(* sx_persist_store — host durable-storage adapter for lib/persist.
   Production twin of `persist/serve` (lib/persist/durable.sx): it answers the
   same `persist/...` IO ops, but backs them with real on-disk storage so writes
   survive a process restart. Stateless-on-disk: every op reads/writes the
   filesystem directly, so a fresh process recovers state with no warm-up — the
   log on disk IS the state.

   On-disk layout under the root dir (default ./persist-data, or $SX_PERSIST_DIR):
     streams/<hex(stream)>.log   append-only, one SX-serialized event per line
     streams/<hex(stream)>.seq   per-stream monotonic high-water counter (int)
     kv/<hex(key)>               one SX-serialized value per key

   Invariants honoured (see plans/persist-on-sx.md Blocker spec):
   1. last-seq is a per-stream monotonic counter stored in .seq, SEPARATE from
      the rows — it keeps climbing across truncate, so a compacted stream never
      reassigns a seq.
   2. append never renumbers — the event already carries its :seq (log.sx does
      last-seq+1); the host only bumps the high-water mark to max(hw, seq).
   3. read returns surviving events in append order with :seq intact.
   4. streams is the set of streams that ever had an append — keyed off the .seq
      file, which truncate never deletes, so it survives full compaction.
   5. values round-trip structurally via the SX serializer/parser. *)

open Sx_types

(* ---- root dir ---------------------------------------------------------- *)

let _root : string option ref = ref None

let set_root dir = _root := Some dir

let root_dir () =
  match !_root with
  | Some d -> d
  | None -> (try Sys.getenv "SX_PERSIST_DIR" with Not_found -> "persist-data")

(* ---- filesystem helpers ------------------------------------------------ *)

let rec ensure_dir dir =
  if dir = "" || dir = "." || dir = "/" || Sys.file_exists dir then ()
  else begin
    ensure_dir (Filename.dirname dir);
    (try Unix.mkdir dir 0o755 with Unix.Unix_error (Unix.EEXIST, _, _) -> ())
  end

let streams_dir () = Filename.concat (root_dir ()) "streams"
let kv_dir () = Filename.concat (root_dir ()) "kv"
let blobs_dir () = Filename.concat (root_dir ()) "blobs"

let read_file path =
  let ic = open_in_bin path in
  let n = in_channel_length ic in
  let s = really_input_string ic n in
  close_in ic;
  s

(* Atomic write: temp file in the same dir then rename over the target. *)
let write_file_atomic path contents =
  ensure_dir (Filename.dirname path);
  let tmp = path ^ ".tmp" in
  let oc = open_out_bin tmp in
  output_string oc contents;
  flush oc;
  close_out oc;
  Sys.rename tmp path

let append_line path line =
  ensure_dir (Filename.dirname path);
  let oc = open_out_gen [Open_append; Open_creat; Open_wronly] 0o644 path in
  output_string oc line;
  output_char oc '\n';
  close_out oc

(* ---- name <-> filename (hex, reversible, fs-safe) ---------------------- *)

let hex_encode s =
  let b = Buffer.create (String.length s * 2) in
  String.iter (fun c -> Buffer.add_string b (Printf.sprintf "%02x" (Char.code c))) s;
  Buffer.contents b

let hex_decode s =
  let n = String.length s / 2 in
  String.init n (fun i -> Char.chr (int_of_string ("0x" ^ String.sub s (i * 2) 2)))

let stream_log stream = Filename.concat (streams_dir ()) (hex_encode stream ^ ".log")
let stream_seq stream = Filename.concat (streams_dir ()) (hex_encode stream ^ ".seq")
let kv_path key = Filename.concat (kv_dir ()) (hex_encode key)

