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 invalid_etag = Error "invalid etag field"

  let has_valid_charset s =
    let f c =
      let n = Char.code c in
      (n >= 0x21 && n <= 0x7E && n <> 0x22) || (n >= 0x80 && n <= 0xFF)
    in
    String.for_all f s

  let of_crockford32 s =
    match has_valid_charset s with false -> invalid_etag | true -> Ok s

  let trim_dquote s =
    let len = String.length s in
    if len >= 2 && s.[0] = '"' && s.[len - 1] = '"' then
      Ok (String.sub s 1 (len - 2))
    else invalid_etag

  (* TODO angstrom parser instead *)
  let parse s =
    if String.trim s = "*" then Ok Any_etag
    else
      String.split_on_char ',' s
      |> List.map String.trim
      |> List.filter (( <> ) "")
      |> Syntax.list_map (fun s ->
             (* "W/" is the weak comparison indicator *)
             let weak, s =
               if String.starts_with ~prefix:"W/" s then
                 (true, String.sub s 2 (String.length s - 2))
               else (false, s)
             in
             Result.bind (trim_dquote s) (fun s ->
                 if has_valid_charset s then Ok { weak; value= s }
                 else invalid_etag))
      |> Result.map (fun l -> Etag_list l)

  (* To evaluate a received If-None-Match header field: - If the field value is
     "*", the condition is false if the origin server has a current
     representation for the target resource. - If the field value is a list of
     entity tags, the condition is false if one of the listed tags matches the
     entity tag of the selected representation. - Otherwise, the condition is
     true. *)
  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