Sosa: Literate Implementation

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(** Sane OCaml String API *)

type ('a, 'b) result = [
  | `Ok of 'a
  | `Error of 'b
]
(** The type [result] is a reusable version the classical [Result.t]
    type. *)

module type OUTPUT_MODEL = sig
  (** A monadic thread model (like [Lwt], [Async]) and an [output]
  function. *)

  type ('a, 'b, 'c) thread
  (** The type of the threads, the type parameters are there in case
      the user needs up to 3 of them.
      
      For instance, if implement with [Lwt], we will have [type ('a, 'b, 'c)
      thread = 'a Lwt.t], but with [Pvem.DEFERRED_RESULT]: [type ('a, 'b, 'c)
      thread = ('a, 'b) Deferred_result.t]. *)


  type ('a, 'b, 'c) channel
  (** The of the channels, channels can have up to 3 type-parameters
      too.  *)

  val return: 'a -> ('a, 'b, 'c) thread
  (** The monadic [return]. *)

  val bind: ('a, 'b, 'c) thread -> ('a -> ('d, 'b, 'c) thread) -> ('d, 'b, 'c) thread
  (** The monadic [bind]. *)

  val output: ('a, 'b, 'c) channel -> String.t -> (unit, 'e, 'f) thread
  (** The function to output a given native string to a channel.  *)

end

module type NATIVE_CONVERSIONS = sig
  (** API definition of conversions from native OCaml strings to a
      given string type or vice-versa. *)

  type t
  (** The string type. *)

  val of_native_string: string -> (t, [> `wrong_char_at of int ]) result
  (** Convert a native string to the current representation.
      [of_native_string] returns [`Error (`wrong_char_at index)]
      when the native string contains a character not representable
      with the type [character]. *)

  val of_native_substring: string -> offset:int -> length:int ->
    (t, [> `wrong_char_at of int | `out_of_bounds ]) result
  (** Convert a native string like [of_native_string] but take a
      subset of the string. *)

  val to_native_string: t -> string
  (** Serialize the string to a native string. *)

end


module type BASIC_CHARACTER = sig
  (** The minimal API implemented by characters. *)

  type t
  (** The type representing the character. *)

  val of_native_char: char -> t option
  (** Import a native [char], returns [None] if the character is not
      representable. *)

  val of_int: int -> t option
  (** Import an integer, returns [None] if there is no character for
      that value. *)

  val to_int: t -> int
  (** Returns the integer representation of the character. *)

  val size: t -> int
  (** Get the size of the character, the exact semantics are
      implementation-specific (c.f. {!write_to_native_string}) *)


  val write_to_native_string: t -> buf:String.t -> index:int -> (int, [> `out_of_bounds]) result
  (** [write_to_native_string c ~buf ~index] serializes
      the character [c] at position [index] in the native string
      [buf] (writing [size c] units). Note, as with {!size} that the
      meaning of [index] is implementation dependent (can be the {i
      index-th} byte, the {i index-th} bit, etc.). *)

  val to_native_string: t -> String.t
  (** [to_native_string c] creates a string containing the
      serialization of the character [c] (if [size c] is not a
      multiple of 8, the end-padding is undefined). *)

  val read_from_native_string: buf:String.t -> index:int -> (t * int) option
  (** Read a character at a given [index] in a native string, returns
      [Some (c, s)], the character [c] and the number of units read [s],
      or [None] if there is no representable/valid character at that
      index. *)

  val to_string_hum: t -> String.t
  (** Convert the character to a human-readable native string (in the
      spirit of [sprintf "%s"]). *)

  val compare: t -> t -> int
  (** Comparison function (as expected by most common functors in the
      ecosystem). *)

  val is_whitespace: t -> bool
  (** Tell whether a character is considered whitespace. *)

end

module type BASIC_STRING = sig
  (** The minimal API implemented by string modules. *)

  type character
  (** A string is a string of characters. *)

  type t
  (** The type of the string. *)

  val empty: t
  (** An “empty” string. *)

  val is_empty: t -> bool
  (** Test whether a string is empty. *)

  val make: int -> character -> t
  (** Build a new string like [String.make]. *)

  val length: t -> int
  (** Get the length of the string (i.e. the number of characters). *)

  val of_character: character -> t
  (** Make a string with one character. *)

  val of_character_list: character list -> t
  (** Make a string out of a list of characters. *)

  val to_character_list: t -> character list
  (** Explode a string into a list of characters. *)


  val get: t -> index:int -> character option
  (** Get the n-th char, indexes are not necessarily bytes, they can
      be bits. [get] returns [None] when [index] is out of bounds. *)

  val set: t -> index:int -> v:character -> t option
  (** [set str ~index ~v] creates a new string equal to [t] with
      character [v] at position [index]. [set] returns [None] when
      [index] is out of bounds. *)

  val get_exn: t -> index:int -> character
  (** Like [get] but fail with an exception *)

  val set_exn: t -> index:int -> v:character -> t
  (** Like [set] but fail with an exception *)

  val concat: ?sep:t -> t list -> t
  (** The classical [concat] function. *)

  include NATIVE_CONVERSIONS with type t := t
  (** By including {!NATIVE_CONVERSIONS}, a
      basic string provides
      {!NATIVE_CONVERSIONS.of_native_string},
      {!NATIVE_CONVERSIONS.of_native_substring}, and
      {!NATIVE_CONVERSIONS.to_native_string}.
  *)

  val to_string_hum: t -> string
  (** Convert the string to a human-readable native string (à la
      [sprintf "%S"]). *)

  val fold: t -> init:'a -> f:('a -> character -> 'a) -> 'a
  (** The standard [fold] function, see [List.fold_left] for example. *)

  val foldi: t -> init:'a -> f:(int -> 'a -> character -> 'a) -> 'a
  (** Pass an accumulator over the string's characters and their ordinals,
      starting with the first; left most. *)

  val fold2_exn: t -> t -> init:'a -> f:('a -> character -> character -> 'a) -> 'a
  (** The standard [fold2] function, see [List.fold_left2] for example. Fails on
  [t]s of different length. *)

  val compare: t -> t -> int
  (** Comparison function (as expected by most common functors in the
      ecosystem). *)

  val sub: t -> index:int -> length:int -> t option
  (** Get the sub-string of size [length] at position [index]. If
      [length] is 0, [sub] returns [Some empty] whichever the other
      parameters are. *)

  val sub_exn: t -> index:int -> length:int -> t
  (** Do like [sub] but throw an exception instead of returning [None] *)

  val slice: ?start:int -> ?finish:int -> t -> t option
  (** Create a sub-string from the [start] (default 0, within \[0,length\))
      position to before the [finish] (default length, within \[0,length\])
      if all of the indices are in bounds.  *)

  val slice_exn: ?start:int -> ?finish:int -> t -> t
  (** Like [slice] but throw an exception instead of returning [None] *)

  val is_prefix: t -> prefix:t -> bool
  (** Does [t] start with [prefix] ? *)

  val is_suffix: t -> suffix:t -> bool
  (** Does [t] end with [suffix] ? *)

  val chop_prefix_exn: t -> prefix:t -> t
  (** Return a copy of [t] with [prefix] removed from the beginning.
      Throws Invalid_argument if [t] does not start with [prefix]. *)

  val chop_prefix: t -> prefix:t -> t option
  (** Like [chop_prefix_exn] but return [None] instead of throwing
      an exception. *)

  val chop_suffix_exn: t -> suffix:t -> t
  (** Return a copy of [t] with [suffix] removed from the end.
      Throws Invalid_argument if [t] does not end with [suffix]. *)

  val chop_suffix: t -> suffix:t -> t option
  (** Like [chop_suffix_exn] but return [None] instead of throwing
      an exception. *)

  val split_at: t -> index:int -> t * t
  (** Return a tuple where the first string is a prefix of the specified length
      and the second is the rest. If index is [=< 0] then the first element is
      empty and the string is returned in the second element, similarly if the
      index is [>= length t] then the first element is [t] and the second is
      [empty]. *)

  val take: t -> index:int -> t
  (** Just the first part of split_at *)

  val drop: t -> index:int -> t
  (** Just the second part of split_at *)

  val compare_substring: t * int * int -> t * int * int -> int
  (** Comparison function for substrings: use as [compare_substring
      (s1, index1, length1) (s2, index2, length2)].

      Note that out-of-bounds accesses will {b not} be reported: for
      performance reasons, if the result can be decided with the
      smallest sub-string then [compare_substring] won't look
      further.

