mini_lsm/mini-lsm-book/src/01-block.md

# Block Builder and Block Iterator

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In this part, you will need to modify:

* `src/block/builder.rs`
* `src/block/iterator.rs`
* `src/block.rs`

You can use `cargo x copy-test day1` to copy our provided test cases to the starter code directory. After you have
finished this part, use `cargo x scheck` to check the style and run all test cases. If you want to write your own
test cases, write a new module `#[cfg(test)] mod user_tests { /* your test cases */ }` in `block.rs`. Remember to remove
`#![allow(...)]` at the top of the modules you modified so that cargo clippy can actually check the styles.

## Task 1 - Block Builder

Block is the minimum read unit in LSM. It is of 4KB size in general, similar database pages. In each block, we will
store a sequence of sorted key value pairs.

You will need to modify `BlockBuilder` to build the encoded data and the offset array. The block contains two parts:
data and offsets.

```
|          data         |           offsets         |
|entry|entry|entry|entry|offset|offset|offset|offset|num_of_elements|
```

When user adds a key-value pair to a block (which is an entry), we will need to serialize it into the following format:

```
|                             entry1                            |
| key_len (2B) | key (varlen) | value_len (2B) | value (varlen) | ... |
```

Key length and value length are 2B, which means their maximum length is 65536.

We assume that keys will never be empty, and values can be empty. An empty value means that the corresponding key has
been deleted in the view of other parts of the system. For the block builder and iterator, we just treat empty value
as-is.

At the end of the block, we will store the offsets of each entry and the total number of entries. For example, if
the first entry is at 0th position of the block, and the second is at 12th position,

```
|offset|offset|num_of_elements|
|   0  |  12  |       2       |
```

The footer of the block will be as above. Each of the number is stored as `u16`.

The block has a size limit, which is `target_size`. Unless the first key-value pair exceeds the target block size, you
should ensure that the encoded block size is always less than or equal to `target_size`.

The `BlockBuilder` will produce the data part and unencoded entry offsets when `build` is called. The information will
be stored in the `Block` struct. As key-value entries are stored in the raw format and offsets are stored in a separate
vector, this reduces unnecessary memory allocations and processing overhead when decoding data -- what you need to do
is to simply copy the raw block data to the `data` vector and decode the entry offsets every 2 bytes, *instead of*
creating something like `Vec<(Vec<u8>, Vec<u8>)>` to store all the key value pairs in one block in memory. This compact
memory layout is very efficient. `Block::encode` and `Block::decode` will encode to / decode from the data layout
illustrated in the above figures.

## Task 2 - Block Iterator

Given a block object, we will need to extract the key-value pairs. To do this, we create an iterator over a block and
find the information we want.

`BlockIterator` can be created with an `Arc<Block>`. If `create_and_seek_to_first` is called, it will be positioned at
the first key in the block. If `create_and_seek_to_key` is called, the iterator will be positioned at the first key which
is `>=` the provided key. For example, if `1, 3, 5` is in a block,

```rust
let mut iter = BlockIterator::create_and_seek_to_key(block, b"2");
assert_eq!(iter.key(), b"3");
```

`seek 2` will make the iterator to be positioned at the next available key of `2`, which is `3`.

The iterator should copy `key` and `value` from the block and store them inside the iterator, so that users can access
the key and the value without any extra copy with `fn key(&self) -> &[u8]`, which directly returns the reference of the
locally-stored key and value.

When `next` is called, the iterator will move to the next position. If we reach the end of the block, we can set `key`
to empty and return `false` from `is_valid`, so that the caller can switch to another block if possible.

After implementing this part, you should be able to pass all tests in `block/tests.rs`.

## Extra Tasks

*Note: Some test cases might not pass after implementing this part. You might need to write your own test cases.*

* Implement block checksum. Verify checksum when decoding the block.
* Compress / uncompress block. Compress on `build` and uncompress on decoding.