(** SX bytecode VM — stack-based interpreter.

    Executes bytecode produced by compiler.sx.
    Designed for speed: array-based stack, direct dispatch,
    no allocation per step (unlike the CEK machine).

    This is the platform-native execution engine. The same bytecode
    runs on all platforms (OCaml, JS, WASM). *)

open Sx_types

(** Code object — compiled function body. *)
type code = {
  arity     : int;
  locals    : int;
  bytecode  : int array;
  constants : value array;
}

(** Upvalue cell — shared mutable reference to a captured variable.
    Open when the variable is still on the stack; closed when the
    enclosing frame returns and the value is moved to the heap. *)
type upvalue_cell = {
  mutable uv_value : value;
}

(** Closure — code + captured upvalues + live env reference. *)
type vm_closure = {
  code     : code;
  upvalues : upvalue_cell array;
  name     : string option;
  env_ref  : (string, value) Hashtbl.t;  (* live global env — NOT a snapshot *)
}

(** Call frame — one per function invocation. *)
type frame = {
  closure   : vm_closure;
  mutable ip : int;
  base      : int;  (* base index in value stack for locals *)
  local_cells : (int, upvalue_cell) Hashtbl.t;  (* slot → shared cell for captured locals *)
}

(** VM state. *)
type vm = {
  mutable stack  : value array;
  mutable sp     : int;
  mutable frames : frame list;
  globals        : (string, value) Hashtbl.t;  (* live reference to kernel env *)
}

let create globals =
  { stack = Array.make 4096 Nil; sp = 0; frames = []; globals }

(** Stack ops — inlined for speed. *)
let push vm v =
  if vm.sp >= Array.length vm.stack then begin
    let ns = Array.make (vm.sp * 2) Nil in
    Array.blit vm.stack 0 ns 0 vm.sp;
    vm.stack <- ns
  end;
  vm.stack.(vm.sp) <- v;
  vm.sp <- vm.sp + 1

let[@inline] pop vm =
  vm.sp <- vm.sp - 1;
  vm.stack.(vm.sp)

let[@inline] peek vm = vm.stack.(vm.sp - 1)

(** Read operands. *)
let[@inline] read_u8 f =
  let v = f.closure.code.bytecode.(f.ip) in
  f.ip <- f.ip + 1; v

let[@inline] read_u16 f =
  let lo = f.closure.code.bytecode.(f.ip) in
  let hi = f.closure.code.bytecode.(f.ip + 1) in
  f.ip <- f.ip + 2;
  lo lor (hi lsl 8)

let[@inline] read_i16 f =
  let v = read_u16 f in
  if v >= 32768 then v - 65536 else v

(** Wrap a VM closure as an SX value (NativeFn). *)
let closure_to_value cl =
  NativeFn ("vm:" ^ (match cl.name with Some n -> n | None -> "anon"),
    fun args -> raise (Eval_error ("VM_CLOSURE_CALL:" ^ String.concat "," (List.map Sx_runtime.value_to_str args))))
  (* Placeholder — actual calls go through vm_call below *)

(** Main execution loop. *)
let rec run vm =
  match vm.frames with
  | [] -> ()  (* no frame = done *)
  | frame :: rest_frames ->
    let bc = frame.closure.code.bytecode in
    let consts = frame.closure.code.constants in
    if frame.ip >= Array.length bc then () (* ran off end *)
    else
    let op = bc.(frame.ip) in
    frame.ip <- frame.ip + 1;
    match op with
    (* ---- Constants ---- *)
    | 1 (* OP_CONST *) ->
      let idx = read_u16 frame in
      push vm consts.(idx);
      run vm
    | 2 (* OP_NIL *)   -> push vm Nil; run vm
    | 3 (* OP_TRUE *)  -> push vm (Bool true); run vm
    | 4 (* OP_FALSE *) -> push vm (Bool false); run vm
    | 5 (* OP_POP *)   -> ignore (pop vm); run vm
    | 6 (* OP_DUP *)   -> push vm (peek vm); run vm

