(*
include String

let empty = ""

let of_string = copy

let to_string = copy

let sub_string = sub

let extend s left right =
  let len = left + length s + right in
  let result = create len in
  let trim_left = max (- left) 0 in
  unsafe_blit s trim_left result (max left 0)
    (length s - trim_left - max (- right) 0);
  result

let blit_string = blit

let cat = ( ^ )

let unsafe_of_string s = s

let unsafe_to_string s = s
let init n f =
  let s = create n in
  for i = 0 to n - 1 do
    s.[i] <- f i
  done;
  s

let mapi f s =
  init (length s) (fun i -> f i (unsafe_get s i))
*)

(*
let iteri f s =
  for i = 0 to length s - 1 do
    f i (unsafe_get s i)
  done

let map f s =
  init (length s) (fun i -> f (unsafe_get s i))

let is_space = function
  | ' ' | '\012' | '\n' | '\r' | '\t' -> true
  | _ -> false

let rec rindex_no_space_from i s =
  if i >= 0 && is_space (unsafe_get s i) then
    rindex_no_space_from (pred i) s
  else
    i

let rec index_no_space_between i j s =
  if i <= j && is_space (unsafe_get s i) then
    index_no_space_between (succ i) j s
  else
    i

let trim s =
  let off_end = rindex_no_space_from (length s - 1) s in
  let off_start = index_no_space_between 0 off_end s in
  if off_start > off_end then
    ""
  else if off_start = 0 && off_end = length s - 1 then
    s
  else
    sub s off_start (off_end - off_start + 1)
*)


include Bytes


(*
let index_opt s c =
  Stdcompat__tools.option_find (index s) c

let rindex_opt s c =
  Stdcompat__tools.option_find (rindex s) c

let index_from_opt s i c =
  Stdcompat__tools.option_find (index_from s i) c

let rindex_from_opt s i c =
  Stdcompat__tools.option_find (rindex_from s i) c
*)

(*
let uppercase_ascii = uppercase

let lowercase_ascii = lowercase

let capitalize_ascii = capitalize

let uncapitalize_ascii = uncapitalize

let equal : t -> t -> bool = ( = )
*)

(*
let of_seq g =
  Stdcompat__buffer.to_bytes (Stdcompat__buffer.of_seq g)

let to_seq s = Stdcompat__tools.vec_to_seq length unsafe_get s

let to_seqi s = Stdcompat__tools.vec_to_seqi length unsafe_get s
*)

(*
let pad_sign i w =
  let pad_width = Stdcompat__sys.int_size - w in
  (i lsl pad_width) asr pad_width

let index_out_of_bounds () =
  invalid_arg "index out of bounds"

let unsafe_get_uint8 b i =
  int_of_char (get b i)

let get_uint8 b i =
  if i < 0 || i >= length b then
    index_out_of_bounds ();
  unsafe_get_uint8 b i

let get_int8 b i =
  pad_sign (get_uint8 b i) 8

let unsafe_get_uint16_le b i =
  let l = unsafe_get_uint8 b i in
  let u = unsafe_get_uint8 b (succ i) in
  l lor u lsl 8

let get_uint16_le b i =
  if i < 0 || succ i >= length b then
    index_out_of_bounds ();
  unsafe_get_uint16_le b i

let unsafe_get_uint16_be b i =
  let u = unsafe_get_uint8 b i in
  let l = unsafe_get_uint8 b (succ i) in
  l lor u lsl 8

let get_uint16_be b i =
  if i < 0 || succ i >= length b then
    index_out_of_bounds ();
  unsafe_get_uint16_be b i

let get_uint16_ne b i =
  if Stdcompat__sys.big_endian then get_uint16_be b i
  else get_uint16_le b i

let get_int16_le b i =
  pad_sign (get_uint16_le b i) 16

let get_int16_be b i =
  pad_sign (get_uint16_be b i) 16

let get_int16_ne b i =
  pad_sign (get_uint16_ne b i) 16

let get_int32_le b i =
  if i < 0 || i + 3 >= length b then
    index_out_of_bounds ();
  let i0 = unsafe_get_uint8 b i in
  let i1 = unsafe_get_uint8 b (i + 1) in
  let i2 = unsafe_get_uint8 b (i + 2) in
  let i3 = unsafe_get_uint8 b (i + 3) in
  Int32.logor (Int32.of_int i0)
    (Int32.logor (Int32.shift_left (Int32.of_int i1) 8)
      (Int32.logor (Int32.shift_left (Int32.of_int i2) 16)
        (Int32.shift_left (Int32.of_int i3) 24)))