      However, if {!compare_substring_strict} returns [Some c] then
      [compare_substring] {i must} return [d] such as [c] = [d] or
      [c] × [d] > 0 (i.e. strictly same sign).

      In other words, is [sub a ~index:ia ~length:la] returns [Some suba] and
      [sub b ~index:ib ~length:lb] returns [Some subb], then
      [compare_substring (a, ia, la) (b, ib, lb)] will behave like
      [compare suba subb] (again, with the same sign).
  *)

  val compare_substring_strict: t * int * int -> t * int * int -> int option
  (** Do like {!compare_substring} but return [Some _] only when it is
      well defined (same validity criteria as {!sub}: if [length]
      is [0], [index] is irrelevant).

      Depending on the backend implementation, this function might be
      significantly slower than [compare_substring] (for example when
      calls to [length] are not {i O(1)}). *)


  val iter: t -> f:(character -> unit) -> unit
  (** Apply [f] on every character successively. *)

  val iteri: t -> f:(int -> character -> unit) -> unit
  (** Apply [f] on every character and its index. *)

  val iter_reverse: t -> f:(character -> unit) -> unit
  (** Apply [f] on every character successively in reverse order. *)

  val rev: t -> t
  (** Reverse the string. **)

  val map: t -> f:(character -> character) -> t
  (** Make a new string by applying [f] to all characters of the
      input. *)

  val mapi: t -> f:(int -> character -> character) -> t
  (** Make a new string by applying [f] to all characters and their indices. *)

  val map2_exn: t -> t -> f:(character -> character -> character) -> t
  (** Make a new string by applying [f] to all pairs of characters of the
      inputs. Fail if strings are not the same length. *)

  val for_all: t -> f:(character -> bool) -> bool
  (** Return [true] if-and-only-if [f] returns [true] on all characters. *)

  val exists: t -> f:(character -> bool) -> bool
  (** Return [true] if-and-only-if [f] returns [true] on at least one
      character. *)

  val take_while: t -> f:(character -> bool) -> t
  (** Take a prefix of the string until [f] returns [false]. *)

  val take_while_with_index: t -> f:(int -> character -> bool) -> t
  (** Like {!take_while} but the function also takes the current index. *)

  val index_of_character: t -> ?from:int -> character -> int option
  (** Find the first occurrence of a character in the string (starting
      at position [from]).
      
      Default value for [from] is [0].
      If [from] is negative, [0] will be used.
      If [from >= length t], [None] will be returned.
  *)

  val index_of_character_reverse: t -> ?from:int -> character -> int option
  (** Do like [index_of_character] but start from the end of the string.

      Default value for [from] is [length t - 1] (end of the string).
      If [from] is negative, [None] will be returned.
      If [from >= length t], [length t - 1] will be used.
  *)

  val index_of_string: ?from:int ->
    ?sub_index:int -> ?sub_length:int -> t -> sub:t -> int option
  (** Find the first occurrence of the substring [(sub, sub_index,
      sub_length)] in a given string, starting at index [from].
  
      The [from] parameter behaves like for {!index_of_character}.

      The [(sub_index, sub_length)] parameters are constrained to [(0, length
      sub)], for example, if [sub] is ["abc"], [(-1, 4)] will be equivalent to
      [(0, 3)], [(1, 3)] will be equivalent to [(1, 2)].

      Searching for an empty string [from] a valid position always succeeds at
      that position.
  *)

  val index_of_string_reverse: ?from:int ->
    ?sub_index:int -> ?sub_length:int -> t -> sub:t -> int option
  (** Do like [index_of_string] but start from the end of the string. 

      The [from] parameter behaves like for {!index_of_character_reverse}.

      The [(sub_index, sub_length)] parameters are constrained like in
      {!index_of_string}.  *)

  val find: ?from:int -> ?length:int -> t -> f:(character -> bool) -> int option
  (** Find the index of the first character [c] for which [f c] is [true]. One
      can restrict to the sub-string [(from, length)] (the
      default is to use the whole string, “out-of-bound” values are restricted
      to the bounds of the string). *)

  val find_reverse:
    ?from:int -> ?length:int -> t -> f:(character -> bool) -> int option
  (** Find the index of the last character [c] for which [f c] is [true]. One
      can restrict to the reverse sub-string [(from, length)] (the
      default is to use the whole string,  “out-of-bound” values are restricted
      to the bounds of the string). *)

  val filter_map: ?from:int -> ?length:int -> t -> 
    f:(character -> character option) -> t
  (** Create a new string with the characters for which [f c] returned 
      [Some c]. One can restrict to the sub-string [(from, length)] (the
      default is to use the whole string, “out-of-bound” values are restricted
      to the bounds of the string). *)

  val filter: ?from:int -> ?length:int -> t -> f:(character -> bool) -> t
  (** Create a new string with the characters for which [f c] is true.
      One can restrict to the sub-string [(from, length)] (the
      default is to use the whole string, “out-of-bound” values are restricted
      to the bounds of the string). *)

  val split: t -> 
    on:[ `Character of character | `String of t ] ->
    t list
  (** Split the string using [on] as separator. 
      Splitting the empty string returns  [[empty]], splitting [on] the empty
      string explodes the string into a list of one-character strings. *)

  val strip: ?on:[`Both | `Left | `Right] ->
    ?whitespace:(character -> bool) -> t -> t
  (** Remove any whitespace characters at the beginning and/or the end of the
      string (default [`Both]).

      The default is to call the {!BASIC_CHARACTER.is_whitespace} function of
      the implemented character.
  *)

  module Make_output: functor (Model: OUTPUT_MODEL) -> sig

    val output:  ('a, 'b, 'c) Model.channel -> t -> (unit, 'e, 'f) Model.thread
    (** Output a string to a channel. *)

  end
  (** [Make_output(Asynchronous_output_model)] provides a function
      {!Make_output.output} given any {!OUTPUT_MODEL}. *)

end

module type UNSAFELY_MUTABLE = sig
  (** This interface defines functions that may be implemented by
      particular string types that are actually mutable.

      They are considered “unsafe” because they break the immutability
      invariants assumed by the rest of this library; you'd better
      know what you're doing. *)

  type t
  type character

  val mutate: t -> index:int -> character -> (unit, [> `out_of_bounds ]) result
  (** Set the [index]-th character of the string. *)

  val mutate_exn: t -> index:int -> character -> unit
  (** Set the [index]-th character of the string, but fail with a
      non-specified exception. *)

  val blit:
    src:t -> src_index:int -> dst:t -> dst_index:int -> length:int ->
    (unit, [> `out_of_bounds ]) result
  (** Copy [length] characters from [src] (starting at [src_index]) to
      [dst] (starting at [dst_index]). *)

  val blit_exn:
    src:t -> src_index:int -> dst:t -> dst_index:int -> length:int -> unit
  (** Like {!blit} but fail with a non-specified exception. *)

end

open Printf

(* Internal “Pervasives” module, to be used in all the following
   implementations. *)
module Internal_pervasives = struct
  module List = ListLabels
  module String = StringLabels
  let (|>) x f = f x
  let return x : (_, _) result = `Ok x
  let fail x : (_, _) result = `Error x
  let bind x f =
    match x with
    | `Ok o -> f o
    | `Error e -> fail e
  let (>>=) = bind
  let dbg fmt = printf ("DBG: " ^^ fmt ^^ "\n%!")

