mte/src/headers_lib.ml

(* independent library for headers fields value *)

(* TODO - test - can still bypass this module and directly set Etag header, but
   its fine *)
module Etag : sig
  (* module to parse etags header fields used by If-Match and If-None-Match
     headers

     https://httpwg.org/specs/rfc9110.html#field.etag *)
  type t
  type header_value

  val parse : string -> (header_value, string) result
  val of_crockford32 : string -> (t, string) result
  val to_raw_string : t -> string
  val to_field_value : t -> string
  val evaluate : t -> header_value -> bool
end = struct
  (* raw etag *)
  type t = string

  (* type for the value of header field *)
  type header_etag_item = {
      weak: bool
    ; value: string
  }

  type header_value =
    | Any_etag
    | Etag_list of header_etag_item list

  let pp_header_etag_item ppf { weak; value } =
    if weak then Fmt.pf ppf {|W/"%s"|} value else Fmt.pf ppf {|"%s"|} value

  let to_raw_string t = t

  let to_field_value t =
    (* always weak comparison for If-None-Match header *)
    let v = { weak= true; value= t } in
    Fmt.str "%a" pp_header_etag_item v

  let is_valid_char c =
    let n = Char.code c in
    (n >= 0x21 && n <= 0x7E && n <> 0x22) || (n >= 0x80 && n <= 0xFF)

  let has_valid_charset s = String.for_all is_valid_char s

  let of_crockford32 s =
    match has_valid_charset s with
    | false -> Error "invalid etag"
    | true -> Ok s

  let parse =
    let open Angstrom in
    let ws = skip_while (function ' ' -> true | _ -> false) in
    let quoted_string =
      char '"' *> take_till (fun c -> c = '"') <* char '"' >>= fun s ->
      if String.for_all is_valid_char s then return s
      else fail "found illegal char"
    in
    let item =
      ws
      *> lift2
           (fun weak value -> { weak; value })
           (option false (string "W/" *> return true))
           quoted_string
      <* ws
    in
    let comma = ws *> char ',' *> ws in
    let list_of_items = sep_by1 comma item in
    let parse_header_value =
      char '*' *> return Any_etag
      <|> (list_of_items >>| fun items -> Etag_list items)
      <* end_of_input
    in
    fun s ->
      match parse_string ~consume:Consume.All parse_header_value s with
      | Error e -> Fmt.error "invalid etag: %s" e
      | Ok v -> Ok v

  let evaluate t header_value =
    match header_value with
    | Any_etag -> false
    | Etag_list l ->
        not @@ List.exists (fun { weak= _; value } -> String.equal t value) l
end