let get_int32_be b i =
  if i < 0 || i + 3 >= length b then
    index_out_of_bounds ();
  let i3 = unsafe_get_uint8 b i in
  let i2 = unsafe_get_uint8 b (i + 1) in
  let i1 = unsafe_get_uint8 b (i + 2) in
  let i0 = unsafe_get_uint8 b (i + 3) in
  Int32.logor (Int32.of_int i0)
    (Int32.logor (Int32.shift_left (Int32.of_int i1) 8)
      (Int32.logor (Int32.shift_left (Int32.of_int i2) 16)
        (Int32.shift_left (Int32.of_int i3) 24)))

let get_int32_ne b i =
  if Stdcompat__sys.big_endian then get_int32_be b i
  else get_int32_le b i

let get_int64_le b i =
  if i < 0 || i + 7 >= length b then
    index_out_of_bounds ();
  let i0 = unsafe_get_uint8 b i in
  let i1 = unsafe_get_uint8 b (i + 1) in
  let i2 = unsafe_get_uint8 b (i + 2) in
  let i3 = unsafe_get_uint8 b (i + 3) in
  let i4 = unsafe_get_uint8 b (i + 4) in
  let i5 = unsafe_get_uint8 b (i + 5) in
  let i6 = unsafe_get_uint8 b (i + 6) in
  let i7 = unsafe_get_uint8 b (i + 7) in
  Int64.logor (Int64.of_int i0)
    (Int64.logor (Int64.shift_left (Int64.of_int i1) 8)
      (Int64.logor (Int64.shift_left (Int64.of_int i2) 16)
        (Int64.logor (Int64.shift_left (Int64.of_int i3) 24)
          (Int64.logor (Int64.shift_left (Int64.of_int i4) 32)
            (Int64.logor (Int64.shift_left (Int64.of_int i5) 40)
              (Int64.logor (Int64.shift_left (Int64.of_int i6) 48)
                (Int64.shift_left (Int64.of_int i7) 56)))))))

let get_int64_be b i =
  if i < 0 || i + 7 >= length b then
    index_out_of_bounds ();
  let i7 = unsafe_get_uint8 b i in
  let i6 = unsafe_get_uint8 b (i + 1) in
  let i5 = unsafe_get_uint8 b (i + 2) in
  let i4 = unsafe_get_uint8 b (i + 3) in
  let i3 = unsafe_get_uint8 b (i + 4) in
  let i2 = unsafe_get_uint8 b (i + 5) in
  let i1 = unsafe_get_uint8 b (i + 6) in
  let i0 = unsafe_get_uint8 b (i + 7) in
  Int64.logor (Int64.of_int i0)
    (Int64.logor (Int64.shift_left (Int64.of_int i1) 8)
      (Int64.logor (Int64.shift_left (Int64.of_int i2) 16)
        (Int64.logor (Int64.shift_left (Int64.of_int i3) 24)
          (Int64.logor (Int64.shift_left (Int64.of_int i4) 32)
            (Int64.logor (Int64.shift_left (Int64.of_int i5) 40)
              (Int64.logor (Int64.shift_left (Int64.of_int i6) 48)
                (Int64.shift_left (Int64.of_int i7) 56)))))))

let get_int64_ne b i =
  if Stdcompat__sys.big_endian then get_int64_be b i
  else get_int64_le b i

let unsafe_set_uint8 b i v =
  unsafe_set b i (char_of_int (v land 0xFF))

let set_uint8 b i v =
  if i < 0 || i >= length b then
    index_out_of_bounds ();
  unsafe_set_uint8 b i v

let set_int8 = set_uint8

let unsafe_set_uint16_le b i v =
  unsafe_set_uint8 b i v;
  unsafe_set_uint8 b (succ i) (v lsr 8)

let set_uint16_le b i v =
  if i < 0 || succ i >= length b then
    index_out_of_bounds ();
  unsafe_set_uint16_le b i v

let unsafe_set_uint16_be b i v =
  unsafe_set_uint8 b i (v lsr 8);
  unsafe_set_uint8 b (succ i) v