  (* The function `List.map` adapted from `Core_kernel`'s way of
     unrolling the loops. *)
  module Core_list_map = struct

    let map_slow l ~f = List.rev (List.rev_map ~f l)

    let rec count_map ~f l ctr =
      match l with
      | [] -> []
      | [x1] ->
        let f1 = f x1 in
        [f1]
      | [x1; x2] ->
        let f1 = f x1 in
        let f2 = f x2 in
        [f1; f2]
      | [x1; x2; x3] ->
        let f1 = f x1 in
        let f2 = f x2 in
        let f3 = f x3 in
        [f1; f2; f3]
      | [x1; x2; x3; x4] ->
        let f1 = f x1 in
        let f2 = f x2 in
        let f3 = f x3 in
        let f4 = f x4 in
        [f1; f2; f3; f4]
      | x1 :: x2 :: x3 :: x4 :: x5 :: tl ->
        let f1 = f x1 in
        let f2 = f x2 in
        let f3 = f x3 in
        let f4 = f x4 in
        let f5 = f x5 in
        f1 :: f2 :: f3 :: f4 :: f5 ::
          (if ctr > 1000
           then map_slow ~f tl
           else count_map ~f tl (ctr + 1))

    let map l ~f = count_map ~f l 0

    let mapi_slow l ~f ~i =
      let _, r = List.fold_left l
        ~f:(fun (i, a) e -> (i + 1, ((f i e)::a)))
        ~init:(i,[])
      in
      List.rev r

    let rec count_mapi ~f l ctr =
      match l with
      | [] -> []
      | [x1] ->
        let f1 = f ctr x1 in
        [f1]
      | [x1; x2] ->
        let f1 = f ctr x1 in
        let f2 = f (ctr + 1) x2 in
        [f1; f2]
      | [x1; x2; x3] ->
        let f1 = f ctr x1 in
        let f2 = f (ctr + 1) x2 in
        let f3 = f (ctr + 2) x3 in
        [f1; f2; f3]
      | [x1; x2; x3; x4] ->
        let f1 = f ctr x1 in
        let f2 = f (ctr + 1) x2 in
        let f3 = f (ctr + 2) x3 in
        let f4 = f (ctr + 3) x4 in
        [f1; f2; f3; f4]
      | x1 :: x2 :: x3 :: x4 :: x5 :: tl ->
        let f1 = f ctr x1 in
        let f2 = f (ctr + 1) x2 in
        let f3 = f (ctr + 2) x3 in
        let f4 = f (ctr + 3) x4 in
        let f5 = f (ctr + 4) x5 in
        f1 :: f2 :: f3 :: f4 :: f5 ::
          (if ctr > 5000
           then mapi_slow ~f ~i:(ctr + 5) tl
           else count_mapi ~f tl (ctr + 5))

    let mapi l ~f = count_mapi ~f l 0

    let map2_slow l1 l2 ~f = List.rev (List.rev_map2 ~f l1 l2)

    let rec count_map2_exn ~f l1 l2 ctr =
      match l1, l2 with
      | [], [] -> []
      | [x1], [y1] ->
        let f1 = f x1 y1 in
        [f1]
      | [x1; x2], [y1; y2] ->
        let f1 = f x1 y1 in
        let f2 = f x2 y2 in
        [f1; f2]
      | [x1; x2; x3], [y1; y2; y3] ->
        let f1 = f x1 y1 in
        let f2 = f x2 y2 in
        let f3 = f x3 y3 in
        [f1; f2; f3]
      | [x1; x2; x3; x4], [y1; y2; y3; y4] ->
        let f1 = f x1 y1 in
        let f2 = f x2 y2 in
        let f3 = f x3 y3 in
        let f4 = f x4 y4 in
        [f1; f2; f3; f4]
      | x1 :: x2 :: x3 :: x4 :: x5 :: tl1,
        y1 :: y2 :: y3 :: y4 :: y5 :: tl2 ->
        let f1 = f x1 y1 in
        let f2 = f x2 y2 in
        let f3 = f x3 y3 in
        let f4 = f x4 y4 in
        let f5 = f x5 y5 in
        f1 :: f2 :: f3 :: f4 :: f5 ::
          (if ctr > 1000
           then map2_slow ~f tl1 tl2
           else count_map2_exn ~f tl1 tl2 (ctr + 1))
      | _, _ -> failwith "count_map2"

    let map2_exn l1 l2 ~f = count_map2_exn ~f l1 l2 0
  end
end
open Internal_pervasives


module type NATIVE_CHARACTER = BASIC_CHARACTER with type t = char

module type NATIVE_STRING = sig

  include BASIC_STRING
    with type t = String.t
    with type character = char

  include UNSAFELY_MUTABLE
    with type t := String.t
    with type character := char
end

module Native_character : NATIVE_CHARACTER = struct

    type t = char

    let of_native_char x = Some x
    let of_int x =
      try Some (char_of_int x) with _ -> None
    let to_int = int_of_char
    let compare = Char.compare

    let size _ = 1

    let is_print t = ' ' <= t && t <= '~'
    let to_native_string x = String.make 1 x
    let to_string_hum x =
      if is_print x then String.make 1 x
      else sprintf "0x%2x" (int_of_char x)

    let write_to_native_string c ~buf ~index =
      try buf.[index] <- c; return 1
      with _ -> fail `out_of_bounds

    let read_from_native_string ~buf ~index =
      try Some (buf.[index], 1)
      with _ -> None

    let is_whitespace = 
      function ' ' | '\t' | '\r' | '\n' -> true | _ -> false
end

module type T_LENGTH_AND_COMPSUB = sig
  type t
  val length: t -> int
  val compare_substring: t * int * int -> t * int * int -> int
end

(* This functor builds a `compare_substring_strict` function out of a
   `compare_substring` function.

   It may not be the optimal algorithm (it may call `length` on both
   strings.)
 *)
module Compare_substring_strict_of_loose (S: T_LENGTH_AND_COMPSUB) = struct
  open S
  let compare_substring_strict (a, idxa, lena) (b, idxb, lenb) =
    let check_a = lazy (idxa >= 0 && lena >= 0 && idxa + lena <= (length a)) in
    let check_b = lazy (idxb >= 0 && lenb >= 0 && idxb + lenb <= (length b)) in
    if lena = 0 && lenb = 0 then Some 0
    else
      (if lena = 0 then (if Lazy.force check_b then Some (-1) else None)
       else
         (if lenb = 0 then (if Lazy.force check_a then Some (1) else None)
          else
            (if not (Lazy.force check_a) || not (Lazy.force check_b) then None
             else
               Some (compare_substring (a, idxa, lena) (b, idxb, lenb)))))
end

module Make_index_of_string (S: T_LENGTH_AND_COMPSUB) = struct
  open S
  let index_of_string ?(from=0) ?(sub_index=0) ?sub_length t ~sub =
    let module With_exn = struct
      exception Found of int

      let f () =
        (* Readjust the arguments: *)
        let length_of_t = length t in
        let from = 
          if from <= 0 then 0 else min length_of_t from in
        let total_length_of_sub = length sub in
        let sub_index =
          if sub_index <= 0 then 0 else sub_index in
        let sub_length =
          let default = max 0 (total_length_of_sub - sub_index) in
          match sub_length with
          | None -> default
          | Some s when s >= default -> default
          | Some s when s < 0 -> 0
          | Some s -> s
        in
        (* dbg "from: %d, length: %d sub_index: %d sub_length: %d" *)
          (* from length_of_t  sub_index sub_length; *)
        if from >= length_of_t then None
        else if length_of_t = 0 then None
        else if sub_length <= 0 then Some from
        else
          begin try
            for i = 0 to length_of_t - from do
              if compare_substring
                  (t, i + from, sub_length)
                  (sub, sub_index, sub_length) = 0
              then raise (Found (i + from))
            done;
            None
          with Found f -> Some f
          end
    end in
    With_exn.f ()

  let index_of_string_reverse ?from ?(sub_index=0) ?sub_length t ~sub =
    let module With_exn = struct
      exception Found of int

      let f () =
        let length_of_t = length t in
        let last = length_of_t - 1 in
        let from = 
          match from with
          | None -> last 
          | Some f when f >= last -> last
          | Some f -> f in
        let total_length_of_sub = length sub in
        let sub_index =
          if sub_index <= 0 then 0 else sub_index in
        let sub_length =
          let default = max 0 (total_length_of_sub - sub_index) in
          match sub_length with
          | None -> default
          | Some s when s >= default -> default
          | Some s when s < 0 -> 0
          | Some s -> s
        in
        (* dbg "from: %d, length: %d sub_index: %d sub_length: %d" *)
          (* from length_of_t  sub_index sub_length; *)
        if from < 0 then None
        else if length_of_t = 0 then None
        else if sub_length <= 0 then Some from
        else
          begin try
            for i = from downto 0 do
              if compare_substring
                  (t, i, sub_length)
                  (sub, sub_index, sub_length) = 0
              then raise (Found (i))
            done;
            None
          with Found f -> Some f
          end
    end in
    With_exn.f ()

end

(* This module type is a subset of `BASIC_STRING` for strings with a `length`
   function, a `sub_exn` function, and the `index_of_*` functions *)
module type T_LENGTH_SUB_AND_SEARCH = sig
  type t
  type character
  val length: t -> int
  val sub_exn: t -> index:int -> length:int -> t
  val index_of_character: t -> ?from:int -> character -> int option
  val index_of_string: ?from:int ->
    ?sub_index:int -> ?sub_length:int -> t -> sub:t -> int option
end

