(*---------------------------------------------------------------------------
  Copyright (c) 2019 Antonio Nuno Monteiro.
  Copyright (c) 2025 Anil Madhavapeddy <anil@recoil.org>.

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 ---------------------------------------------------------------------------*)

(** HTTP/2 Stream State Machine per RFC 9113 Section 5.1.

    Implements the stream lifecycle:
    - idle -> open -> half-closed -> closed
    - With reserved states for server push (PUSH_PROMISE)

    See {{:https://datatracker.ietf.org/doc/html/rfc9113#section-5.1}RFC 9113 Section 5.1}. *)

(* ============================================================
   Stream States
   ============================================================ *)

(** Why a stream was closed. *)
type closed_reason =
  | Finished
      (** Both endpoints sent END_STREAM. *)
  | ResetByUs of H2_frame.error_code
      (** We sent RST_STREAM. *)
  | ResetByThem of H2_frame.error_code
      (** Peer sent RST_STREAM. *)

(** Stream state per RFC 9113 Section 5.1. *)
type state =
  | Idle
      (** Initial state. Transitions to open on send/recv HEADERS,
          or to reserved on PUSH_PROMISE. *)
  | Reserved_local
      (** We sent PUSH_PROMISE. Can only send HEADERS (-> half_closed_remote)
          or RST_STREAM (-> closed). *)
  | Reserved_remote
      (** Peer sent PUSH_PROMISE. Can only receive HEADERS (-> half_closed_local)
          or send/recv RST_STREAM (-> closed). *)
  | Open
      (** Both sides can send frames. Transitions to half_closed on END_STREAM,
          or closed on RST_STREAM. *)
  | Half_closed_local
      (** We sent END_STREAM. We can only receive frames.
          Transitions to closed on recv END_STREAM or RST_STREAM. *)
  | Half_closed_remote
      (** Peer sent END_STREAM. We can only send frames.
          Transitions to closed on send END_STREAM or RST_STREAM. *)
  | Closed of closed_reason
      (** Terminal state. *)

(** Events that cause state transitions. *)
type event =
  | Send_headers of { end_stream : bool }
  | Recv_headers of { end_stream : bool }
  | Send_data of { end_stream : bool }
  | Recv_data of { end_stream : bool }
  | Send_push_promise
  | Recv_push_promise
  | Send_rst_stream of H2_frame.error_code
  | Recv_rst_stream of H2_frame.error_code
  | Send_end_stream
  | Recv_end_stream

(** Result of applying an event to a state. *)
type transition_result =
  | Transition_ok of state
  | Transition_error of H2_frame.error_code * string

(* ============================================================
   State Transition Logic
   ============================================================ *)

(** [transition state event] applies [event] to [state] and returns
    the new state or an error.

    Implements RFC 9113 Section 5.1 state machine. *)
let transition state event =
  match state, event with
  (* === Idle state transitions === *)
  | Idle, Send_headers { end_stream = false } ->
    Transition_ok Open
  | Idle, Send_headers { end_stream = true } ->
    Transition_ok Half_closed_local
  | Idle, Recv_headers { end_stream = false } ->
    Transition_ok Open
  | Idle, Recv_headers { end_stream = true } ->
    Transition_ok Half_closed_remote
  | Idle, Send_push_promise ->
    Transition_ok Reserved_local
  | Idle, Recv_push_promise ->
    Transition_ok Reserved_remote
  | Idle, _ ->
    Transition_error (H2_frame.Protocol_error,
           "Invalid frame on idle stream")

  (* === Reserved (local) state transitions === *)
  | Reserved_local, Send_headers { end_stream = false } ->
    Transition_ok Half_closed_remote
  | Reserved_local, Send_headers { end_stream = true } ->
    (* Send HEADERS with END_STREAM on reserved -> closed *)
    Transition_ok (Closed Finished)
  | Reserved_local, Send_rst_stream code ->
    Transition_ok (Closed (ResetByUs code))
  | Reserved_local, Recv_rst_stream code ->
    Transition_ok (Closed (ResetByThem code))
  | Reserved_local, _ ->
    Transition_error (H2_frame.Protocol_error,
           "Invalid frame on reserved (local) stream")