let set_uint16_be b i v =
  if i < 0 || succ i >= length b then
    index_out_of_bounds ();
  unsafe_set_uint16_be b i v

let set_uint16_ne b i v =
  if Stdcompat__sys.big_endian then set_uint16_be b i v
  else set_uint16_le b i v

let set_int16_le = set_uint16_le

let set_int16_be = set_uint16_be

let set_int16_ne = set_uint16_ne

let set_int32_le b i v =
  if i < 0 || i + 3 >= length b then
    index_out_of_bounds ();
  unsafe_set_uint8 b i (Int32.to_int v);
  unsafe_set_uint8 b (i + 1) (Int32.to_int (Int32.shift_right v 8));
  unsafe_set_uint8 b (i + 2) (Int32.to_int (Int32.shift_right v 16));
  unsafe_set_uint8 b (i + 3) (Int32.to_int (Int32.shift_right v 24))

let set_int32_be b i v =
  if i < 0 || i + 3 >= length b then
    index_out_of_bounds ();
  unsafe_set_uint8 b i (Int32.to_int (Int32.shift_right v 24));
  unsafe_set_uint8 b (i + 1) (Int32.to_int (Int32.shift_right v 16));
  unsafe_set_uint8 b (i + 2) (Int32.to_int (Int32.shift_right v 8));
  unsafe_set_uint8 b (i + 3) (Int32.to_int v)

let set_int32_ne b i v =
  if Stdcompat__sys.big_endian then set_int32_be b i v
  else set_int32_le b i v

let set_int64_le b i v =
  if i < 0 || i + 7 >= length b then
    index_out_of_bounds ();
  unsafe_set_uint8 b i (Int64.to_int v);
  unsafe_set_uint8 b (i + 1) (Int64.to_int (Int64.shift_right v 8));
  unsafe_set_uint8 b (i + 2) (Int64.to_int (Int64.shift_right v 16));
  unsafe_set_uint8 b (i + 3) (Int64.to_int (Int64.shift_right v 24));
  unsafe_set_uint8 b (i + 4) (Int64.to_int (Int64.shift_right v 32));
  unsafe_set_uint8 b (i + 5) (Int64.to_int (Int64.shift_right v 40));
  unsafe_set_uint8 b (i + 6) (Int64.to_int (Int64.shift_right v 48));
  unsafe_set_uint8 b (i + 7) (Int64.to_int (Int64.shift_right v 56))

let set_int64_be b i v =
  if i < 0 || i + 7 >= length b then
    index_out_of_bounds ();
  unsafe_set_uint8 b i (Int64.to_int (Int64.shift_right v 56));
  unsafe_set_uint8 b (i + 1) (Int64.to_int (Int64.shift_right v 48));
  unsafe_set_uint8 b (i + 2) (Int64.to_int (Int64.shift_right v 40));
  unsafe_set_uint8 b (i + 3) (Int64.to_int (Int64.shift_right v 32));
  unsafe_set_uint8 b (i + 4) (Int64.to_int (Int64.shift_right v 24));
  unsafe_set_uint8 b (i + 5) (Int64.to_int (Int64.shift_right v 16));
  unsafe_set_uint8 b (i + 6) (Int64.to_int (Int64.shift_right v 8));
  unsafe_set_uint8 b (i + 7) (Int64.to_int v)

let set_int64_ne b i v =
  if Stdcompat__sys.big_endian then set_int64_be b i v
  else set_int64_le b i v
*)


(*
external unsafe_get_uint8 : bytes -> int -> int = "%bytes_unsafe_get"
external unsafe_get_uint16_ne : bytes -> int -> int = "%caml_bytes_get16u"
external get_uint8 : bytes -> int -> int = "%bytes_safe_get"
external get_uint16_ne : bytes -> int -> int = "%caml_bytes_get16"
external get_int32_ne : bytes -> int -> int32 = "%caml_bytes_get32"
external get_int64_ne : bytes -> int -> int64 = "%caml_bytes_get64"

external unsafe_set_uint8 : bytes -> int -> int -> unit = "%bytes_unsafe_set"
external unsafe_set_uint16_ne : bytes -> int -> int -> unit
                              = "%caml_bytes_set16u"
external set_int8 : bytes -> int -> int -> unit = "%bytes_safe_set"
external set_int16_ne : bytes -> int -> int -> unit = "%caml_bytes_set16"
external set_int32_ne : bytes -> int -> int32 -> unit = "%caml_bytes_set32"
external set_int64_ne : bytes -> int -> int64 -> unit = "%caml_bytes_set64"