(* This functor implements the `BASIC_STRING.split` function out of a
   `T_LENGTH_AND_SEARCH` *)
module Make_split_function (S: T_LENGTH_SUB_AND_SEARCH) = struct

  let split t ~on =
    let length_of_t = S.length t in
    begin match on with
    | `Character c ->
      let rec loop acc from =
        match S.index_of_character t ~from c with
        | Some index ->
          loop (S.sub_exn t ~index:from ~length:(index - from) :: acc) 
            (index + 1)
        | None ->
          (S.sub_exn t ~index:from ~length:(length_of_t - from) :: acc)
      in
      List.rev (loop [] 0)
    | `String s ->
      let length_of_s = S.length s in
      let rec loop acc from =
        match S.index_of_string t ~from ~sub:s with
        | Some index ->
          loop (S.sub_exn t ~index:from ~length:(index - from) :: acc) 
            (index + length_of_s)
        | None ->
          (S.sub_exn t ~index:from ~length:(length_of_t - from) :: acc)
      in
      if length_of_s > 0 
      then List.rev (loop [] 0)
      else if length_of_t = 0
      then [ t ]
      else begin
        let res = ref [] in
        for index = length_of_t - 1 downto 0 do
          res := S.sub_exn t ~index ~length:1 :: !res
        done;
        !res
      end
    end

end

module Make_strip_function (S:
   sig
     type t
     type character
     val empty : t
     val is_whitespace: character -> bool
     val length: t -> int
     val find:
       ?from:int -> ?length:int -> t -> f:(character -> bool) -> int option
     val find_reverse:
       ?from:int -> ?length:int -> t -> f:(character -> bool) -> int option
     val sub_exn: t -> index:int -> length:int -> t
   end) = struct

  let strip ?(on=`Both) ?(whitespace=S.is_whitespace) t =
    let open S in
    let first_non () =
      match find t ~f:(fun c -> not (whitespace c)) with
      | None -> raise Not_found | Some s -> s in
    let last_non () =
      match find_reverse t ~f:(fun c -> not (whitespace c)) with
      | None -> raise Not_found | Some s -> s in
    try
      match on with
      | `Both ->
        let index = first_non () in
        let last = last_non () in
        sub_exn t ~index ~length:(last - index + 1)
      | `Left ->
        let index = first_non () in
        sub_exn t ~index ~length:(length t - index)
      | `Right ->
        let last = last_non () in
        sub_exn t ~index:0 ~length:(last + 1)
    with 
    | Not_found -> empty
end


module Make_prefix_suffix_array (A:
  sig
    type t
    type character
    val get : t -> int -> character
    val length: t -> int
    val sub_exn: t -> index:int -> length:int -> t
  end) = struct

  let rec sub_same_tl t ~comp ~len ~off =
    let rec loop i =
      i = len || (A.get t (off + i) = A.get comp i) && loop (i + 1)
    in
    (A.length t >= len) && loop 0

  let is_prefix t ~prefix =
    let len = A.length prefix in
    sub_same_tl t ~comp:prefix ~len ~off:0

  let is_suffix t ~suffix =
    let len = A.length suffix and lt = A.length t in
    sub_same_tl t ~comp:suffix ~len ~off:(lt - len)

  let chop_prefix_exn t ~prefix =
    let len = A.length prefix and lt = A.length t in
    if sub_same_tl t ~comp:prefix ~len ~off:0
    then A.sub_exn t ~index:len ~length:(lt - len)
    else raise (Invalid_argument "not a prefix")

  let chop_prefix t ~prefix =
    try Some (chop_prefix_exn t prefix)
    with _ -> None

  let chop_suffix_exn t ~suffix =
    let len = A.length suffix and lt = A.length t in
    if sub_same_tl t ~comp:suffix ~len ~off:(lt - len)
    then A.sub_exn t ~index:0 ~length:(lt - len)
    else raise (Invalid_argument "not a suffix")

  let chop_suffix t ~suffix =
    try Some (chop_suffix_exn t suffix)
    with _ -> None

end

module Make_split_at_index_functions (A:
    sig
      type t
      type character
      val empty : t
      val length : t -> int
      val sub_exn : t -> index:int -> length:int -> t
    end) = struct

  let split_at t ~index =
    let l = A.length t in
    if index < 0 then (A.empty, t)
    else if index >= l then (t, A.empty)
         else (A.sub_exn t ~index:0 ~length:index),
              (A.sub_exn t ~index:index ~length:(l - index))

  let take t ~index =
    let l = A.length t in
    if index < 0 then A.empty
    else if index >= l then t
         else A.sub_exn t ~index:0 ~length:index

  let drop t ~index =
    let l = A.length t in
    if index < 0 then t
    else if index >= l then A.empty
         else (A.sub_exn t ~index:index ~length:(l - index))


  end

module Native_string : NATIVE_STRING = struct

  include StringLabels
  type character = char

  let empty = ""
  let is_empty t = (compare "" t = 0)

  let of_character = String.make 1
  let of_character_list cl =
    let length = List.length cl in
    let buf = String.make length '\x00' in
    List.iteri cl ~f:(fun i c -> buf.[i] <- c);
    buf

  let to_character_list s =
    let res = ref [] in
    for i = length s - 1 downto 0 do
      res := s.[i] :: !res
    done;
    !res

  let get s ~index =
    try Some (s.[index])
    with _ -> None

  let set s ~index ~v =
    if index > String.length s - 1
    then None
    else begin
      let cop = String.copy s in
      cop.[index] <- v;
      Some cop
    end
  let get_exn s ~index = s.[index]
  let set_exn s ~index ~v =
    match set s ~index ~v with None -> failwith "set_exn" | Some s -> s

  let compare_substring (a, idxa, lena) (b, idxb, lenb) =
    let module With_exns = struct
      exception Return of int
      exception Left_out of int
      exception Right_out of int
      let f () =
        try
          let shortest = min lena lenb in
          for i = 0 to shortest - 1 do
            let ca = try a.[idxa + i] with _ -> raise (Left_out i) in
            let cb = try b.[idxb + i] with _ -> raise (Right_out i) in
            let c = Char.compare  ca cb in
            if c <> 0
            then raise (Return c)
            else ()
          done;
          (Pervasives.compare (lena : int) lenb)
        with
        | Return c -> c
        | Left_out c -> (* a went out of bounds at 'c + idxa' *) -1
        | Right_out _ -> (* b went out of bounds at 'c + idxb' *)
          (* so, a is “longer” *) 1
    end in
    With_exns.f ()

  type s = t
  module T_length_and_compsub = struct
    type t = s let length = length let compare_substring = compare_substring
  end
  include Compare_substring_strict_of_loose(T_length_and_compsub)
  include Make_index_of_string(T_length_and_compsub)


  let to_native_string x = String.copy x
  let of_native_string x = return (String.copy x)
  let of_native_substring x ~offset ~length =
    if length = 0 then return ""
    else
      try return (String.sub x offset length)
      with e -> fail `out_of_bounds

  let to_string_hum x = sprintf "%S" x

  let concat ?(sep="") sl = concat ~sep sl

  let fold t ~init ~f =
    let res = ref init in
    for i = 0 to String.length t - 1 do
      res := f !res t.[i];
    done;
    !res
  
  let foldi t ~init ~f =
    let res = ref init in
    for i = 0 to String.length t - 1 do
      res := f i !res t.[i];
    done;
    !res

  let fold2_exn t1 t2 ~init ~f =
    let lgth1 = (length t1) in
    let lgth2 = (length t2) in
    match lgth1, lgth2 with
    | 0, 0 -> init
    | _, _ when lgth1 <> lgth2 -> failwith "fold2_exn"
    | lgth1, lgth2 ->
       let res = ref init in
       for i = 0 to lgth1 - 1 do
         res := f !res t1.[i] t2.[i];
       done;
       !res

  let sub_exn t ~index ~length =
    if length = 0 then empty else String.sub t index length

  let sub t ~index ~length =
    if length = 0 then Some empty else
      try Some (String.sub t index length)
      with e -> None

  let slice_exn ?(start=0) ?finish t =
    let length_of_t = String.length t in
    let bound_check strict m x =
      let out_of_ub = if strict then x > length_of_t else x >= length_of_t in
      if x < 0 || (not (is_empty t) && out_of_ub) then
        ksprintf failwith "slice_exn: invalid %s %d" m x
      else x
    in
    let _      = bound_check false "start" start
    and finish =
      match finish with
      | None   -> length_of_t
      | Some f -> bound_check true "finish" f
    in
    sub_exn t ~index:start ~length:(finish - start)

