diff --git a/rust/treedirstate/src/dirstate.rs b/rust/treedirstate/src/dirstate.rs
new file mode 100644
--- /dev/null
+++ b/rust/treedirstate/src/dirstate.rs
@@ -0,0 +1,282 @@
+// Copyright Facebook, Inc. 2017
+//! Directory State.
+
+use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
+use errors::*;
+use filestore::FileStore;
+use std::io::{Cursor, Read, Write};
+use std::path::Path;
+use store::{BlockId, NullStore, Store, StoreView};
+use tree::{Key, KeyRef, Storable, Tree};
+
+/// Marker indicating that a block is probably a root node.
+const MAGIC: &[u8] = b"////";
+const MAGIC_LEN: usize = 4;
+
+/// Selected backend implementation for the dirstate.
+enum Backend {
+    /// The dirstate is not currently backed by a file.
+    Empty(NullStore),
+
+    /// The dirstate is backed by a file on disk.
+    File(FileStore),
+}
+
+impl Backend {
+    pub fn store<'a>(&'a mut self) -> &'a mut Store {
+        match *self {
+            Backend::Empty(ref mut _null) => panic!("attempt to write to uninitialized dirstate"),
+            Backend::File(ref mut file) => file,
+        }
+    }
+
+    pub fn store_view<'a>(&'a self) -> &'a StoreView {
+        match *self {
+            Backend::Empty(ref null) => null,
+            Backend::File(ref file) => file,
+        }
+    }
+}
+
+/// A dirstate object.  This contains the state of all files in the dirstate, stored in tree
+/// structures, and backed by an append-only store on disk.
+pub struct Dirstate<T> {
+    /// The store currently in use by the Dirstate.
+    store: Backend,
+
+    /// The tree of tracked files.
+    tracked: Tree<T>,
+
+    /// The tree of removed files.
+    removed: Tree<T>,
+
+    /// The ID of the root block.
+    root_id: Option<BlockId>,
+}
+
+impl<T: Storable + Clone> Dirstate<T> {
+    /// Create a new, empty dirstate, with no backend store.
+    pub fn new() -> Dirstate<T> {
+        Dirstate {
+            store: Backend::Empty(NullStore::new()),
+            tracked: Tree::new(),
+            removed: Tree::new(),
+            root_id: None,
+        }
+    }
+
+    /// Open an existing dirstate file.  The entries in the file will not be materialized from
+    /// the disk until they are accessed.
+    pub fn open<P: AsRef<Path>>(&mut self, filename: P, root_id: BlockId) -> Result<()> {
+        let store = FileStore::open(filename)?;
+        {
+            let root_data = store.read(root_id)?;
+            let mut root = Cursor::new(root_data);
+
+            // Sanity check that this is a root
+            let mut buffer = [0; MAGIC_LEN];
+            root.read_exact(&mut buffer)?;
+            if buffer != MAGIC {
+                bail!(ErrorKind::InvalidStoreId(root_id.0));
+            }
+
+            let tracked_root_id = BlockId(root.read_u64::<BigEndian>()?);
+            let tracked_file_count = root.read_u32::<BigEndian>()?;
+            let removed_root_id = BlockId(root.read_u64::<BigEndian>()?);
+            let removed_file_count = root.read_u32::<BigEndian>()?;
+            self.tracked = Tree::open(tracked_root_id, tracked_file_count);
+            self.removed = Tree::open(removed_root_id, removed_file_count);
+        }
+        self.store = Backend::File(store);
+        self.root_id = Some(root_id);
+        Ok(())
+    }
+
+    /// Write a new root block to the store.  This contains the identities of the tree roots
+    /// and the tree sizes.