  (* === Reserved (remote) state transitions === *)
  | Reserved_remote, Recv_headers { end_stream = false } ->
    Transition_ok Half_closed_local
  | Reserved_remote, Recv_headers { end_stream = true } ->
    (* Recv HEADERS with END_STREAM on reserved -> closed *)
    Transition_ok (Closed Finished)
  | Reserved_remote, Send_rst_stream code ->
    Transition_ok (Closed (ResetByUs code))
  | Reserved_remote, Recv_rst_stream code ->
    Transition_ok (Closed (ResetByThem code))
  | Reserved_remote, _ ->
    Transition_error (H2_frame.Protocol_error,
           "Invalid frame on reserved (remote) stream")

  (* === Open state transitions === *)
  | Open, Send_headers { end_stream = true } ->
    Transition_ok Half_closed_local
  | Open, Send_headers { end_stream = false } ->
    Transition_ok Open (* Trailers without END_STREAM, unusual but valid *)
  | Open, Recv_headers { end_stream = true } ->
    Transition_ok Half_closed_remote
  | Open, Recv_headers { end_stream = false } ->
    Transition_ok Open (* Trailers without END_STREAM *)
  | Open, Send_data { end_stream = true } ->
    Transition_ok Half_closed_local
  | Open, Send_data { end_stream = false } ->
    Transition_ok Open
  | Open, Recv_data { end_stream = true } ->
    Transition_ok Half_closed_remote
  | Open, Recv_data { end_stream = false } ->
    Transition_ok Open
  | Open, Send_end_stream ->
    Transition_ok Half_closed_local
  | Open, Recv_end_stream ->
    Transition_ok Half_closed_remote
  | Open, Send_rst_stream code ->
    Transition_ok (Closed (ResetByUs code))
  | Open, Recv_rst_stream code ->
    Transition_ok (Closed (ResetByThem code))
  | Open, Send_push_promise | Open, Recv_push_promise ->
    (* PUSH_PROMISE is sent on an existing stream but creates a new reserved stream *)
    Transition_ok Open

  (* === Half-closed (local) state transitions === *)
  | Half_closed_local, Recv_headers { end_stream = true } ->
    Transition_ok (Closed Finished)
  | Half_closed_local, Recv_headers { end_stream = false } ->
    Transition_ok Half_closed_local
  | Half_closed_local, Recv_data { end_stream = true } ->
    Transition_ok (Closed Finished)
  | Half_closed_local, Recv_data { end_stream = false } ->
    Transition_ok Half_closed_local
  | Half_closed_local, Recv_end_stream ->
    Transition_ok (Closed Finished)
  | Half_closed_local, Send_rst_stream code ->
    Transition_ok (Closed (ResetByUs code))
  | Half_closed_local, Recv_rst_stream code ->
    Transition_ok (Closed (ResetByThem code))
  | Half_closed_local, (Send_headers _ | Send_data _ | Send_end_stream) ->
    Transition_error (H2_frame.Stream_closed,
           "Cannot send on half-closed (local) stream")
  | Half_closed_local, _ ->
    Transition_ok Half_closed_local (* WINDOW_UPDATE, PRIORITY allowed *)

