(* A blob is a one-byte constructor tag followed by its payload. The
   tag makes the encoding self-describing so [value_of_blob] can
   recover the constructor:
   - String  ['\000'] ++ raw utf-8 bytes
   - Integer ['\001'] ++ 8-byte big-endian int64 *)
let tag_string = '\000'
let tag_integer = '\001'

let blob_of_value = function
  | Value.String s ->
      let b = Blob.create (1 + String.length s) in
      Blob.set b 0 tag_string;
      Blob.blit_string s 0 b 1 (String.length s);
      b
  | Value.Integer n ->
      let b = Blob.create 9 in
      Blob.set b 0 tag_integer;
      Blob.set_int64_be b 1 (Int64.of_int n);
      b

let value_of_blob (b : Blob.t) =
  let n = Blob.length b in
  if n = 0 then invalid_arg "value_of_blob: empty blob";
  match Blob.get b 0 with
  | c when c = tag_string -> Value.String (Blob.sub_string b 1 (n - 1))
  | c when c = tag_integer ->
      if n <> 9 then invalid_arg "value_of_blob: malformed integer payload";
      Value.Integer (Int64.to_int (Blob.get_int64_be b 1))
  | c -> invalid_arg (Printf.sprintf "value_of_blob: unknown tag %C" c)

 module Error = struct
  open Condition

  let unexpected_end =
    condition "codec-unexpected-end" "Unexpected end of input while decoding" empty

  let malformed_integer digits =
    condition "codec-malformed-integer" "A byte run is not a valid integer"
      ("digits" |=| Value.String digits)

  let unexpected_byte c =
    condition "codec-unexpected-byte" "Unexpected byte while decoding"
      ("byte" |=| Value.String (String.make 1 c))

  let string_out_of_bounds length =
    condition "codec-string-out-of-bounds" "A string length runs past the end of input"
      ("length" |=| Value.Integer length)

  let trailing_bytes =
    condition "codec-trailing-bytes" "Bytes remain after a complete value" empty

  let tag_mismatch ~expected ~actual =
    condition "codec-tag-mismatch" "A value with a different tag was expected"
      ("expected" |=| Value.String (String.make 1 expected) &
       "actual" |=| Value.String (String.make 1 actual))

  let type_mismatch ~expected =
    condition "codec-type-mismatch" "A Bencode value of a different shape was expected"
      ("expected" |=| Value.String expected)

  let missing_field key =
    condition "codec-missing-field" "A required field is absent"
      ("field" |=| Value.String key)

  let unexpected_object ~expected ~actual =
    condition "codec-unexpected-object" "The decoded object is not of the expected kind"
      ("expected" |=| Value.String expected & "actual" |=| Value.String actual)
end

(* A self-describing serialization used as the uniform wire format for
   durable objects. Chosen over an ad-hoc key=value scheme because it
   nests (a multigroup holds a map of relation hashes) and has a
   single byte form for a given value (dict keys are emitted sorted),
   which matters once these bytes are content-addressed.

   Grammar:
   - Int    "i" <decimal> "e" (negatives allowed)
   - String <length> ":" <raw bytes>
   - List   "l" <element>* "e"
   - Dict   "d" (<String key> <value>)* "e" keys sorted on encode *)
module Bencode = struct
  type t = Tagged of char * t | Int of int | String of string | List of t list | Dict of (string * t) list

  let rec encode buf = function
    | Tagged (tag, value) ->
       Buffer.add_char buf 't';
       Buffer.add_char buf tag;
       encode buf value
    | Int n ->
       Buffer.add_char buf 'i';
       Buffer.add_string buf (string_of_int n);
       Buffer.add_char buf 'e'
    | String s ->
       Buffer.add_string buf (string_of_int (String.length s));
       Buffer.add_char buf ':';
       Buffer.add_string buf s
    | List xs ->
       Buffer.add_char buf 'l';
       List.iter (encode buf) xs;
       Buffer.add_char buf 'e'
    | Dict kvs ->
       Buffer.add_char buf 'd';
       List.iter
         (fun (k, v) ->
           encode buf (String k);
           encode buf v )
         (List.sort (fun (a, _) (b, _) -> String.compare a b) kvs);
       Buffer.add_char buf 'e'

  let to_bytes (value : t) : bytes =
    let buf = Buffer.create 256 in
    encode buf value;
    Buffer.to_bytes buf

  let as_string = function String s -> Ok s | _ -> Error (Error.type_mismatch ~expected:"string")
  let as_int = function Int n -> Ok n | _ -> Error (Error.type_mismatch ~expected:"integer")
  let as_list = function List l -> Ok l | _ -> Error (Error.type_mismatch ~expected:"list")
  let as_dict = function Dict d -> Ok d | _ -> Error (Error.type_mismatch ~expected:"dict")