  let slice ?start ?finish t =
    try Some (slice_exn ?start ?finish t)
    with _ -> None

  let mutate_exn t ~index c = String.set t index c

  let mutate t ~index c =
    try String.set t index c; return () with _ -> fail `out_of_bounds

  let blit_exn ~src ~src_index ~dst ~dst_index ~length =
    blit ~src ~src_pos:src_index ~dst ~dst_pos:dst_index ~len:length

  let blit ~src ~src_index ~dst ~dst_index ~length =
    try blit_exn ~src ~src_index ~dst ~dst_index ~length; return ()
    with _ -> fail `out_of_bounds

  let iter t ~f = String.iter t ~f
  let iteri t ~f = String.iteri t ~f
  let iter_reverse t ~f =
    for i = length t -1 downto 0 do
      f (get_exn t i)
    done

  let rev t =
    let lgth = length t in
    match lgth with
    | 0 -> empty
    | lgth ->
       let res = make lgth (String.get t 0) in
       for i = 0 to lgth - 1 do
         String.set res i (String.get t (lgth - 1 - i))
       done;
       res

  let map t ~f = String.map t ~f

  let map2_exn t1 t2 ~f =
    let lgth1 = (length t1) in
    let lgth2 = (length t2) in
    match lgth1, lgth2 with
    | 0, 0 -> empty
    | _, _ when lgth1 <> lgth2 -> failwith "map2_exn"
    | lgth1, lgth2 ->
       let res = make lgth1 (String.get t1 0) in
       for i = 0 to lgth1 - 1 do
         String.set res i (f (String.get t1 i) (String.get t2 i))
       done;
       res

  let mapi t ~f =
    let buffer = String.create (String.length t) in
    let ()     = String.iteri t ~f:(fun i c -> String.set buffer i (f i c)) in
    buffer
  (* TODO: Change this to 
    let mapi t ~f = String.mapi t ~f
    once we switch to 4.02 *)

  let for_all t ~f =
    try (iter t (fun x -> if not (f x) then raise Not_found else ()); true)
    with Not_found -> false

  let exists t ~f =
    try (iter t (fun x -> if f x then raise Not_found else ()); false)
    with Not_found -> true

  let index_of_character t ?(from=0) c =
    let from = if from <= 0 then 0 else min (length t) from in
    try Some (String.index_from t from c)
    with _ -> None

  let index_of_character_reverse t ?from c =
    let from =
      let length_of_t = length t in
      match from with
      | None -> length_of_t - 1
      | Some s when s < 0 -> -1
      | Some s when s > length_of_t - 1 -> length_of_t - 1
      | Some s -> s
    in
    try Some (String.rindex_from t from c)
    with _ -> None

  let resize_from_length ~from ?length ~length_of_s =
    let from = if from <= 0 then 0 else min length_of_s from in
    let length =
      match length with
      | None -> length_of_s - from
      | Some lg when lg <= 0 -> 0
      | Some lg -> min (length_of_s - from) lg
    in
    (from, length)

  let find ?(from=0) ?length s ~f =
    let length_of_s = String.length s in
    let from, length = resize_from_length ~from ?length ~length_of_s in
    let found = ref None in
    let i = ref 0 in
    while !found = None && !i  < length do
      if f (get_exn s (!i + from))
      then found := Some (!i + from)
      else incr i
    done;
    !found

  let find_reverse ?from ?length s ~f =
    let length_of_s = String.length s in
    if length_of_s = 0 then None
    else begin
      let from = 
        match from with
        | None -> length_of_s - 1 
        | Some s when s < 0 -> -1
        | Some s when s >= length_of_s - 1 -> length_of_s - 1
        | Some s -> s
      in
      let length =
        match length with
        | None -> from + 1
        | Some l when l <= 0 -> 0
        | Some l when l >= from + 1 -> from + 1
        | Some l -> l
      in
      let found = ref None in
      let i = ref from in
      while !found = None && !i >= from - length + 1 do
        (* dbg "i: %d from: %d length: %d" !i from length; *)
        if f (get_exn s !i)
        then found := Some (!i)
        else decr i
      done;
      !found
    end

  let filter_map ?(from=0) ?length s ~f =
    let length_of_s = String.length s in
    let from, length = resize_from_length ~from ?length ~length_of_s in
    if length = 0 then empty
    else begin
      let b = Buffer.create length in
      for i = 0 to length - 1 do
        match f (get_exn s (i + from)) with
        | Some c -> Buffer.add_char b c
        | None -> ()
      done;
      Buffer.contents b
    end

  let filter ?from ?length s ~f =
      filter_map ?from ?length s ~f:(fun c -> if f c then Some c else None)

  include Make_strip_function (struct
      type t = string
      type character = char
      let empty = empty
      let length = length
      let sub_exn = sub_exn
      let find = find
      let find_reverse = find_reverse
      let is_whitespace = Native_character.is_whitespace
    end)

  include Make_split_function(struct
      type t = string
      type character = char
      let length = length
      let sub_exn = sub_exn
      let index_of_string = index_of_string
      let index_of_character = index_of_character
    end)


  include Make_prefix_suffix_array (struct
      type t = string
      type character = char
      let length = length
      let get = (fun s i -> s.[i])
      let sub_exn = sub_exn
    end)

  include Make_split_at_index_functions(struct
      type t = string
      type character = char
      let empty = empty
      let length = length
      let sub_exn = sub_exn
    end)

  module Make_output (Model: OUTPUT_MODEL) = Model

  let take_while_with_index t ~f =
    let buf = Buffer.create (length t) in
    let rec loop idx =
      match get t idx with
      | Some c when f idx c -> Buffer.add_char buf c; loop (idx + 1)
      | _ -> ()
    in
    loop 0;
    Buffer.contents buf

  let take_while t ~f = take_while_with_index t ~f:(fun _ c -> f c)

end

(* Module to help build `{of,to}_native_[sub]string` functions.

   It is most useful while using variable sized characters. *)
module Make_native_conversions = struct


  let of_native_substring
      ~empty ~init ~on_new_character ~finalize
      ~read_character_from_native_string
      s ~offset ~length =
    if length = 0 then return empty
    else
      begin
        (if offset + length > String.length s
         then fail `out_of_bounds
         else return ())
        >>= fun () ->
        let module With_exn = struct
          exception WChar of int
          let f buf =
            let x = init () in
            try
              let rec loop index =
                if index < offset + length
                then
                  begin match read_character_from_native_string ~buf ~index with
                  | Some (s, size) when index + size <= offset + length ->
                    on_new_character x s;
                    loop (index + size)
                  | Some (_, _ (* too big size *))
                  | None -> raise (WChar index)
                  end
                else ()
              in
              loop offset;
              return (finalize x)
            with
            | WChar c -> fail (`wrong_char_at c)
        end in
        With_exn.f s
      end

  let of_native_string of_native_substring s =
    match of_native_substring s ~offset:0 ~length:(String.length s) with
    | `Ok o -> return o
    | `Error (`wrong_char_at c) -> fail (`wrong_char_at c)
    | `Error `out_of_bounds -> (* There is a bug ! *) assert false


  let to_native_string_knowing_size
      ~future_size ~iter ~write_char_to_native_string l =
    let length = future_size l in
    let buf = String.make length 'B' in
    let index = ref 0 in
    iter l ~f:begin fun c ->
      match write_char_to_native_string c ~buf ~index:!index with
      | `Ok siz ->  index := !index + siz
      | `Error `out_of_bounds ->
        failwith "Bug in Make_native_conversions.to_native_string"
    end;
    buf

end





module List_of (Char: BASIC_CHARACTER) :
  BASIC_STRING
  with type character = Char.t
  with type t = Char.t list = struct

  type character = Char.t

  type t = character list

  let empty = []
  let is_empty = (=) []

  let make length c =
    let rec loop n acc =
      if n >= length then acc else loop (n + 1) (c :: acc)
    in
    loop 0 []

  let of_character c = [c]
  let of_character_list cl = cl
  let to_character_list cl = cl

  let get sl ~index =
    try Some (List.nth sl index) with _ -> None

  let set s ~index ~v =
    let rec loop n acc = function
    | [] -> None
    | q :: t when n = index ->
      Some (List.rev_append acc (v :: t))
    | q :: t ->
      loop (n + 1) (q :: acc) t
    in
    loop 0 [] s