  (* === Half-closed (remote) state transitions === *)
  | Half_closed_remote, Send_headers { end_stream = true } ->
    Transition_ok (Closed Finished)
  | Half_closed_remote, Send_headers { end_stream = false } ->
    Transition_ok Half_closed_remote
  | Half_closed_remote, Send_data { end_stream = true } ->
    Transition_ok (Closed Finished)
  | Half_closed_remote, Send_data { end_stream = false } ->
    Transition_ok Half_closed_remote
  | Half_closed_remote, Send_end_stream ->
    Transition_ok (Closed Finished)
  | Half_closed_remote, Send_rst_stream code ->
    Transition_ok (Closed (ResetByUs code))
  | Half_closed_remote, Recv_rst_stream code ->
    Transition_ok (Closed (ResetByThem code))
  | Half_closed_remote, (Recv_headers _ | Recv_data _ | Recv_end_stream) ->
    Transition_error (H2_frame.Stream_closed,
           "Received data on half-closed (remote) stream")
  | Half_closed_remote, _ ->
    Transition_ok Half_closed_remote (* WINDOW_UPDATE, PRIORITY allowed *)

  (* === Closed state - terminal === *)
  | Closed reason, Recv_rst_stream _ ->
    (* Can receive RST_STREAM on closed stream (race condition) *)
    Transition_ok (Closed reason)
  | Closed _, _ ->
    Transition_error (H2_frame.Stream_closed,
           "Stream is closed")

(* ============================================================
   Stream Type
   ============================================================ *)

(** Flow control state for a stream. *)
type flow_control = {
  mutable send_window : int;
      (** Bytes we're allowed to send. *)
  mutable recv_window : int;
      (** Bytes we've advertised we can receive. *)
  initial_send_window : int;
      (** Initial send window from SETTINGS. *)
  initial_recv_window : int;
      (** Initial receive window from SETTINGS. *)
}

(** A single HTTP/2 stream. *)
type t = {
  id : int32;
      (** Stream identifier (odd = client-initiated, even = server-initiated). *)
  mutable state : state;
      (** Current stream state. *)
  flow : flow_control;
      (** Flow control windows. *)
  mutable request_headers : H2_hpack.header list option;
      (** Request headers once received/sent. *)
  mutable response_headers : H2_hpack.header list option;
      (** Response headers once received/sent. *)
  mutable trailers : H2_hpack.header list option;
      (** Trailing headers. *)
}

(** Default initial flow control window size per RFC 9113. *)
let default_initial_window_size = 65535

(** Create a new stream with the given ID. *)
let create ?(initial_send_window = default_initial_window_size)
           ?(initial_recv_window = default_initial_window_size)
           id =
  { id;
    state = Idle;
    flow = {
      send_window = initial_send_window;
      recv_window = initial_recv_window;
      initial_send_window;
      initial_recv_window;
    };
    request_headers = None;
    response_headers = None;
    trailers = None;
  }

(** Get the stream ID. *)
let id t = t.id

(** Get the current state. *)
let state t = t.state

(** Helper for state predicate checks. *)
let state_matches pred t = pred t.state

(** Check if stream is in idle state. *)
let is_idle = state_matches (function Idle -> true | _ -> false)

(** Check if stream is open (including half-closed). *)
let is_active = state_matches (function
  | Open | Half_closed_local | Half_closed_remote -> true
  | _ -> false)

(** Check if stream is fully open (both directions). *)
let is_open = state_matches (function Open -> true | _ -> false)

(** Check if stream is closed. *)
let is_closed = state_matches (function Closed _ -> true | _ -> false)

(** Check if we can send on this stream. *)
let can_send = state_matches (function
  | Open | Half_closed_remote | Reserved_local -> true
  | _ -> false)

(** Check if we can receive on this stream. *)
let can_recv = state_matches (function
  | Open | Half_closed_local | Reserved_remote -> true
  | _ -> false)

(** Apply an event to the stream, updating its state.
    Returns Ok () on success, or Error with an error code and message. *)
let apply_event t event =
  match transition t.state event with
  | Transition_ok new_state ->
    t.state <- new_state;
    Ok ()
  | Transition_error (code, msg) ->
    Error (code, msg)

(* ============================================================
   Flow Control
   ============================================================ *)

(** Consume bytes from the send window.
    Returns the number of bytes actually consumed (may be less if window exhausted). *)
let consume_send_window t bytes =
  let available = min bytes t.flow.send_window in
  t.flow.send_window <- t.flow.send_window - available;
  available