  let of_bytes (bytes : bytes) : (t, Condition.condition) result =
    let open Utilities.Result in
    let s = Bytes.to_string bytes in
    let n = String.length s in
    let pos = ref 0 in
    let peek () = if !pos >= n then Error (Error.unexpected_end) else Ok (s.[!pos]) in
    let advance () = incr pos in
    (* Read the decimal run up to (and consuming) [term]. *)
    let read_int_from start term =
      let rec scan () =
        let* c = peek () in
        if c = term then begin
          let digits = String.sub s start (!pos - start) in
          advance ();
          match int_of_string_opt digits with
          | Some i -> Ok i
          | None -> Error (Error.malformed_integer digits)
        end
        else begin
          advance ();
          scan ()
        end
      in
      scan ()
    in
    let read_int term = read_int_from !pos term in
    let read_char () =
      let* c = peek () in
      advance ();
      Ok c
    in
    let rec parse () =
      let* c = peek () in
      advance ();
      match c with
      | 't' ->
         let* tag = read_char () in
         let* data = parse () in
         Ok (Tagged (tag, data))
      | 'i' ->
         let* n = read_int 'e' in
         Ok (Int n)
      | 'l' -> parse_list []
      | 'd' -> parse_dict []
      | c when c >= '0' && c <= '9' -> parse_string_from (!pos - 1)
      | c -> Error (Error.unexpected_byte c)
    and parse_string () =
      parse_string_from !pos
    and parse_string_from start =
      let* len = read_int_from start ':' in
      if len < 0 || !pos + len > n then
        Error (Error.string_out_of_bounds len)
      else
        let str = String.sub s !pos len in
        pos := !pos + len;
        Ok (String str)
    and parse_list acc =
      let* c = peek () in
      if c = 'e' then (advance (); Ok (List (List.rev acc)))
      else
        let* result = parse () in
        parse_list (result :: acc)
    and parse_dict acc =
      let* c = peek () in
      if c = 'e' then (advance (); Ok (Dict (List.rev acc)))
      else
        let* key = parse_string () in
        let* key = as_string key in
        let* value = parse () in
        parse_dict ((key, value) :: acc)
    in
    let* value = parse () in
    if !pos <> n then Error Error.trailing_bytes else
      Ok value

  let to_blob x = to_bytes x |> Blob.blob_of_bytes
  let of_blob b = Blob.bytes_of_blob b |> of_bytes

  let with_tag t = function
    | Tagged (t', v) ->
       if t = t' then
         Ok v
       else
         Error (Error.tag_mismatch ~expected:t ~actual:t')
    | _ -> Error (Error.type_mismatch ~expected:"tagged")

  let field key = function
    | Dict kvs -> (
      match List.assoc_opt key kvs with
      | Some v -> Ok v
      | None -> Error (Error.missing_field key) )
    | _ -> Error (Error.type_mismatch ~expected:"dict")
end

module Field = struct
  let string value = Bencode.String value
  let address address = Bencode.String (Hash.to_raw_string address)

  let require_string key data =
    Bencode.field key data |> Utilities.Result.fmap Bencode.as_string

  let require_address ~malformed key data =
    let open Utilities.Result in
    let* raw = require_string key data in
    if String.length raw = Hash.size then Ok (Hash.of_raw_string raw)
    else Error (malformed (String.length raw))
end

module Record = struct
  module type S = sig
    type t

    val equal : t -> t -> bool

    val to_bencode : t -> Bencode.t
    val of_bencode : Bencode.t -> (t, Condition.condition) result

    val to_blob : t -> Blob.t
    val of_blob : Blob.t -> (t, Condition.condition) result

    val to_bytes : t -> bytes
    val of_bytes : bytes -> (t, Condition.condition) result
  end

  module type BODY = sig
    type t

    val tag : char
    val malformed : unit -> Condition.condition
    val equal : t -> t -> bool
    val fields : t -> (string * Bencode.t) list
    val of_fields : Bencode.t -> (t, Condition.condition) result
  end

  module Make (B : BODY) = struct
    type t = B.t

    let equal = B.equal
    let to_bencode object_ = Bencode.Tagged (B.tag, Bencode.Dict (B.fields object_))

    let of_bencode = function
      | Bencode.Tagged (tag, (Bencode.Dict _ as data)) when Char.equal tag B.tag ->
         B.of_fields data
      | _ -> Error (B.malformed ())

    let to_blob object_ = to_bencode object_ |> Bencode.to_blob
    let of_blob blob = Bencode.of_blob blob |> Utilities.Result.fmap of_bencode

    let to_bytes object_ = to_blob object_ |> Blob.bytes_of_blob
    let of_bytes bytes = Bencode.of_bytes bytes |> Utilities.Result.fmap of_bencode
  end
end