  let get_exn s ~index =
    match get s ~index with None -> failwith "get_exn" | Some s -> s
  let set_exn s ~index ~v =
    match set s ~index ~v with None -> failwith "set_exn" | Some s -> s

  let iter t ~f = List.iter t ~f
  let iteri t ~f = List.iteri t ~f
  let iter_reverse t ~f =
    List.iter (List.rev t) ~f

  let rev t = List.rev t

  let fold t ~init ~f = List.fold_left t ~init ~f
  let foldi t ~init ~f = 
    snd (List.fold_left t ~init:(0,init)
          ~f:(fun (i,a) c -> (i+1,f i a c)))
  let fold2_exn t1 t2 ~init ~f = List.fold_left2 t1 t2 ~init ~f
  let map = Core_list_map.map
  let mapi = Core_list_map.mapi
  let map2_exn = Core_list_map.map2_exn
  let for_all t ~f = List.for_all t ~f
  let exists t ~f = List.exists t ~f

  let compare (a : Char.t list) (b: Char.t list) = compare a b
  let of_native_substring s ~offset ~length =
    Make_native_conversions.of_native_substring
      ~empty ~init:(fun () -> ref [])
      ~on_new_character:(fun x c -> x := c :: !x)
      ~finalize:(fun x -> List.rev !x)
      ~read_character_from_native_string:Char.read_from_native_string
      s ~offset ~length

  let of_native_string s =
    Make_native_conversions.of_native_string
      of_native_substring s

  let to_native_string l =
    Make_native_conversions.to_native_string_knowing_size
      ~future_size:(fun l ->
          List.fold_left l ~init:0 ~f:(fun sum c -> sum + Char.size c))
      ~iter ~write_char_to_native_string:Char.write_to_native_string
      l

  let to_string_hum l = sprintf "%S" (to_native_string l)

  let concat ?(sep=[]) ll =
    match ll with
    | [] -> []
    | hh :: tt ->
      let x = ref (List.rev hh) in
      List.iter tt ~f:(fun l ->
        x := List.rev_append sep !x;
        x := List.rev_append l !x;
        );
      List.rev !x

  let length = List.length

  let sub t ~index ~length =
    let r = ref [] in
    let c = ref 0 in
    try
      List.iteri t ~f:(fun i a ->
          if i >= index + length then raise Not_found;
          if index <= i then (
            r:= a :: !r;
            incr c;
          );
        );
      if !c = length then Some (List.rev !r) else None
    with
    | Not_found -> Some (List.rev !r)

  let sub_exn t ~index ~length =
    match sub t ~index ~length with
    | Some s -> s
    | None -> ksprintf failwith "sub_exn(%d,%d)" index length

  let slice_exn ?(start=0) ?finish t =
    let length_of_t = List.length t in
    if start < 0 || (not (is_empty t) && start >= length_of_t) then
      ksprintf failwith "slice_exn: invalid start %d" start
    else
      match finish with
      | None   -> sub_exn t ~index:start ~length:(length_of_t - start)
      | Some f -> if f < 0 || f > length_of_t then
                    ksprintf failwith "slice_exn: invalid finish %d" f
                  else
                    sub_exn t ~index:start ~length:(f - start)

  let slice ?start ?finish t =
    try Some (slice_exn ?start ?finish t)
    with _ -> None


  let rec comp_loop p lst_pair =
    if p
    then match lst_pair with
         | (i,[])           -> Some i
         | ([],j)           -> None
         | (i::is), (j::js) -> comp_loop (i = j) (is,js)
    else None

  let is_prefix t ~prefix =
    match comp_loop true (t,prefix) with
    | Some _  -> true
    | None    -> false

  let is_suffix t ~suffix =
    is_prefix (List.rev t) ~prefix:(List.rev suffix)

  let chop_prefix_exn t ~prefix =
    match comp_loop true (t,prefix) with
    | Some r  -> r
    | None    -> raise (Invalid_argument "chop_prefix_exn: not a prefix")

  let chop_prefix t ~prefix =
    try Some (chop_prefix_exn t prefix)
    with _ -> None

  let chop_suffix_exn t ~suffix =
    List.rev (chop_prefix_exn (List.rev t) ~prefix:(List.rev suffix))

  let chop_suffix t ~suffix =
    try Some (chop_suffix_exn t suffix)
    with _ -> None

  let unrevSplit t n =
    if n < 0
    then [],t
    else let rec offset i ((l,r) as p) =
           if i = n
           then p
           else match r with
                | []   -> p
                | h::t -> offset (i + 1) (h::l,t)
         in
         offset 0 ([],t)

  let split_at t ~index =
    let l,r = unrevSplit t index in
    List.rev l, r

  let take t ~index = fst (split_at t index)

  let drop t ~index =
    let l,r = unrevSplit t index in
    r


  let index_of_character t ?(from=0) c =
    let index = ref 0 in
    try begin
      List.iter t ~f:(fun x ->
          if !index >= from
          then
            if x = c
            then failwith "found"
            else incr index
          else incr index);
      None
    end
    with _ -> Some !index

  let index_of_character_reverse t ?from c =
    let length_of_t, rev =
      let rec loop lgth acc = function
      | [] -> (lgth, acc)
      | h :: t -> loop (lgth + 1) (h :: acc) t in
      loop 0 [] t
    in
    let from =
      match from with
      | None -> length_of_t - 1
      | Some s when s < 0 -> -1
      | Some s when s > length_of_t - 1 -> length_of_t - 1
      | Some s -> s
    in
    match index_of_character rev ~from:(length_of_t - from - 1) c with
    | Some c -> Some (length_of_t - c - 1)
    | None -> None

  let compare_substring (a, idxa, lena) (b, idxb, lenb) =
    let module With_exns = struct
      exception Left
      exception Right
      let rec drop_until ~exn idx l =
        match idx, l with
        | 0, l -> l
        | more, [] -> raise exn
        | more, h :: t -> drop_until ~exn (more - 1) t
      let f () =
        begin try
          let rec cmp l1 l2 len1 len2 =
            if len1 < 0 then raise Left;
            if len2 < 0 then raise Right;
            match l1, l2 with
            | _, _ when len1 = 0 && len2 = 0 -> 0
            | _, _ when len1 = 0 -> -1
            | _, _ when len2 = 0 -> 1
            | [], [] when len1 = 0 || len2 = 0 -> Pervasives.compare lena lenb
            | [], _ when len1 > 0 -> raise Left
            | _, [] when len2 > 0 -> raise Right
            | h1 :: t1, h2 :: t2 when Char.compare h1 h2 = 0 ->
              cmp t1 t2 (len1 - 1) (len2 - 1)
            | h1 :: _, h2 :: _ -> Char.compare h1 h2
            | _, _ -> assert false (* calming down the warnings.. *)
          in
          if lena = 0 && lenb = 0 then 0
          else (
            let aa = drop_until ~exn:Left idxa a in
            let bb = drop_until ~exn:Right idxb b in
            (cmp aa bb lena lenb)
          )
        with
        | Left -> -1
        | Right -> 1
        | Failure s -> 1
          (* dbg "(%d, %d/%d) Vs (%d, %d/%d) %s" idxa lena (length a) idxb lenb (length b) s; *)
        end
    end in
    With_exns.f ()

  type s = t
  module T_length_and_compsub = struct
    type t = s let length = length let compare_substring = compare_substring
  end
  include Compare_substring_strict_of_loose(T_length_and_compsub)
  include Make_index_of_string(T_length_and_compsub)

  let find ?(from=0) ?length s ~f =
    (* index and virtual_length are maybe a bit redundant but I favor
       readability of the branches of the match *)
    let from = if from <= 0 then 0 else from in
    let rec find_from index virtual_length l =
      match l, length with
      | [], _ -> None
      | _, Some lgth when lgth <= virtual_length -> None
      | h :: t, _ when index < from -> find_from (index + 1) virtual_length t
      | h :: t, _ when index >= from && f h -> Some index
      | h :: t, _ -> find_from (index + 1) (virtual_length + 1) t
    in
    find_from 0 0 s 

  let find_reverse ?from ?length s ~f =
    let length_of_s = List.length s in
    let from = 
      match from with
      | None -> None
      | Some s when s < 0 -> Some length_of_s
      | Some s when s > length_of_s - 1 -> Some 0
      | Some s -> Some (length_of_s - 1 - s)
    in
    match find ?from ?length (List.rev s) ~f with
    | None -> None
    | Some i -> Some (length_of_s - 1 - i)

