module type HANDLER = sig
  type t
  type handle
  type cursor

  val open_ : t -> path:Concepts.Path.t -> (handle, Concepts.Condition.condition) result

  val open_cursor :
    handle ->
    args:Concepts.Value.value BatFingerTree.t ->
    (cursor, Concepts.Condition.condition) result

  val next : cursor -> (Concepts.Tuple.t option, Concepts.Condition.condition) result
  val close : cursor -> unit
end

let not_a_relation () : Concepts.Condition.condition =
  Concepts.Condition.condition "not-a-relation"
    "handle does not refer to a stored or ephemeral relation" Concepts.Condition.empty

module Make (Store : Abstract.Storage.STORAGE) = struct
  module Cur = Cursor.Make (Store)

  type t = {objects: Object.rnt_object_tree; txn: Store.transaction}
  type handle = {object_: Object.registry; txn: Store.transaction}
  type cursor = Cur.cursor

  let create objects txn = {objects; txn}

  let open_ t ~path =
    let object_ = Object.find path t.objects in
    Ok {object_; txn= t.txn}

  let relation_descriptor (object_ : Object.registry) :
      (Cursor.descriptor, Concepts.Condition.condition) result =
    let {Object.entry; _} = object_ in
    match entry.Object.kind with
    | Object.Relation {merkle_root} -> Ok (Cursor.Stored {merkle_root})
    | Object.EphemeralRelation {merkle_root; dependencies} ->
        Ok (Cursor.Ephemeral {merkle_root; dependencies})
    | _ -> Error (not_a_relation ())

  let open_cursor handle ~args =
    let open Utilities.Result in
    let* descriptor = relation_descriptor handle.object_ in
    Cur.open_ handle.txn descriptor ~args

  let next = Cur.next
  let close = Cur.close
  let object_ (handle : handle) = handle.object_
end