external swap16 : int -> int = "%bswap16"

let unsafe_get_uint16_le b i =
  if Sys.big_endian then
    swap16 (unsafe_get_uint16_ne b i)
  else
    unsafe_get_uint16_ne b i

let unsafe_get_uint16_be b i =
  if Sys.big_endian then
    unsafe_get_uint16_ne b i
  else
    swap16 (unsafe_get_uint16_ne b i)

let unsafe_set_uint16_le b i x =
  if Sys.big_endian then
    unsafe_set_uint16_ne b i (swap16 x)
  else
    unsafe_set_uint16_ne b i x

let unsafe_set_uint16_be b i x =
  if Sys.big_endian then
    unsafe_set_uint16_ne b i x
  else
    unsafe_set_uint16_ne b i (swap16 x)
*)


(*

external unsafe_blit_string :
  string -> int -> bytes -> int -> int -> unit = "caml_blit_string"[@@noalloc]

(*

external unsafe_blit_string :
  string -> int -> Stdcompat__init.bytes -> int -> int -> unit =
    "caml_blit_string" "noalloc"

(*
external unsafe_blit_string :
  string -> int -> Stdcompat__init.bytes -> int -> int -> unit =
    "blit_string" "noalloc"
*)
*)
*)

(*
let rec fold_left_rec f init bytes i =
  if i >= length bytes then
    init
  else
    fold_left_rec f (f init (unsafe_get bytes i)) bytes (succ i)

let fold_left f init bytes =
  fold_left_rec f init bytes 0

let rec fold_right_rec f bytes init i =
  if i = 0 then
    init
  else
    let j = pred i in
    fold_right_rec f bytes (f (unsafe_get bytes j) init) j

let fold_right f bytes init =
  fold_right_rec f bytes init (length bytes)

let rec for_all_rec f bytes i =
  if i >= length bytes then
    true
  else if f (unsafe_get bytes i) then
    for_all_rec f bytes (succ i)
  else
    false

let for_all f bytes =
  for_all_rec f bytes 0

let rec exists_rec f bytes i =
  if i >= length bytes then
    false
  else if f (unsafe_get bytes i) then
    true
  else
    exists_rec f bytes (succ i)

let exists f bytes =
  exists_rec f bytes 0

let rec unsafe_sub_equal bytes0 off0 bytes1 off1 length =
  if length = 0 then
    true
  else if unsafe_get bytes0 off0 = unsafe_get bytes1 off1 then
    unsafe_sub_equal bytes0 (succ off0) bytes1 (succ off1) (pred length)
  else
    false

let starts_with ~prefix bytes =
  length prefix <= length bytes &&
  unsafe_sub_equal bytes 0 prefix 0 (length prefix)

let ends_with ~suffix bytes =
  length suffix <= length bytes &&
  unsafe_sub_equal bytes (length bytes - length suffix) suffix 0 (length suffix)

let split_on_char c s =
  let previous_index = ref (length s) in
  let accu = ref [] in
  for i = length s - 1 downto 0 do
    if unsafe_get s i = c then
      begin
        accu := sub s (i + 1) (!previous_index - i - 1) :: !accu;
        previous_index := i
      end
  done;
  sub s 0 !previous_index :: !accu
*)

(*
let dec_invalid = Stdcompat__uchar.utf_decode_invalid
let dec_ret n u = Stdcompat__uchar.utf_decode n (Stdcompat__uchar.unsafe_of_int u)

(* In case of decoding error, if we error on the first byte, we
   consume the byte, otherwise we consume the [n] bytes preceeding
   the erroring byte.

