(* Branch: the effectful session layer.
   Owns the RNT branch handle and the in-memory mirror of every multigroup
   bound to that branch.  Each mutation names its target mg explicitly. *)

module Make (NT : Nt.S) = struct
  module Error = struct
    open Condition
    (* TODO: more structure *)
    let not_supported msg = condition "not-supported" msg empty
    let handle_error msg = condition "handle-error" msg empty
  end

  let ( let* ) = Result.bind

  type mode =
    | Live                (* tracking branch tip; mutations allowed *)
    | Detached of string  (* pinned to snapshot hash; read-only    *)

  class branch
    ~(init_bh   : Nt.branch_handle)
    ~(init_name : string)
    ~(init_tip  : string)
    ~(init_mgs  : (string * Management.Multigroup.multigroup) list)
    ~(init_mode : mode) =
  object (self)
    val mutable bh   : Nt.branch_handle = init_bh
    val          name : string           = init_name
    val mutable tip  : string            = init_tip
    val mutable mode : mode              = init_mode
    val          mgs : (string, Management.Multigroup.multigroup) Hashtbl.t =
      let t = Hashtbl.create 8 in
      List.iter (fun (n, mg) -> Hashtbl.replace t n mg) init_mgs;
      t

    method name           = name
    method tip            = tip
    method snapshot       = tip
    method mode           = mode
    method branch_handle  = bh

    method mg_of (n : string) : Management.Multigroup.multigroup option =
      Hashtbl.find_opt mgs n

    method multigroups : (string * Management.Multigroup.multigroup) list =
      Hashtbl.to_seq mgs |> List.of_seq

    (* Register a fresh mg under this branch.  Idempotent. *)
    method add_multigroup ~(name : string) : unit =
      if not (Hashtbl.mem mgs name) then
        Hashtbl.add mgs name (new Management.Multigroup.multigroup ~name)

    (* Replace the cached mg value (e.g. after an NT mutation returned a
       new mg).  Idempotent insert if the mg was not present. *)
    method set_mg ~(name : string) (mg : Management.Multigroup.multigroup) : unit =
      Hashtbl.replace mgs name mg

    (* Path for a relation in this branch (Live) or its pinned snapshot
       (Detached).  Pass [~branch] to reference a relation on a different
       live branch (cross-multigroup reads). *)
    method path ?(branch : string option) (fqn : Qualified_name.t) : string =
      match branch with
      | Some b ->
          "/system/branches/" ^ b ^ "/multigroups/" ^ fqn.mg
          ^ "/relations/" ^ fqn.name
      | None ->
          match mode with
          | Live ->
              "/system/branches/" ^ name ^ "/multigroups/" ^ fqn.mg
              ^ "/relations/" ^ fqn.name
          | Detached hash ->
              "/system/snapshots/" ^ hash ^ "/relations/" ^ fqn.name

    method private assert_live op =
      match mode with
      | Live         -> Ok ()
      | Detached _   ->
          Error (Error.not_supported (op ^ ": detached snapshot is read-only"))

    method private sync_tip () =
      match mode with
      | Detached _ -> ()
      | Live ->
          match NT.branch_target_of_handle bh with
          | Error _  -> ()
          | Ok h     -> tip <- h

    method private get_mg op (mg_name : string) :
        (Management.Multigroup.multigroup, Condition.t) result =
      match Hashtbl.find_opt mgs mg_name with
      | Some mg -> Ok mg
      | None    -> Error (Error.handle_error
                            (op ^ ": multigroup not found: " ^ mg_name))

    method create_relation ~(mg : string) ~(rel_name : string) ~schema =
      let* () = self#assert_live "create_relation" in
      let* mg_val = self#get_mg "create_relation" mg in
      let* (bh', mg') =
        NT.create_relation bh mg_val
          ~branch_name:name ~mg_name:mg ~name:rel_name ~schema
      in
      bh <- bh'; Hashtbl.replace mgs mg mg'; self#sync_tip (); Ok ()

    method retract_relation ~(mg : string) ~(rel_name : string) =
      let* () = self#assert_live "retract_relation" in
      let* mg_val = self#get_mg "retract_relation" mg in
      let* (bh', mg') = NT.retract_relation bh mg_val ~name:rel_name in
      bh <- bh'; Hashtbl.replace mgs mg mg'; self#sync_tip (); Ok ()

    method clear_relation ~(mg : string) ~(rel_name : string) =
      let* () = self#assert_live "clear_relation" in
      let* mg_val = self#get_mg "clear_relation" mg in
      match mg_val#get_relation rel_name with
      | None     -> Error (Error.handle_error ("relation not found: " ^ rel_name))
      | Some rel ->
          let* (bh', mg') =
            NT.clear_relation bh mg_val
              ~branch_name:name ~mg_name:mg rel
          in
          bh <- bh'; Hashtbl.replace mgs mg mg'; self#sync_tip (); Ok ()

