Free chunk list

src/development/design/filesystem.md

@@ -105,7 +105,7 @@ Also, we are able to verify data. Before passing off the data, we re-hash it usi
 If it does, we simply pass it along like normal. If not, we refuse, and send an error [message](/development/design/actor.md#messages).
 
 ### Writing
-Writing uses a similar process. An *Actor* can request to write data. If it has proper capabilties, we serialize the data, allocate a free chunk[^free_chunk], and write to it.
+Writing uses a similar process. An *Actor* can request to write data. If it has proper capabilties, we serialize the data, allocate a free chunk, and write to it.
 We *hash* the data first to generate a checksum, and set proper metadata.
 
 ### Permissions
@@ -119,12 +119,15 @@ will be determined via [capabilities](/development/design/actor.md#ocap)
 ### Indexing
 Created in-memory on startup, modified directly whenever the filesystem is modified.
 It's saved in the *Index Sector* (which is at a known offset), allowing it to be read in easily on boot.
-It again simply uses `bincode` and compression.
 
 The index is simply an `alloc::` [BTreeMap](https://doc.rust-lang.org/stable/alloc/collections/btree_map/struct.BTreeMap.html).
 
+We also have a simple `Vec` of the chunks that are free, which we modify in reverse.
+
 ```rust
-let mut index = BTreeMap::new();
+let mut index = BTreeMap::new(); // Basic Actor index
+let mut free_index = Vec<u64>; // Index of free chunks
+
 struct Location {
     partition: Uuid, // Partition identified via Uuid
     chunks: Vec<u64>, // Which chunk(s) in the partition it is
@@ -136,9 +139,16 @@ let new_data_location = Location {
 }
 
 index.entry(&actor.uuid).or_insert(&new_data_location); // Insert an Actor's storage location if it's not already stored
+for i in &new_data_location.chunks {
+    free_index.pop(&i); // Remove used chunks from the free chunks list
+}
+
 index.contains_key(&actor.uuid); // Check if the index contains an Actor's data
 index.get(&actor.uuid); // Get the Location of the actor
 index.remove(&actor.uuid); // Remove an Actor's data from the index (e.g. on deletion)
+for i in &new_data_location.chunks {
+    free_index.push(&i); // Add back the now free chunks
+}
 ```
 
 This then allows the index to be searched easily to find the data location of a specific `Uuid`.
@@ -179,5 +189,3 @@ struct PackedExecutable {
 ```
 
 [^encryption]: Specific details to be figured out later
-
-[^free_chunk]: Need to figure out how to efficiently do this. **XFS** seems to just keep another index of free chunks. It also uses a **B+Tree** rather than a hashmap - to look into.