feat(code): part 3 iterators

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

mini-lsm-starter/src/mem_table.rs

@@ -0,0 +1,79 @@
+#![allow(unused_variables)] // TODO(you): remove this lint after implementing this mod
+#![allow(dead_code)] // TODO(you): remove this lint after implementing this mod
+
+use std::ops::Bound;
+
+use anyhow::Result;
+use bytes::Bytes;
+use crossbeam_skiplist::SkipMap;
+
+use crate::iterators::impls::StorageIterator;
+use crate::table::SsTableBuilder;
+
+/// A basic mem-table based on crossbeam-skiplist
+pub struct MemTable {
+    map: SkipMap<Bytes, Bytes>,
+}
+
+impl MemTable {
+    /// Create a new mem-table.
+    pub fn create() -> Self {
+        unimplemented!()
+    }
+
+    /// Get a value by key.
+    pub fn get(&self, key: &[u8]) -> Result<Option<Bytes>> {
+        unimplemented!()
+    }
+
+    /// Put a key-value pair into the mem-table.
+    pub fn put(&self, key: &[u8], value: &[u8]) -> Result<()> {
+        unimplemented!()
+    }
+
+    /// Get an iterator over a range of keys.
+    pub fn scan(&self, lower: Bound<&[u8]>, upper: Bound<&[u8]>) -> Result<MemTableIterator> {
+        unimplemented!()
+    }
+
+    /// Flush the mem-table to SSTable.
+    pub fn flush(&self, builder: &mut SsTableBuilder) -> Result<()> {
+        unimplemented!()
+    }
+}
+
+type SkipMapRangeIter<'a> =
+    crossbeam_skiplist::map::Range<'a, Bytes, (Bound<Bytes>, Bound<Bytes>), Bytes, Bytes>;
+
+/// An iterator over a range of `SkipMap`.
+pub struct MemTableIterator<'a> {
+    _phantom: std::marker::PhantomData<&'a ()>,
+}
+
+impl<'a> MemTableIterator<'a> {
+    fn new(iter: SkipMapRangeIter<'a>) -> Self {
+        unimplemented!()
+    }
+}
+
+impl StorageIterator for MemTableIterator<'_> {
+    fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    fn next(&mut self) -> Result<()> {
+        unimplemented!()
+    }
+}
+
+#[cfg(test)]
+#[path = "mem_table_test.rs"]
+mod tests;