  let filter_map ?(from=0) ?length t ~f =
    let rec filter_map_rec acc index virtual_length l = 
      match l, length with
      | [], _ -> List.rev acc
      | _, Some lgth when lgth <= virtual_length -> List.rev acc
      | h :: t, _ when index < from ->
        filter_map_rec acc (index + 1) virtual_length t
      | h :: t, _ (* when index >= from  *) ->
        begin match f h with
        | Some o -> filter_map_rec (o :: acc) (index + 1) (virtual_length + 1) t
        | None -> filter_map_rec acc (index + 1) (virtual_length + 1) t
        end
    in
    filter_map_rec [] 0 0 t

  let filter ?from ?length t ~f =
      filter_map ?from ?length t ~f:(fun c -> if f c then Some c else None)

  include Make_strip_function (struct
      type t = Char.t list
      type character = Char.t
      let empty = empty
      let length = length
      let sub_exn = sub_exn
      let find = find
      let find_reverse = find_reverse
      let is_whitespace = Char.is_whitespace
    end)

  include Make_split_function(struct
      type t = Char.t list
      type character = Char.t
      let length = length
      let sub_exn = sub_exn
      let index_of_string = index_of_string
      let index_of_character = index_of_character
    end)


  module Make_output (Model: OUTPUT_MODEL) = struct

    let (>>=) = Model.bind

    let output chan l =
      List.fold_left l ~init:(Model.return ()) ~f:(fun prev_m c ->
          prev_m >>= fun () ->
          Model.output chan (Char.to_native_string c))

  end

  let take_while_with_index t ~f =
    let rec loop idx acc =
      function
      | h :: t when f idx h -> loop (idx + 1) (h :: acc) t
      | []
      | _ :: _ -> List.rev acc
    in
    loop 0 [] t
  let take_while t ~f = take_while_with_index t ~f:(fun _ c -> f c)


end

module Int_utf8_character : BASIC_CHARACTER with type t = int = struct

    type t = int

    let of_native_char x = Some (int_of_char x)

    let compare (i: int) (j : int) = compare i j
    let of_int x =
      if x land 0x7FFF_FFFF = x then Some x else None
    let to_int c = c
    let size x =
      if x <=        0x7f then 1 else
      if x <=       0x7ff then 2 else
      if x <=      0xffff then 3 else
      if x <=   0x1f_ffff then 4 else
      if x <=  0x3ff_ffff then 5 else
      if x <= 0x7fff_ffff then 6 else 0

    let is_print t = int_of_char ' ' <= t && t <= int_of_char '~'

    let to_string_hum x =
      if is_print x then String.make 1 (char_of_int x)
      else sprintf "&#x%X;" x

    let write_to_native_string c ~buf ~index =
      let sz = size c in
      try
        let first_byte =
          match sz with
          | 1 -> ((c lsr  0) land 0b0111_1111) lor 0b0000_0000
          | 2 -> ((c lsr  6) land 0b0001_1111) lor 0b1100_0000
          | 3 -> ((c lsr 12) land 0b0000_1111) lor 0b1110_0000
          | 4 -> ((c lsr 18) land 0b0000_0111) lor 0b1111_0000
          | 5 -> ((c lsr 24) land 0b0000_0011) lor 0b1111_1000
          | 6 -> ((c lsr 30) land 0b0000_0001) lor 0b1111_1100
          | _ -> assert false in
        buf.[index] <- char_of_int first_byte;
        for i = 2 to sz  do
          let ith_byte =
            ((c lsr (6 * (i - 2))) land 0b0011_1111) lor 0b1000_0000 in
          buf.[index + sz - i + 1] <- char_of_int ith_byte;
        done;
        return sz
      with _ -> fail `out_of_bounds

    let read_from_native_string ~buf ~index =
      try
        let first_char = buf.[index] |> int_of_char in
        let size, mask =
          if first_char lsr 7 = 0 then (1, 0b0111_1111)
          else if first_char lsr     5 =     0b110 then (2, 0b0001_1111)
          else if first_char lsr     4 =    0b1110 then (3, 0b0000_1111)
          else if first_char lsr     3 =   0b11110 then (4, 0b0000_0111)
          else if first_char lsr     2 =  0b111110 then (5, 0b0000_0011)
          else if first_char lsr     1 = 0b1111110 then (6, 0b0000_0001)
          else raise Not_found
        in
        let the_int = ref (first_char land mask) in
        for i = 1 to size - 1 do
          let the_char = buf.[index + i] |> int_of_char in
          if (the_char lsr 6) = 0b10
          then (
            the_int := (!the_int lsl 6) lor (the_char land 0b0011_1111);
          ) else raise Not_found;
        done;
        Some (!the_int, size)
      with _ -> None

    let to_native_string x =
      let buf = String.make (size x) 'B' in
      begin match write_to_native_string x ~buf ~index:0 with
      | `Ok _ -> ()
      | `Error e ->
        dbg "buf: %S siz: %d x: %d" buf (size x) x;
        assert false
      end;
      buf

    let is_whitespace c = 
      try 
        match char_of_int c with
        | ' ' | '\t' | '\r' | '\n' -> true | _ -> false
      with _ -> false

end


module type MINIMALISTIC_MUTABLE_STRING = sig
  type character
  type t

  val empty: t
  val make: int -> character -> t
  val length: t -> int
  val compare: t -> t -> int
  val compare_char: character -> character -> int
  val get: t -> int -> character
  val set: t -> int -> character -> unit
  val blit: src:t -> src_pos:int -> dst:t -> dst_pos:int -> len:int -> unit

  val is_whitespace: character -> bool
  include NATIVE_CONVERSIONS with type t := t
end

module Of_mutable
    (S: MINIMALISTIC_MUTABLE_STRING) :
  BASIC_STRING
  with type character = S.character
  with type t = S.t = struct

  include S
  let is_empty s =
    try ignore (S.get s 0); false with _ -> true

  let get t ~index = try Some (get t index) with _ -> None
  let set t ~index ~v:c =
    let lgth = length t in
    if index < 0 || lgth <= index then None
    else Some (
        let res = make lgth (S.get t 0) in
        blit ~dst:res ~dst_pos:0 ~src:t ~src_pos:0 ~len:lgth;
        S.set res index c;
        res)

  let get_exn s ~index = S.get s index
  let set_exn s ~index ~v =
    match set s ~index ~v with None -> failwith "set_exn" | Some s -> s

  let of_character c = make 1 c

  let of_character_list cl =
    match cl with
    | [] -> empty
    | one :: more ->
      let res = make (List.length cl) one in
      List.iteri more ~f:(fun  i c ->
          S.set res (i + 1) c);
      res

  let to_character_list s =
    let res = ref [] in
    for i = S.length s - 1 downto 0 do
      res := S.get s i :: !res
    done;
    !res

  let rec concat  ?(sep=empty) tl =
    match tl with
    | [] -> empty
    | one :: more ->
      begin try
        let first_char =
          try S.get one 0
          with _ -> S.get sep 0
        in
        let sep_length = S.length sep in
        let total_length =
          List.fold_left ~init:(S.length one) more ~f:(fun prev s ->
              prev + sep_length + S.length s) in
        let dst = make total_length first_char in
        let index = ref 0 in
        blit ~dst ~dst_pos:!index ~src:one ~src_pos:0 ~len:(length one);
        index := !index + (length one);
        List.iter more ~f:(fun s ->
            blit ~dst ~dst_pos:!index ~src:sep ~src_pos:0 ~len:sep_length;
            index := !index + sep_length;
            blit ~dst ~dst_pos:!index ~src:s ~src_pos:0 ~len:(length s);
            index := !index + (length s);
          );
        dst
      with _ ->
        concat more ~sep (* both one and sep are empty *)
      end

  let iter t ~f =
    for i = 0 to length t - 1 do
      f (S.get t i)
    done

  let iteri t ~f =
    for i = 0 to length t - 1 do
      f i (S.get t i)
    done

  let iter_reverse t ~f =
    for i = length t -1 downto 0 do
      f (S.get t i)
    done

  let fold t ~init ~f =
    let x = ref init in
    for i = 0 to length t - 1 do
      x := f !x (S.get t i)
    done;
    !x