   This means that if a client uses decodes without caring about
   validity it naturally replace bogus data with Stdcompat__uchar.rep according
   to the WHATWG Encoding standard. Other schemes are possible by
   consulting the number of used bytes on invalid decodes. For more
   details see https://hsivonen.fi/broken-utf-8/

   For this reason in [get_utf_8_uchar] we gradually check the next
   byte is available rather than doing it immediately after the
   first byte. Contrast with [is_valid_utf_8]. *)

(* UTF-8 *)

let not_in_x80_to_xBF b = b lsr 6 <> 0b10
let not_in_xA0_to_xBF b = b lsr 5 <> 0b101
let not_in_x80_to_x9F b = b lsr 5 <> 0b100
let not_in_x90_to_xBF b = b < 0x90 || 0xBF < b
let not_in_x80_to_x8F b = b lsr 4 <> 0x8

let utf_8_uchar_2 b0 b1 =
  ((b0 land 0x1F) lsl 6) lor
  ((b1 land 0x3F))

let utf_8_uchar_3 b0 b1 b2 =
  ((b0 land 0x0F) lsl 12) lor
  ((b1 land 0x3F) lsl 6) lor
  ((b2 land 0x3F))

let utf_8_uchar_4 b0 b1 b2 b3 =
  ((b0 land 0x07) lsl 18) lor
  ((b1 land 0x3F) lsl 12) lor
  ((b2 land 0x3F) lsl 6) lor
  ((b3 land 0x3F))

let get_utf_8_uchar b i =
  let b0 = get_uint8 b i in (* raises if [i] is not a valid index. *)
  let get = unsafe_get_uint8 in
  let max = length b - 1 in
  match Char.unsafe_chr b0 with (* See The Unicode Standard, Table 3.7 *)
  | '\x00' .. '\x7F' -> dec_ret 1 b0
  | '\xC2' .. '\xDF' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x80_to_xBF b1 then dec_invalid 1 else
      dec_ret 2 (utf_8_uchar_2 b0 b1)
  | '\xE0' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_xA0_to_xBF b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      dec_ret 3 (utf_8_uchar_3 b0 b1 b2)
  | '\xE1' .. '\xEC' | '\xEE' .. '\xEF' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x80_to_xBF b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      dec_ret 3 (utf_8_uchar_3 b0 b1 b2)
  | '\xED' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x80_to_x9F b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      dec_ret 3 (utf_8_uchar_3 b0 b1 b2)
  | '\xF0' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x90_to_xBF b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      let i = i + 1 in if i > max then dec_invalid 3 else
      let b3 = get b i in if not_in_x80_to_xBF b3 then dec_invalid 3 else
      dec_ret 4 (utf_8_uchar_4 b0 b1 b2 b3)
  | '\xF1' .. '\xF3' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x80_to_xBF b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      let i = i + 1 in if i > max then dec_invalid 3 else
      let b3 = get b i in if not_in_x80_to_xBF b3 then dec_invalid 3 else
      dec_ret 4 (utf_8_uchar_4 b0 b1 b2 b3)
  | '\xF4' ->
      let i = i + 1 in if i > max then dec_invalid 1 else
      let b1 = get b i in if not_in_x80_to_x8F b1 then dec_invalid 1 else
      let i = i + 1 in if i > max then dec_invalid 2 else
      let b2 = get b i in if not_in_x80_to_xBF b2 then dec_invalid 2 else
      let i = i + 1 in if i > max then dec_invalid 3 else
      let b3 = get b i in if not_in_x80_to_xBF b3 then dec_invalid 3 else
      dec_ret 4 (utf_8_uchar_4 b0 b1 b2 b3)
  | _ -> dec_invalid 1

let set_utf_8_uchar b i u =
  let set = unsafe_set_uint8 in
  let max = length b - 1 in
  match Stdcompat__uchar.to_int u with
  | u when u < 0 -> assert false
  | u when u <= 0x007F ->
      set_uint8 b i u;
      1
  | u when u <= 0x07FF ->
      let last = i + 1 in
      if last > max then 0 else
      (set_uint8 b i (0xC0 lor (u lsr 6));
       set b last (0x80 lor (u land 0x3F));
       2)
  | u when u <= 0xFFFF ->
      let last = i + 2 in
      if last > max then 0 else
      (set_uint8 b i (0xE0 lor (u lsr 12));
       set b (i + 1) (0x80 lor ((u lsr 6) land 0x3F));
       set b last (0x80 lor (u land 0x3F));
       3)
  | u when u <= 0x10FFFF ->
      let last = i + 3 in
      if last > max then 0 else
      (set_uint8 b i (0xF0 lor (u lsr 18));
       set b (i + 1) (0x80 lor ((u lsr 12) land 0x3F));
       set b (i + 2) (0x80 lor ((u lsr 6) land 0x3F));
       set b last (0x80 lor (u land 0x3F));
       4)
  | _ -> assert false