(* ---- value <-> SX text (round-trips through Sx_parser) ----------------- *)

let escape_str s =
  let len = String.length s in
  let buf = Buffer.create (len + 16) in
  for i = 0 to len - 1 do
    match s.[i] with
    | '"'  -> Buffer.add_string buf "\\\""
    | '\\' -> Buffer.add_string buf "\\\\"
    | '\n' -> Buffer.add_string buf "\\n"
    | '\r' -> Buffer.add_string buf "\\r"
    | '\t' -> Buffer.add_string buf "\\t"
    | c    -> Buffer.add_char buf c
  done;
  Buffer.contents buf

let rec serialize = function
  | Nil -> "nil"
  | Bool true -> "true"
  | Bool false -> "false"
  | Integer n -> string_of_int n
  | Number n -> format_number n
  | String s -> "\"" ^ escape_str s ^ "\""
  | Symbol s -> "(quote " ^ s ^ ")"
  | Keyword k -> ":" ^ k
  | List items | ListRef { contents = items } ->
    "(list" ^ (List.fold_left (fun acc v -> acc ^ " " ^ serialize v) "" items) ^ ")"
  | Dict d ->
    let pairs = Hashtbl.fold (fun k v acc ->
      (Printf.sprintf ":%s %s" k (serialize v)) :: acc) d [] in
    "{" ^ String.concat " " (List.sort String.compare pairs) ^ "}"
  | _ -> "nil"

(* Parse one serialized value back. Empty / blank -> Nil. *)
let rec deserialize line =
  let line = String.trim line in
  if line = "" then Nil
  else match Sx_parser.parse_all line with
    | v :: _ -> eval_quote_lists v
    | [] -> Nil

(* serialize emits lists as `(list ...)` and symbols as `(quote s)` so the
   parser yields data, not a call — but the parser leaves those as AST. Walk
   the parsed AST and collapse `(list ...)`/`(quote s)` back to values. *)
and eval_quote_lists v =
  match v with
  | List (Symbol "quote" :: x :: []) -> x
  | List (Symbol "list" :: rest) -> List (List.map eval_quote_lists rest)
  | List items -> List (List.map eval_quote_lists items)
  | ListRef { contents = items } -> List (List.map eval_quote_lists items)
  | Dict d ->
    let d' = Hashtbl.create (Hashtbl.length d) in
    Hashtbl.iter (fun k v -> Hashtbl.replace d' k (eval_quote_lists v)) d;
    Dict d'
  | other -> other

(* ---- seq counter ------------------------------------------------------- *)

let read_seq stream =
  let p = stream_seq stream in
  if Sys.file_exists p then (try int_of_string (String.trim (read_file p)) with _ -> 0)
  else 0

let write_seq stream n = write_file_atomic (stream_seq stream) (string_of_int n)

let value_to_int = function
  | Integer n -> n
  | Number n -> int_of_float n
  | _ -> 0

let event_seq ev =
  match ev with
  | Dict d -> (match Hashtbl.find_opt d "seq" with Some v -> value_to_int v | None -> 0)
  | _ -> 0

(* ---- ops --------------------------------------------------------------- *)

let do_append stream ev =
  ensure_dir (streams_dir ());
  (* bump the monotonic high-water mark; create .seq on first append so the
     stream shows up in `streams` and survives later truncation. *)
  let hw = read_seq stream in
  let s = event_seq ev in
  write_seq stream (max hw s);
  append_line (stream_log stream) (serialize ev)

let do_read stream =
  let p = stream_log stream in
  if not (Sys.file_exists p) then List []
  else begin
    let content = read_file p in
    let lines = String.split_on_char '\n' content in
    let evs = List.filter_map (fun l ->
      if String.trim l = "" then None else Some (deserialize l)) lines in
    List evs
  end

let do_last_seq stream = Number (float_of_int (read_seq stream))

let list_dir_suffix dir suffix =
  if not (Sys.file_exists dir) then []
  else
    Array.to_list (Sys.readdir dir)
    |> List.filter (fun f -> Filename.check_suffix f suffix)
    |> List.map (fun f -> hex_decode (Filename.chop_suffix f suffix))
    |> List.sort String.compare

let do_streams () = List (List.map (fun s -> String s) (list_dir_suffix (streams_dir ()) ".seq"))