  let foldi t ~init ~f =
    let x = ref init in
    for i = 0 to length t - 1 do
      x := f i !x (S.get t i)
    done;
    !x

  let fold2_exn t1 t2 ~init ~f =
    let lgth1 = (length t1) in
    let lgth2 = (length t2) in
    match lgth1, lgth2 with
    | 0, 0 -> init
    | _, _ when lgth1 <> lgth2 -> failwith "fold2_exn"
    | lgth1, lgth2 ->
       let res = ref init in
       for i = 0 to lgth1 - 1 do
         res := f !res (S.get t1 i) (S.get t2 i);
       done;
       !res

  let rev t =
    let lgth = length t in
    match lgth with
    | 0 -> empty
    | lgth ->
       let res = make lgth (S.get t 0) in
       for i = 0 to lgth - 1 do
         S.set res i (S.get t (lgth - 1 - i))
       done;
       res

  let map t ~f =
    let lgth = (length t) in
    if lgth = 0
    then empty
    else begin
      let res = make lgth (S.get t 0) in
      for i = 0 to lgth - 1 do
        S.set res i (f (S.get t i))
      done;
      res
    end

  let mapi t ~f =
    let lgth = (length t) in
    if lgth = 0
    then empty
    else begin
      let res = make lgth (S.get t 0) in
      for i = 0 to lgth - 1 do
        S.set res i (f i (S.get t i))
      done;
      res
    end

  let map2_exn t1 t2 ~f =
    let lgth1 = (length t1) in
    let lgth2 = (length t2) in
    match lgth1, lgth2 with
    | 0, 0 -> empty
    | _, _ when lgth1 <> lgth2 -> failwith "map2_exn"
    | lgth1, lgth2 ->
       let res = make lgth1 (S.get t1 0) in
       for i = 0 to lgth1 - 1 do
         S.set res i (f (S.get t1 i) (S.get t2 i))
       done;
       res

  let for_all t ~f =
    try
      iter t (fun c -> if not (f c) then raise Not_found);
      true
    with _ -> false

  let exists t ~f =
    try
      iter t (fun c -> if (f c) then raise Not_found);
      false
    with _ -> true

  let sub t ~index ~length =
    if length = 0 then Some empty else
      begin
        let lgth = S.length t in
        if lgth = 0
        then None (* `length <> 0` *)
        else begin
          try
            let res = make length (S.get t index) in
            for i = 1 to length - 1 do
              S.set res i (S.get t (index + i))
            done;
            Some res
          with _ -> None
        end
      end

  let sub_exn t ~index ~length =
    match sub t ~index ~length with
    | Some s -> s
    | None -> ksprintf failwith "sub_exn(%d,%d)" index length

  let slice_exn ?(start=0) ?finish t =
    let length_of_t = S.length t in
    if start < 0 || (not (is_empty t) && start >= length_of_t) then
      ksprintf failwith "slice_exn: invalid start %d" start
    else
      match finish with
      | None   -> sub_exn t ~index:start ~length:(length_of_t - start)
      | Some f -> if f < 0 || f > length_of_t then
                    ksprintf failwith "slice_exn: invalid finish %d" f
                  else
                    sub_exn t ~index:start ~length:(f - start)

  let slice ?start ?finish t =
    try Some (slice_exn ?start ?finish t)
    with _ -> None

  let to_string_hum t = to_native_string t |> sprintf "%S"

  let index_of_character t ?(from=0) c =
    let from = if from <= 0 then 0 else min (length t) from in
    let res = ref None in
    try
      for i = from to length t - 1 do
        if S.get t i = c then (res:= Some i; raise Not_found)
      done;
      None
    with _ -> !res

  let index_of_character_reverse t ?from c =
    let from =
      let length_of_t = length t in
      match from with
      | None -> length_of_t - 1
      | Some s when s < 0 -> -1
      | Some s when s > length_of_t - 1 -> length_of_t - 1
      | Some s -> s
    in
    let res = ref None in
    try
      for i = from downto 0 do
        if S.get t i = c then (res:= Some i; raise Not_found)
      done;
      None
    with _ -> !res

  let compare_substring (a, idxa, lena) (b, idxb, lenb) =
    let module With_exns = struct
      exception Return of int
      exception Left_out of int
      exception Right_out of int
      let f () =
        try
          let shortest = min lena lenb in
          for i = 0 to shortest - 1 do
            let ca = try S.get a (idxa + i) with _ -> raise (Left_out i) in
            let cb = try S.get b (idxb + i) with _ -> raise (Right_out i) in
            let c = S.compare_char  ca cb in
            if c <> 0
            then raise (Return c)
            else ()
          done;
          (Pervasives.compare (lena : int) lenb)
        with
        | Return c -> c
        | Left_out c -> (* a went out of bounds at 'c + idxa' *) -1
        | Right_out _ -> (* b went out of bounds at 'c + idxb' *)
          (* so, a is “longer” *) 1
    end in
    With_exns.f ()

  type s = t
  module T_length_and_compsub = struct
    type t = s let length = length let compare_substring = compare_substring
  end
  include Compare_substring_strict_of_loose(T_length_and_compsub)
  include Make_index_of_string(T_length_and_compsub)


  let resize_from_length ~from ?length ~length_of_s =
    let from = if from <= 0 then 0 else min length_of_s from in
    let length =
      match length with
      | None -> length_of_s - from
      | Some lg when lg <= 0 -> 0
      | Some lg -> min (length_of_s - from) lg
    in
    (from, length)

  let find ?(from=0) ?length s ~f =
    let length_of_s = S.length s in
    let from, length = resize_from_length ~from ?length ~length_of_s in
    let found = ref None in
    let i = ref 0 in
    while !found = None && !i  < length do
      if f (get_exn s (!i + from))
      then found := Some (!i + from)
      else incr i
    done;
    !found

  let find_reverse ?from ?length s ~f =
    let length_of_s = S.length s in
    if length_of_s = 0 then None
    else begin
      let from = 
        match from with
        | None -> length_of_s - 1 
        | Some s when s < 0 -> -1
        | Some s when s >= length_of_s - 1 -> length_of_s - 1
        | Some s -> s
      in
      let length =
        match length with
        | None -> from + 1
        | Some l when l <= 0 -> 0
        | Some l when l >= from + 1 -> from + 1
        | Some l -> l
      in
      let found = ref None in
      let i = ref from in
      while !found = None && !i >= from - length + 1 do
        (* dbg "i: %d from: %d length: %d" !i from length; *)
        if f (get_exn s !i)
        then found := Some (!i)
        else decr i
      done;
      !found
    end

  let filter_map ?(from=0) ?length s ~f =
    let length_of_s = S.length s in
    let from, length = resize_from_length ~from ?length ~length_of_s in
    if length = 0 then empty
    else begin
      let res = ref [] in
      for i = length - 1 downto 0 do
        match f (get_exn s (i + from)) with
        | Some c -> res := c :: !res
        | None -> ()
      done;
      of_character_list !res
    end

  let filter ?from ?length s ~f =
      filter_map ?from ?length s ~f:(fun c -> if f c then Some c else None)

  include Make_strip_function (struct
      type t = S.t
      type character = S.character
      let empty = empty
      let length = length
      let sub_exn = sub_exn
      let find = find
      let find_reverse = find_reverse
      let is_whitespace = S.is_whitespace
    end)

  include Make_split_function(struct
      type t = S.t
      type character = S.character
      let length = length
      let sub_exn = sub_exn
      let index_of_string = index_of_string
      let index_of_character = index_of_character
    end)


  include Make_prefix_suffix_array (struct
    type t = S.t
    type character = S.character
    let length = S.length
    let get = S.get
    let sub_exn = sub_exn
    end)


  include Make_split_at_index_functions(struct
      type t = S.t
      type character = S.character
      let empty = empty
      let length = length
      let sub_exn t ~index ~length = sub_exn t index length
    end)



  module Make_output (Model: OUTPUT_MODEL) = struct

    let (>>=) = Model.bind

    let output chan t =
      Model.output chan (to_native_string t)

  end

  let take_while_with_index t ~f =
    if length t = 0 then empty
    else (
      let buf = make (length t) (S.get t 0) in
      let rec loop idx =
        match get t idx with
        | Some c when f idx c -> S.set buf idx c; loop (idx + 1)
        | _ -> idx
      in
      let new_length = loop 0 in
      sub_exn buf ~index:0 ~length:new_length
    )

  let take_while t ~f = take_while_with_index t ~f:(fun _ c -> f c)

end