    method register_domain ~(mg : string) domain =
      let* () = self#assert_live "register_domain" in
      let* mg_val = self#get_mg "register_domain" mg in
      let* (bh', mg') = NT.register_domain bh mg_val domain in
      bh <- bh'; Hashtbl.replace mgs mg mg'; Ok ()

    method create_immutable_relation ~(mg : string) ~name:rel_name ~schema
        ~generator ~membership_criteria ~cardinality ~producer =
      let* () = self#assert_live "create_immutable_relation" in
      let* mg_val = self#get_mg "create_immutable_relation" mg in
      let* (bh', mg', rel) =
        NT.create_immutable_relation bh mg_val ~name:rel_name ~schema ~generator
          ~membership_criteria ~cardinality ~producer
      in
      bh <- bh'; Hashtbl.replace mgs mg mg'; Ok rel

    method register_constraint ~(mg : string) ~constraint_name ~relation_name
        ~body =
      let* () = self#assert_live "register_constraint" in
      let* mg_val = self#get_mg "register_constraint" mg in
      let* (bh', mg') =
        NT.register_constraint bh mg_val ~constraint_name ~relation_name ~body
      in
      bh <- bh'; Hashtbl.replace mgs mg mg'; Ok ()

    method insert_tuple ~(mg : string) ~(rel_name : string) attrs =
      let* () = self#assert_live "insert_tuple" in
      let* hash =
        NT.insert_tuple ~branch_name:name ~mg_name:mg ~rel_name attrs
      in
      self#sync_tip (); Ok hash

    method create_tuple ~(mg : string) ~(rel_name : string) tuple =
      let* () = self#assert_live "create_tuple" in
      let* hash =
        NT.create_tuple ~branch_name:name ~mg_name:mg ~rel_name tuple
      in
      self#sync_tip (); Ok hash

    method create_tuples ~(mg : string) ~(rel_name : string) tuples =
      let* () = self#assert_live "create_tuples" in
      let* hash =
        NT.create_tuples ~branch_name:name ~mg_name:mg ~rel_name tuples
      in
      self#sync_tip (); Ok hash

    method retract_tuple ~(mg : string) ~(rel_name : string) ~tuple_hash =
      let* () = self#assert_live "retract_tuple" in
      let* () =
        NT.retract_tuple ~branch_name:name ~mg_name:mg ~rel_name tuple_hash
      in
      self#sync_tip (); Ok ()

    (* Applies a Transition delta from a sublanguage executor.  Each entry
       replaces the named mg in the cache, then the tip is re-read. *)
    method apply_delta (delta : (string * Management.Multigroup.multigroup) list) =
      List.iter
        (fun (mg_name, mg_val) -> Hashtbl.replace mgs mg_name mg_val)
        delta;
      self#sync_tip ()

    method close () = NT.close_branch bh
  end

  (* Default mg name seeded into an unborn branch so the session can write
     into something.  Real catalog seeding (under "sakura") is the prelude's
     responsibility. *)
  let default_mg_name = "public"

  let open_branch claims branch_name =
    let* (bh, _legacy_mg) = NT.open_branch claims branch_name in
    let tip = match NT.branch_target_of_handle bh with
      | Ok h    -> h
      | Error _ -> ""
    in
    let* mg_entries = NT.list_branch_multigroups branch_name in
    let mgs =
      List.map
        (fun (mg_name, _mg_hash) ->
          (* For each bound mg, enumerate its relations from the kernel and
             rebuild the OCaml mirror.  Schemas are not persisted in the
             snapshot codec — they are repopulated by DDL replay / catalog
             reads later. *)
          let rels =
            match NT.list_relations ~branch_name ~mg_name with
            | Ok pairs -> pairs
            | Error _  -> []
          in
          let mg =
            List.fold_left
              (fun mg (rel_name, root) ->
                let rel =
                  (new Relation.stored
                    ~name:rel_name
                    ~schema:[]
                    ~constraints:None
                    ~cardinality:Conventions.Cardinality.AlephZero
                    ~lineage:(Some (Relation.Lineage.Base rel_name))
                    ~provenance:(Some (Relation.Provenance.base
                                         ~relation:rel_name ~attributes:[]))
                    ~membership_criteria:(fun _ -> true))
                    #set_tree_pointer (Some root)
                in
                mg#add_relation (rel :> Relation.relation))
              (new Management.Multigroup.multigroup ~name:mg_name)
              rels
          in
          (mg_name, mg))
        mg_entries
    in
    let mgs =
      if mgs = [] then
        [ (default_mg_name,
           new Management.Multigroup.multigroup ~name:default_mg_name) ]
      else mgs
    in
    Ok (new branch
          ~init_bh:bh ~init_name:branch_name ~init_tip:tip ~init_mgs:mgs
          ~init_mode:Live)

  let open_snapshot claims ~branch_name ~snapshot_hash =
    let* (bh, mg) = NT.open_snapshot claims branch_name snapshot_hash in
    Ok (new branch
          ~init_bh:bh ~init_name:branch_name ~init_tip:snapshot_hash
          ~init_mgs:[ (mg#name, mg) ]
          ~init_mode:(Detached snapshot_hash))

end