checkin part 2 solution

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

mini-lsm/src/compact/simple_leveled.rs

@@ -28,6 +28,9 @@ impl SimpleLeveledCompactionController {
         Self { options }
     }
 
+    /// Generates a compaction task.
+    ///
+    /// Returns `None` if no compaction needs to be scheduled. The order of SSTs in the compaction task id vector matters.
     pub fn generate_compaction_task(
         &self,
         snapshot: &LsmStorageState,
@@ -68,6 +71,13 @@ impl SimpleLeveledCompactionController {
         None
     }
 
+    /// Apply the compaction result.
+    ///
+    /// The compactor will call this function with the compaction task and the list of SST ids generated. This function applies the
+    /// result and generates a new LSM state. The functions should only change `l0_sstables` and `levels` without changing memtables
+    /// and `sstables` hash map. Though there should only be one thread running compaction jobs, you should think about the case
+    /// where an L0 SST gets flushed while the compactor generates new SSTs, and with that in mind, you should do some sanity checks
+    /// in your implementation.
     pub fn apply_compaction_result(
         &self,
         snapshot: &LsmStorageState,