+    fn write_root(&mut self) -> Result<()> {
+        let store = self.store.store();
+        let mut data = Vec::new();
+        data.write(MAGIC)?;
+        data.write_u64::<BigEndian>(self.tracked.root_id().unwrap().0)?;
+        data.write_u32::<BigEndian>(self.tracked.file_count())?;
+        data.write_u64::<BigEndian>(self.removed.root_id().unwrap().0)?;
+        data.write_u32::<BigEndian>(self.removed.file_count())?;
+        self.root_id = Some(store.append(&data)?);
+        store.flush()?;
+        Ok(())
+    }
+
+    /// Write a full copy of the dirstate out to a new file.
+    pub fn write_full<P: AsRef<Path>>(&mut self, filename: P) -> Result<()> {
+        {
+            let mut store = FileStore::create(filename)?;
+            {
+                let old_store = self.store.store_view();
+                self.tracked.write_full(&mut store, old_store)?;
+                self.removed.write_full(&mut store, old_store)?;
+            }
+            self.store = Backend::File(store);
+        }
+        self.write_root()
+    }
+
+    /// Write updated entries in the dirstate to the store.
+    pub fn write_delta(&mut self) -> Result<()> {
+        {
+            let store = self.store.store();
+            self.tracked.write_delta(store)?;
+            self.removed.write_delta(store)?;
+        }
+        self.write_root()
+    }
+
+    /// Clears all entries from the dirstate.
+    pub fn clear(&mut self) {
+        self.tracked.clear();
+        self.removed.clear();
+    }
+
+    /// Returns the ID of the root block.
+    pub fn root_id(&self) -> Option<BlockId> {
+        self.root_id
+    }
+
+    /// Add or update a file entry in the dirstate.
+    pub fn add_file(&mut self, name: KeyRef, state: &T) -> Result<()> {
+        let store = self.store.store_view();
+        self.removed.remove(store, name)?;
+        self.tracked.add(store, name, state)?;
+        Ok(())
+    }
+
+    /// Mark a file as removed in the dirstate.
+    pub fn remove_file(&mut self, name: KeyRef, state: &T) -> Result<()> {
+        let store = self.store.store_view();
+        self.tracked.remove(store, name)?;
+        self.removed.add(store, name, state)?;
+        Ok(())
+    }
+
+    /// Drop a file from the dirstate.
+    pub fn drop_file(&mut self, name: KeyRef) -> Result<bool> {
+        let store = self.store.store_view();
+        let tracked = self.tracked.remove(store, name)?;
+        let removed = self.removed.remove(store, name)?;
+        Ok(tracked || removed)
+    }
+
+    pub fn tracked_count(&self) -> u32 {
+        self.tracked.file_count()
+    }
+
+    pub fn removed_count(&self) -> u32 {
+        self.removed.file_count()
+    }
+
+    /// Get an entry from the tracked tree.
+    pub fn get_tracked<'a>(&'a mut self, name: KeyRef) -> Result<Option<&'a T>> {
+        self.tracked.get(self.store.store_view(), name)
+    }
+
+    /// Get the name and state of the first file in the tracked tree.
+    pub fn get_first_tracked<'a>(&'a mut self) -> Result<Option<(Key, &'a T)>> {
+        self.tracked.get_first(self.store.store_view())
+    }
+
+    /// Get the name and state of the next file in the tracked tree after the named file.
+    pub fn get_next_tracked<'a>(&'a mut self, name: KeyRef) -> Result<Option<(Key, &'a T)>> {
+        self.tracked.get_next(self.store.store_view(), name)
+    }
+
+    pub fn has_tracked_dir(&mut self, name: KeyRef) -> Result<bool> {
+        self.tracked.has_dir(self.store.store_view(), name)
+    }
+
+    /// Get an entry from the removed tree.
+    pub fn get_removed<'a>(&'a mut self, name: KeyRef) -> Result<Option<&'a T>> {
+        self.removed.get(self.store.store_view(), name)
+    }
+
+    /// Get the name and state of the first file in the removed tree.
+    pub fn get_first_removed<'a>(&'a mut self) -> Result<Option<(Key, &'a T)>> {
+        self.removed.get_first(self.store.store_view())
+    }
+
+    /// Get the name and state of the next file in the removed tree after the named file.