    (* ---- Variable access ---- *)
    | 16 (* OP_LOCAL_GET *) ->
      let slot = read_u8 frame in
      (* Check if this local is captured — read from shared cell *)
      let v = match Hashtbl.find_opt frame.local_cells slot with
        | Some cell -> cell.uv_value
        | None -> vm.stack.(frame.base + slot)
      in
      push vm v;
      run vm
    | 17 (* OP_LOCAL_SET *) ->
      let slot = read_u8 frame in
      let v = peek vm in
      (* Write to shared cell if captured, else to stack *)
      (match Hashtbl.find_opt frame.local_cells slot with
       | Some cell -> cell.uv_value <- v
       | None -> vm.stack.(frame.base + slot) <- v);
      run vm
    | 18 (* OP_UPVALUE_GET *) ->
      let idx = read_u8 frame in
      push vm frame.closure.upvalues.(idx).uv_value;
      run vm
    | 19 (* OP_UPVALUE_SET *) ->
      let idx = read_u8 frame in
      frame.closure.upvalues.(idx).uv_value <- peek vm;
      run vm
    | 20 (* OP_GLOBAL_GET *) ->
      let idx = read_u16 frame in
      let name = match consts.(idx) with String s -> s | _ -> "" in
      let v = try Hashtbl.find vm.globals name with Not_found ->
        try Sx_primitives.get_primitive name
        with _ -> raise (Eval_error ("VM undefined: " ^ name))
      in
      push vm v; run vm
    | 21 (* OP_GLOBAL_SET *) ->
      let idx = read_u16 frame in
      let name = match consts.(idx) with String s -> s | _ -> "" in
      Hashtbl.replace vm.globals name (peek vm);
      run vm

    (* ---- Control flow ---- *)
    | 32 (* OP_JUMP *) ->
      let offset = read_i16 frame in
      frame.ip <- frame.ip + offset;
      run vm
    | 33 (* OP_JUMP_IF_FALSE *) ->
      let offset = read_i16 frame in
      let v = pop vm in
      if not (sx_truthy v) then frame.ip <- frame.ip + offset;
      run vm
    | 34 (* OP_JUMP_IF_TRUE *) ->
      let offset = read_i16 frame in
      let v = pop vm in
      if sx_truthy v then frame.ip <- frame.ip + offset;
      run vm

    (* ---- Function calls ---- *)
    | 48 (* OP_CALL *) ->
      let argc = read_u8 frame in
      let args = Array.init argc (fun _ -> pop vm) in
      let f = pop vm in
      let args_list = List.rev (Array.to_list args) in
      vm_call vm f args_list;
      run vm
    | 49 (* OP_TAIL_CALL *) ->
      let argc = read_u8 frame in
      let args = Array.init argc (fun _ -> pop vm) in
      let f = pop vm in
      let args_list = List.rev (Array.to_list args) in
      (* Tail call: pop current frame, reuse stack space *)
      vm.frames <- rest_frames;
      vm.sp <- frame.base;
      vm_call vm f args_list;
      run vm
    | 50 (* OP_RETURN *) ->
      let result = pop vm in
      vm.frames <- rest_frames;
      vm.sp <- frame.base;
      push vm result
      (* Return — don't recurse, let caller continue *)
    | 51 (* OP_CLOSURE *) ->
      let idx = read_u16 frame in
      let code_val = consts.(idx) in
      let code = code_from_value code_val in
      (* Read upvalue descriptors from bytecode *)
      let uv_count = match code_val with
        | Dict d -> (match Hashtbl.find_opt d "upvalue-count" with
            | Some (Number n) -> int_of_float n | _ -> 0)
        | _ -> 0
      in
      let upvalues = Array.init uv_count (fun _ ->
        let is_local = read_u8 frame in
        let index = read_u8 frame in
        if is_local = 1 then begin
          (* Capture from enclosing frame's local slot.
             Create a shared cell — both parent and closure
             read/write through this cell. *)
          let cell = match Hashtbl.find_opt frame.local_cells index with
            | Some existing -> existing  (* reuse existing cell *)
            | None ->
              let c = { uv_value = vm.stack.(frame.base + index) } in
              Hashtbl.replace frame.local_cells index c;
              c
          in
          cell
        end else
          (* Capture from enclosing frame's upvalue — already a shared cell *)
          frame.closure.upvalues.(index)
      ) in
      let cl = { code; upvalues; name = None; env_ref = vm.globals } in
      (* Wrap as NativeFn that calls back into the VM *)
      let fn = NativeFn ("vm-closure", fun args ->
        call_closure cl args vm.globals)
      in
      push vm fn;
      run vm
    | 52 (* OP_CALL_PRIM *) ->
      let idx = read_u16 frame in
      let argc = read_u8 frame in
      let name = match consts.(idx) with String s -> s | _ -> "" in
      let args = List.init argc (fun _ -> pop vm) |> List.rev in
      let result =
        (match Sx_primitives.get_primitive name with
         | NativeFn (_, fn) -> fn args
         | _ -> Nil)
      in
      push vm result;
      run vm

