Garbage Collect empty storage keys (Neo 3)

[igormcoelho]

I think a very nice policy for Neo is to automatically garbage-collect empty keys.

Example:

Storage.Put("x", 10);
Storage.Put("x", Storage.Get("x") - 10);

This is used a lot on NEP-5 tokens, and usually we have to put an extra if, because if key is not empty (or zero-valued), we call Storage.Delete. It could be done automatically, which is more simple for users, and more efficient for the network (no empty keys recorded, and they take space).

Note that, one slight change may be necessary (proposed on neo-vm project): we must ensure that BigInteger format is always expressed in its most compressed format... meaning that BigInteger(0) will now be expressed as an empty array (not "0x00"). This is easier to understand and more space efficient (and automatically help us with this garbage collection).

[igormcoelho]

See: neo-project/neo-vm#170

[igormcoelho]

This could also be useful for: #814, to prevent unnecessary Storage.Delete operations on NEP-5.

[igormcoelho]

Implementation is simple: every time we do a Put (or PutEx), if value is an empty array, we call Storage.Delete automatically.

[shargon]

Then we can't do this neo-project/neo-vm#170 :)

[erikzhang]

neo/neo/SmartContract/InteropService.cs

Line 473 in fe3ab04

engine.CurrentContext.EvaluationStack.Push(item?.Value ?? new byte[0]);

For Storage.Get, if the key does not exist, it returns an empty bytes.

So, for Storage.Put, if the value is an empty bytes, we can just remove the entry.

This seems quite reasonable.

[igormcoelho]

Then we can't do this neo-project/neo-vm#170 :)

In fact, it's the opposite @shargon , we need that! I'll show you on the tests there... it's all connected ;)