(** Lock-free buffer pool using Kcas and Eio.

    Provides pre-allocated buffers for zero-copy I/O operations. Uses
    Kcas_data.Queue for lock-free buffer storage and Eio.Semaphore for blocking
    acquire when pool is exhausted. *)

type t = {
  buffers : Cstruct.t Kcas_data.Queue.t;
  buf_size : int;
  total : int;
  semaphore : Eio.Semaphore.t;
}

let create ~size ~count =
  let buffers = Kcas_data.Queue.create () in
  for _ = 1 to count do
    Kcas.Xt.commit
      {
        tx =
          (fun ~xt -> Kcas_data.Queue.Xt.add ~xt (Cstruct.create size) buffers);
      }
  done;
  {
    buffers;
    buf_size = size;
    total = count;
    semaphore = Eio.Semaphore.make count;
  }

let acquire t =
  Eio.Semaphore.acquire t.semaphore;
  let buf_opt =
    Kcas.Xt.commit
      { tx = (fun ~xt -> Kcas_data.Queue.Xt.take_opt ~xt t.buffers) }
  in
  match buf_opt with
  | Some buf ->
      Cstruct.memset buf 0;
      buf
  | None ->
      (* Should not happen if semaphore is properly synchronized,
         but handle gracefully by allocating a new buffer *)
      Cstruct.create t.buf_size

let try_acquire t =
  (* Check if semaphore has available permits without blocking *)
  if Eio.Semaphore.get_value t.semaphore > 0 then begin
    (* Race condition possible here - another fiber might acquire between
       get_value and acquire. In that case, acquire will block briefly.
       For truly non-blocking behavior, we'd need atomic CAS on semaphore. *)
    Eio.Semaphore.acquire t.semaphore;
    let buf_opt =
      Kcas.Xt.commit
        { tx = (fun ~xt -> Kcas_data.Queue.Xt.take_opt ~xt t.buffers) }
    in
    match buf_opt with
    | Some buf ->
        Cstruct.memset buf 0;
        Some buf
    | None -> Some (Cstruct.create t.buf_size)
  end
  else None

let release t buf =
  Kcas.Xt.commit { tx = (fun ~xt -> Kcas_data.Queue.Xt.add ~xt buf t.buffers) };
  Eio.Semaphore.release t.semaphore

let with_buffer t f =
  let buf = acquire t in
  Fun.protect ~finally:(fun () -> release t buf) (fun () -> f buf)

let available t = Eio.Semaphore.get_value t.semaphore
let total t = t.total
let size t = t.buf_size