(** Add bytes to the send window (from WINDOW_UPDATE). *)
let credit_send_window t increment =
  let new_window = t.flow.send_window + increment in
  if new_window > 0x7FFFFFFF then
    Error (H2_frame.Flow_control_error,
           "Flow control window overflow")
  else begin
    t.flow.send_window <- new_window;
    Ok ()
  end

(** Consume bytes from the receive window.
    Call this when receiving DATA frames. *)
let consume_recv_window t bytes =
  t.flow.recv_window <- t.flow.recv_window - bytes

(** Credit the receive window (we're sending WINDOW_UPDATE). *)
let credit_recv_window t increment =
  t.flow.recv_window <- t.flow.recv_window + increment

(** Get available send window. *)
let send_window t = t.flow.send_window

(** Get available receive window. *)
let recv_window t = t.flow.recv_window

(** Get initial receive window size. *)
let initial_recv_window t = t.flow.initial_recv_window

(** Update initial window size (from SETTINGS).
    Adjusts the current window by the delta. *)
let update_initial_window_size t new_initial_size =
  let delta = new_initial_size - t.flow.initial_send_window in
  let new_window = t.flow.send_window + delta in
  if new_window > 0x7FFFFFFF || new_window < 0 then
    Error (H2_frame.Flow_control_error,
           "Flow control window overflow after SETTINGS update")
  else begin
    t.flow.send_window <- new_window;
    Ok ()
  end

(* ============================================================
   Stream Identifier Management
   ============================================================ *)

(** Check if stream ID is client-initiated (odd). *)
let is_client_initiated id = Int32.(logand id 1l = 1l)

(** Check if stream ID is server-initiated (even, non-zero). *)
let is_server_initiated id = Int32.(id > 0l && logand id 1l = 0l)

(** Check if stream ID is valid (non-zero). *)
let is_valid_id id = Int32.compare id 0l > 0

(** Connection-level stream ID is 0. *)
let connection_stream_id = 0l

(** Check if this is the connection-level stream. *)
let is_connection_stream id = Int32.equal id 0l

(* ============================================================
   Pretty Printing
   ============================================================ *)

let pp_closed_reason fmt = function
  | Finished -> Format.fprintf fmt "Finished"
  | ResetByUs code ->
    Format.fprintf fmt "ResetByUs(%a)" H2_frame.pp_error_code code
  | ResetByThem code ->
    Format.fprintf fmt "ResetByThem(%a)" H2_frame.pp_error_code code

let pp_state fmt = function
  | Idle -> Format.fprintf fmt "Idle"
  | Reserved_local -> Format.fprintf fmt "Reserved(local)"
  | Reserved_remote -> Format.fprintf fmt "Reserved(remote)"
  | Open -> Format.fprintf fmt "Open"
  | Half_closed_local -> Format.fprintf fmt "HalfClosed(local)"
  | Half_closed_remote -> Format.fprintf fmt "HalfClosed(remote)"
  | Closed reason -> Format.fprintf fmt "Closed(%a)" pp_closed_reason reason

let state_to_string state =
  Format.asprintf "%a" pp_state state

let pp fmt t =
  Format.fprintf fmt "Stream{id=%ld; state=%a; send_window=%d; recv_window=%d}"
    t.id pp_state t.state t.flow.send_window t.flow.recv_window

(* ============================================================
   Headers Management
   ============================================================ *)

(** Set request headers. *)
let set_request_headers t headers = t.request_headers <- Some headers

(** Get request headers. *)
let request_headers t = t.request_headers

(** Set response headers. *)
let set_response_headers t headers = t.response_headers <- Some headers

(** Get response headers. *)
let response_headers t = t.response_headers

(** Set trailers. *)
let set_trailers t headers = t.trailers <- Some headers

(** Get trailers. *)
let trailers t = t.trailers

(** Reset the stream with an error code. *)
let reset t code =
  ignore (apply_event t (Send_rst_stream code))