checkin initial MVCC codebase

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-mvcc/src/block.rs

@@ -0,0 +1,43 @@
+mod builder;
+mod iterator;
+
+pub use builder::BlockBuilder;
+use bytes::{Buf, BufMut, Bytes};
+pub use iterator::BlockIterator;
+
+pub(crate) const SIZEOF_U16: usize = std::mem::size_of::<u16>();
+
+/// A block is the smallest unit of read and caching in LSM tree. It is a collection of sorted
+/// key-value pairs.
+pub struct Block {
+    pub(crate) data: Vec<u8>,
+    pub(crate) offsets: Vec<u16>,
+}
+
+impl Block {
+    pub fn encode(&self) -> Bytes {
+        let mut buf = self.data.clone();
+        let offsets_len = self.offsets.len();
+        for offset in &self.offsets {
+            buf.put_u16(*offset);
+        }
+        // Adds number of elements at the end of the block
+        buf.put_u16(offsets_len as u16);
+        buf.into()
+    }
+
+    pub fn decode(data: &[u8]) -> Self {
+        // get number of elements in the block
+        let entry_offsets_len = (&data[data.len() - SIZEOF_U16..]).get_u16() as usize;
+        let data_end = data.len() - SIZEOF_U16 - entry_offsets_len * SIZEOF_U16;
+        let offsets_raw = &data[data_end..data.len() - SIZEOF_U16];
+        // get offset array
+        let offsets = offsets_raw
+            .chunks(SIZEOF_U16)
+            .map(|mut x| x.get_u16())
+            .collect();
+        // retrieve data
+        let data = data[0..data_end].to_vec();
+        Self { data, offsets }
+    }
+}