(* drop events with seq <= n; the .seq high-water counter is untouched. *)
let do_truncate stream n =
  let p = stream_log stream in
  if Sys.file_exists p then begin
    let evs = match do_read stream with List l -> l | _ -> [] in
    let kept = List.filter (fun ev -> event_seq ev > n) evs in
    let body = String.concat "" (List.map (fun ev -> serialize ev ^ "\n") kept) in
    write_file_atomic p body
  end

let do_kv_get key =
  let p = kv_path key in
  if Sys.file_exists p then deserialize (read_file p) else Nil

let do_kv_put key v =
  ensure_dir (kv_dir ());
  write_file_atomic (kv_path key) (serialize v)

let do_kv_delete key =
  let p = kv_path key in
  if Sys.file_exists p then (try Sys.remove p with _ -> ())

let do_kv_has key = Bool (Sys.file_exists (kv_path key))

let do_kv_keys () =
  if not (Sys.file_exists (kv_dir ())) then List []
  else
    List (
      Array.to_list (Sys.readdir (kv_dir ()))
      |> List.map hex_decode
      |> List.sort String.compare
      |> List.map (fun s -> String s))

(* ---- blob store (content-addressed) ------------------------------------ *)
(* Same pattern as the persist ops, but a SEPARATE adapter: large objects live
   in a content-addressed directory keyed by a CIDv1 (raw codec, sha2-256).
   persist only ever stores the returned ref ({:cid :size :mime}), never bytes.
   blob/put is idempotent — identical bytes hash to the same cid + same file. *)

let codec_raw = 0x55

let blob_cid bytes =
  let digest = Sx_cid.unhex (Sx_sha2.sha256_hex bytes) in
  Sx_cid.cidv1 codec_raw (Sx_cid.multihash Sx_cid.mh_sha2_256 digest)

let blob_path cid = Filename.concat (blobs_dir ()) cid

let do_blob_put bytes =
  let cid = blob_cid bytes in
  let p = blob_path cid in
  if not (Sys.file_exists p) then write_file_atomic p bytes;
  String cid

let do_blob_get cid =
  let p = blob_path cid in
  if Sys.file_exists p then String (read_file p) else Nil

let do_blob_has cid = Bool (Sys.file_exists (blob_path cid))

(* ---- dispatch ---------------------------------------------------------- *)

let arglist = function
  | List l | ListRef { contents = l } -> l
  | Nil -> []
  | v -> [v]

(* Returns Some response if op is a persist op this store owns, None otherwise. *)
let handle_op op args =
  let a = arglist args in
  let str = function String s -> s | v -> value_to_string v in
  match op with
  | "persist/append" ->
    (match a with stream :: ev :: _ -> do_append (str stream) ev | _ -> ()); Some Nil
  | "persist/read" ->
    (match a with stream :: _ -> Some (do_read (str stream)) | _ -> Some (List []))
  | "persist/last-seq" ->
    (match a with stream :: _ -> Some (do_last_seq (str stream)) | _ -> Some (Number 0.0))
  | "persist/streams" -> Some (do_streams ())
  | "persist/truncate" ->
    (match a with stream :: n :: _ -> do_truncate (str stream) (value_to_int n) | _ -> ()); Some Nil
  | "persist/kv-get" ->
    (match a with key :: _ -> Some (do_kv_get (str key)) | _ -> Some Nil)
  | "persist/kv-put" ->
    (match a with key :: v :: _ -> do_kv_put (str key) v | _ -> ()); Some Nil
  | "persist/kv-delete" ->
    (match a with key :: _ -> do_kv_delete (str key) | _ -> ()); Some Nil
  | "persist/kv-has?" ->
    (match a with key :: _ -> Some (do_kv_has (str key)) | _ -> Some (Bool false))
  | "persist/kv-keys" -> Some (do_kv_keys ())
  | "blob/put" ->
    (match a with bytes :: _ -> Some (do_blob_put (str bytes)) | _ -> Some Nil)
  | "blob/get" ->
    (match a with cid :: _ -> Some (do_blob_get (str cid)) | _ -> Some Nil)
  | "blob/has?" ->
    (match a with cid :: _ -> Some (do_blob_has (str cid)) | _ -> Some (Bool false))
  | _ -> None