2 files changed, 68 insertions, 0 deletions
diff --git a/lib/runtime/Context.ml b/lib/runtime/Context.ml
new file mode 100644
index 0000000..10422e4
--- /dev/null
+++ b/lib/runtime/Context.ml
@@ -0,0 +1,60 @@
+module Error = struct
+  open Concepts.Condition
+
+  let budget_exhausted ~requested ~remaining =
+    condition "budget-exhausted" "An evaluation asked for more budget than it had left"
+      ("requested" |=| Concepts.Value.Integer requested
+      & "remaining" |=| Concepts.Value.Integer remaining)
+
+  let not_a_directory segment =
+    condition "not-a-directory" "A path segment named an object with no Directory protocol"
+      ("segment" |=| Concepts.Value.String segment)
+end
+
+type budget = int Atomic.t
+
+let charge_budget budget n =
+  let open Utilities.Result in
+  let* _ =
+    Utilities.Atomic.mswap budget (fun remaining ->
+        if remaining >= n then Ok (remaining - n)
+        else Error (Error.budget_exhausted ~requested:n ~remaining) )
+  in
+  Ok ()
+
+class context ~root ~budget ~cancelled =
+  object (self)
+    val root : Protocols.Handle.t = root
+    val budget : budget = budget
+    val cancelled : unit -> bool = cancelled
+
+    method resolve (name : string) :
+        (Protocols.Handle.t option, Concepts.Condition.condition) result =
+      let open Utilities.Result in
+      let rec walk handle = function
+        | [] -> Ok (Protocols.Handle.copy handle)
+        | segment :: rest ->
+            let* directory = Protocols.Directory.from handle
+              |> Option.to_result ~none:(Error.not_a_directory segment)
+            in
+            let* found = Protocols.Directory.find directory segment in
+            match found with
+            | None -> Ok None
+            | Some child ->
+                Fun.protect ~finally:(fun () -> Protocols.Handle.release child)
+                  (fun () -> walk child rest)
+      in
+      String.split_on_char '/' name |> List.filter (fun segment -> segment <> "") |> walk root
+
+    method cancelled = cancelled ()
+    method charge n = charge_budget budget n
+    method nest ?authority:(_ : string BatSet.t option) () =
+      (new context ~root ~budget ~cancelled :> Protocols.Context.implementation)
+
+    initializer ignore self
+  end
+
+let over ~root ?(budget = max_int) ?(cancelled = fun () -> false) () =
+  Protocols.Context.make (new context ~root ~budget:(Atomic.make budget) ~cancelled)
+
+let remaining (_ : Protocols.Context.t) : int option = failwith "not implemented"
diff --git a/lib/runtime/Context.mli b/lib/runtime/Context.mli
new file mode 100644
index 0000000..31ef194
--- /dev/null
+++ b/lib/runtime/Context.mli
@@ -0,0 +1,8 @@
+val over :
+  root:Protocols.Handle.t ->
+  ?budget:int ->
+  ?cancelled:(unit -> bool) ->
+  unit ->
+  Protocols.Context.t
+
+val remaining : Protocols.Context.t -> int option