collisions). What about this simplissim solution?
As the number of buckets is a power of two in Lua, this'd produce a poor results on pointers which tend to have patterns in powers of two. So Lua does a modulo of the number of buckets minus 1, which works well.

-6- By exploring this topic, I discovered (on my computer) a minimal offset of 8 bytes between addresses (I mean they are assigned modulo 8). (Thus, addresses must be divided per 8 before beeing hashed.) Has this something to do with Lua?
This has everything to do with the system you're compiling for. Lua just uses malloc/realloc, which are supposed to return a pointer with the property that it's aligned for any use. As doubles on x86 are slow when not 8 byte aligned, malloc should always return pointers that are 8 byte aligned. On another system, this might not be the case.

(*) What's the common name for "return value of a hash func" in english?
(In french we commonly say "empreinte", meaning _print_ like in "finger print", but there are other words depending on use case.)
We tend to just call it "the hash" or "hash value".