Add estimated_size for Block structure

[ppdogg]

There has already been a function estimated_size in SsTableBuilder. I think it is fair to add the same function to BlockBuilder structure:

impl BlockBuilder {
     pub fn estimated_size(&self) -> usize {}
}

Doing this has two benefits:

1. Making the estimated_size in SsTableBuilder more precise:

pub fn estimated_size(&self) -> usize {
    self.data.len() + self.builder.estimated_size()
}

Because during the process of building table, there may be some data in block builder that are not taken into count.

2. In function compact of compact.rs, we can use estimated_size of SsTableBuilder to find if there are some data left:

if builder.estimated_size() > 0 {}

I have read your code in checkpoint, and I think this eliminates the need to use Option as a wrapper of SsTableBuilder.

[ppdogg]

Also: In concat_iterator.rs you use next_sst_idx; In lsm_storage.rs, you use next_sst_id

I think it'd better to use the same name.

[ppdogg]

PS: Maybe you could add a Q&A in each section. There must be some questions that have been frequently asked, which you've already answered in discord.

[skyzh]

+1 :) I'd like to postpone API updates to the next major update of the course. I'll leave this open so that I don't forget.

[ppdogg]

+1 :) I'd like to postpone API updates to the next major update of the course. I'll leave this open so that I don't forget.

If you want to add estimate_size for BlockBuilder, there is something you need to modify:

pub fn estimated_size(&self) -> usize {
    self.data.len() + SIZEOF_U16 * self.offsets.len()
}

rather than

pub fn estimated_size(&self) -> usize {
    self.data.len() + SIZEOF_U16 * self.offsets.len() + SIZEOF_U16
}