chore(SharedMemory): small internal refactoring

[thedevbirb]

• current_len is redundant -> replaced with last_checkpoint with minor performance boost
• some renaming and refactoring done, the code is a little cleaner (imo)
• added a couple of tests

[DaniPopes]

resize already allocates this, actually for new_size > 4*1024 it would allocate twice.
Do you mean usize::min?

[thedevbirb]

My idea was to reserve a minimum of 4096 bytes to avoid many mini-allocations, but I agree that if new_size > 4 * 1024 then this line is useless

[DaniPopes]

Yes, then it should be min not max

[thedevbirb]

Hmm I'm not sure: suppose new_size equals 32, if we use min then:

assert_eq!(usize::min(32, 4*1024), 32);

and we reserve 32 more bytes instead of 4096, which is not what we want.
I don't understand if I'm missing something here from what you said

[DaniPopes]

Ah you're right. This should be removed because Vec already does exponential allocations so this is redundant since we're starting already with 4*1024

[rakita]

To be precise it should be something like .reserve(self.last_checkpoint+max(new_size,4*1024)) to allocate at least 4096 okay ignore this. As @DaniPopes said resize doubles capacity every time so it is good to rely on.

[thedevbirb]

Thanks for the review! Now it should be good.

[rakita]

lgtm