+    pub fn get_next_removed<'a>(&'a mut self, name: KeyRef) -> Result<Option<(Key, &'a T)>> {
+        self.removed.get_next(self.store.store_view(), name)
+    }
+
+    pub fn has_removed_dir(&mut self, name: KeyRef) -> Result<bool> {
+        self.removed.has_dir(self.store.store_view(), name)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use dirstate::Dirstate;
+    use tempdir::TempDir;
+    use tree::Storable;
+    use std::io::{Read, Write};
+    use byteorder::{ReadBytesExt, WriteBytesExt};
+    use errors::*;
+
+    #[derive(PartialEq, Clone, Debug)]
+    struct State(char);
+
+    impl Storable for State {
+        fn write(&self, w: &mut Write) -> Result<()> {
+            w.write_u8(self.0 as u8)?;
+            Ok(())
+        }
+
+        fn read(r: &mut Read) -> Result<State> {
+            Ok(State(r.read_u8()? as char))
+        }
+    }
+
+    #[test]
+    fn goodpath() {
+        let dir = TempDir::new("dirstate_test").expect("create temp dir");
+        let p = dir.path().join("store");
+        let mut ds = Dirstate::<State>::new();
+        ds.write_full(&p).expect("can write full empty dirstate");
+        ds.add_file(b"dirA/file1", &State('n')).expect("can add");
+        ds.remove_file(b"dirA/file2", &State('r'))
+            .expect("can remove");
+        ds.write_delta().expect("can write delta");
+        ds.add_file(b"dirA/file2", &State('n')).expect("can add");
+        ds.remove_file(b"dirA/file1", &State('r'))
+            .expect("can remove");
+        ds.write_delta().expect("can write delta");
+        let ds_root = ds.root_id().unwrap();
+        drop(ds);
+        let mut ds2 = Dirstate::<State>::new();
+        ds2.open(&p, ds_root).expect("can re-open");
+        ds2.add_file(b"dirB/file3", &State('m')).expect("can add");
+        ds2.remove_file(b"dirC/file4", &State('r'))
+            .expect("can remove");
+        assert_eq!(ds2.get_tracked(b"dirA/file1").expect("can get"), None);
+        assert_eq!(
+            ds2.get_tracked(b"dirA/file2").expect("can get"),
+            Some(&State('n'))
+        );
+        assert_eq!(
+            ds2.get_removed(b"dirA/file1").expect("can get"),
+            Some(&State('r'))
+        );
+        assert_eq!(ds2.get_removed(b"dirA/file2").expect("can get"), None);
+        assert_eq!(ds2.tracked_count(), 2);
+        assert_eq!(ds2.removed_count(), 2);
+        ds2.drop_file(b"dirA/file1").expect("can drop");
+        ds2.drop_file(b"dirA/file2").expect("can drop");
+        ds2.write_delta().expect("can write delta");
+        assert_eq!(ds2.tracked_count(), 1);
+        assert_eq!(ds2.removed_count(), 1);
+        ds2.clear();
+        ds2.write_delta().expect("can write delta");
+        assert_eq!(ds2.tracked_count(), 0);
+        assert_eq!(ds2.removed_count(), 0);
+    }
+}
diff --git a/rust/treedirstate/src/lib.rs b/rust/treedirstate/src/lib.rs
--- a/rust/treedirstate/src/lib.rs
+++ b/rust/treedirstate/src/lib.rs
@@ -14,9 +14,6 @@
 
 extern crate byteorder;
 
-#[cfg(test)]
-extern crate tempdir;
-
 #[macro_use]
 extern crate error_chain;
 
@@ -27,6 +24,10 @@
 #[macro_use]
 extern crate quickcheck;
 
+#[cfg(test)]
+extern crate tempdir;
+
+pub mod dirstate;
 pub mod errors;
 pub mod filestate;
 pub mod filestore;