1 files changed, 63 insertions, 0 deletions
diff --git a/lib/kernel/BranchManager.ml b/lib/kernel/BranchManager.ml
new file mode 100644
index 0000000..03a2765
--- /dev/null
+++ b/lib/kernel/BranchManager.ml
@@ -0,0 +1,63 @@
+module Error = struct
+  open Concepts.Condition
+
+  let invalid_root head =
+    condition "invalid-root" "The root hash for the database state is missing from the backend. Either your storage is corrupted, or this is a bug in RNT!"
+      ("hash" |=| (Concepts.Value.String (Concepts.Hash.to_hum_string head)))
+
+  let no_such_branch branch_name =
+    condition "no-such-branch" "A branch with the specified name was not found"
+      ("branch"    |=| Concepts.Value.String branch_name)
+
+  let comparison_failed branch_name reference head =
+    condition "comparison-failed" "The HEAD of the branch did not match what was expected"
+      ("branch"    |=| Concepts.Value.String branch_name &
+       "reference" |=| Concepts.Value.String (Concepts.Hash.to_hum_string reference) &
+       "head"      |=| Concepts.Value.String (Concepts.Hash.to_hum_string head))
+end
+
+module Make (S : Abstract.Storage.STORAGE) = struct
+  module M = Merkle.Make (S) (Merkle.StringKey)
+  module SI = Storage.Make (S)
+
+  type manager = { storage: S.connection;
+                   label : string;
+                   head: M.node Atomic.t }
+
+  let root_for tx label =
+    let open Utilities.Result in
+    let* label = S.get tx (S.Label label) in
+    let label = Option.map Concepts.Hash.hash_of_blob label in
+    Ok label
+
+  let initialize storage label =
+    let open Utilities.Result in
+    SI.with_transaction storage (fun tx ->
+        let* addr = root_for tx label in
+        match addr with
+        | None -> failwith "TODO"
+        | Some addr ->
+           let* head = M.find tx addr in
+           let* head = Option.to_result ~none:(Error.invalid_root addr) head in
+           Ok { storage; label; head = Atomic.make head })
+
+  let fetch { storage; head; _ } branch_name =
+    SI.with_transaction storage (fun tx ->
+        let head = Atomic.get head in
+        M.lookup tx branch_name head)
+
+  let update { storage; head; label } branch_name reference new_branch =
+    let open Utilities.Result in
+    let* _ = SI.with_transaction storage (fun tx ->
+                 Utilities.Atomic.mswap head (fun head ->
+                     let* branch = M.lookup tx branch_name head
+                                   |> Result.map (Option.to_result ~none:(Error.no_such_branch branch_name))
+                                   |> Result.join in
+                     if branch = reference then
+                       let* new_head = M.insert tx label new_branch head in
+                       Ok new_head
+                     else
+                       Error (Error.comparison_failed branch_name reference branch))) in
+    Ok ()
+
+end