    (* ---- Collections ---- *)
    | 64 (* OP_LIST *) ->
      let count = read_u16 frame in
      let items = List.init count (fun _ -> pop vm) |> List.rev in
      push vm (List items);
      run vm
    | 65 (* OP_DICT *) ->
      let count = read_u16 frame in
      let d = Hashtbl.create count in
      for _ = 1 to count do
        let v = pop vm in
        let k = pop vm in
        let key = match k with String s -> s | Keyword s -> s | _ -> Sx_runtime.value_to_str k in
        Hashtbl.replace d key v
      done;
      push vm (Dict d);
      run vm

    (* ---- String ops ---- *)
    | 144 (* OP_STR_CONCAT *) ->
      let count = read_u8 frame in
      let parts = List.init count (fun _ -> pop vm) |> List.rev in
      let s = String.concat "" (List.map Sx_runtime.value_to_str parts) in
      push vm (String s);
      run vm

    (* ---- Define ---- *)
    | 128 (* OP_DEFINE *) ->
      let idx = read_u16 frame in
      let name = match consts.(idx) with String s -> s | _ -> "" in
      let v = peek vm in
      Hashtbl.replace vm.globals name v;
      run vm

    | opcode ->
      (* Unknown opcode — fall back to CEK machine *)
      raise (Eval_error (Printf.sprintf "VM: unknown opcode %d at ip=%d"
                           opcode (frame.ip - 1)))

(** Call a value as a function — dispatch by type. *)
and vm_call vm f args =
  match f with
  | NativeFn (_name, fn) ->
    let result = fn args in
    push vm result
  | Lambda _ | Component _ | Island _ ->
    (* Fall back to CEK machine for SX-defined functions *)
    let result = Sx_ref.cek_call f (List args) in
    push vm result
  | _ ->
    raise (Eval_error ("VM: not callable: " ^ Sx_runtime.value_to_str f))

(** Convert compiler output (SX dict) to a code object. *)
and code_from_value v =
  match v with
  | Dict d ->
    let bc_list = match Hashtbl.find_opt d "bytecode" with
      | Some (List l | ListRef { contents = l }) ->
        Array.of_list (List.map (fun x -> match x with Number n -> int_of_float n | _ -> 0) l)
      | _ -> [||]
    in
    let entries = match Hashtbl.find_opt d "constants" with
      | Some (List l | ListRef { contents = l }) -> Array.of_list l
      | _ -> [||]
    in
    let constants = Array.map (fun entry ->
      match entry with
      | Dict ed when Hashtbl.mem ed "bytecode" -> entry  (* nested code — convert lazily *)
      | _ -> entry
    ) entries in
    let arity = match Hashtbl.find_opt d "arity" with
      | Some (Number n) -> int_of_float n | _ -> 0
    in
    { arity; locals = arity + 16; bytecode = bc_list; constants }
  | _ -> { arity = 0; locals = 16; bytecode = [||]; constants = [||] }

(** Execute a closure with arguments — creates a new VM frame.
    The closure carries its upvalue cells for captured variables. *)
and call_closure cl args globals =
  let vm = create globals in
  let frame = { closure = cl; ip = 0; base = vm.sp; local_cells = Hashtbl.create 4 } in
  (* Push args as locals *)
  List.iter (fun a -> push vm a) args;
  (* Pad remaining locals with nil *)
  for _ = List.length args to cl.code.locals - 1 do push vm Nil done;
  vm.frames <- [frame];
  run vm;
  pop vm

(** Execute a compiled module (top-level bytecode). *)
let execute_module code globals =
  let cl = { code; upvalues = [||]; name = Some "module"; env_ref = globals } in
  let vm = create globals in
  let frame = { closure = cl; ip = 0; base = 0; local_cells = Hashtbl.create 4 } in
  for _ = 0 to code.locals - 1 do push vm Nil done;
  vm.frames <- [frame];
  run vm;
  pop vm