let is_valid_utf_8 b =
  let rec loop max b i =
    if i > max then true else
    let get = unsafe_get_uint8 in
    match Char.unsafe_chr (get b i) with
    | '\x00' .. '\x7F' -> loop max b (i + 1)
    | '\xC2' .. '\xDF' ->
        let last = i + 1 in
        if last > max
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xE0' ->
        let last = i + 2 in
        if last > max
        || not_in_xA0_to_xBF (get b (i + 1))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xE1' .. '\xEC' | '\xEE' .. '\xEF' ->
        let last = i + 2 in
        if last > max
        || not_in_x80_to_xBF (get b (i + 1))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xED' ->
        let last = i + 2 in
        if last > max
        || not_in_x80_to_x9F (get b (i + 1))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xF0' ->
        let last = i + 3 in
        if last > max
        || not_in_x90_to_xBF (get b (i + 1))
        || not_in_x80_to_xBF (get b (i + 2))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xF1' .. '\xF3' ->
        let last = i + 3 in
        if last > max
        || not_in_x80_to_xBF (get b (i + 1))
        || not_in_x80_to_xBF (get b (i + 2))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | '\xF4' ->
        let last = i + 3 in
        if last > max
        || not_in_x80_to_x8F (get b (i + 1))
        || not_in_x80_to_xBF (get b (i + 2))
        || not_in_x80_to_xBF (get b last)
        then false
        else loop max b (last + 1)
    | _ -> false
  in
  loop (length b - 1) b 0

(* UTF-16BE *)

let get_utf_16be_uchar b i =
  let get = unsafe_get_uint16_be in
  let max = length b - 1 in
  if i < 0 || i > max then invalid_arg "index out of bounds" else
  if i = max then dec_invalid 1 else
  match get b i with
  | u when u < 0xD800 || u > 0xDFFF -> dec_ret 2 u
  | u when u > 0xDBFF -> dec_invalid 2
  | hi -> (* combine [hi] with a low surrogate *)
      let last = i + 3 in
      if last > max then dec_invalid (max - i + 1) else
      match get b (i + 2) with
      | u when u < 0xDC00 || u > 0xDFFF -> dec_invalid 2 (* retry here *)
      | lo ->
          let u = (((hi land 0x3FF) lsl 10) lor (lo land 0x3FF)) + 0x10000 in
          dec_ret 4 u

let set_utf_16be_uchar b i u =
  let set = unsafe_set_uint16_be in
  let max = length b - 1 in
  if i < 0 || i > max then invalid_arg "index out of bounds" else
  match Stdcompat__uchar.to_int u with
  | u when u < 0 -> assert false
  | u when u <= 0xFFFF ->
      let last = i + 1 in
      if last > max then 0 else (set b i u; 2)
  | u when u <= 0x10FFFF ->
      let last = i + 3 in
      if last > max then 0 else
      let u' = u - 0x10000 in
      let hi = (0xD800 lor (u' lsr 10)) in
      let lo = (0xDC00 lor (u' land 0x3FF)) in
      set b i hi; set b (i + 2) lo; 4
  | _ -> assert false

let is_valid_utf_16be b =
  let rec loop max b i =
    let get = unsafe_get_uint16_be in
    if i > max then true else
    if i = max then false else
    match get b i with
    | u when u < 0xD800 || u > 0xDFFF -> loop max b (i + 2)
    | u when u > 0xDBFF -> false
    | _hi ->
        let last = i + 3 in
        if last > max then false else
        match get b (i + 2) with
        | u when u < 0xDC00 || u > 0xDFFF -> false
        | _lo -> loop max b (i + 4)
  in
  loop (length b - 1) b 0

(* UTF-16LE *)

let get_utf_16le_uchar b i =
  let get = unsafe_get_uint16_le in
  let max = length b - 1 in
  if i < 0 || i > max then invalid_arg "index out of bounds" else
  if i = max then dec_invalid 1 else
  match get b i with
  | u when u < 0xD800 || u > 0xDFFF -> dec_ret 2 u
  | u when u > 0xDBFF -> dec_invalid 2
  | hi -> (* combine [hi] with a low surrogate *)
      let last = i + 3 in
      if last > max then dec_invalid (max - i + 1) else
      match get b (i + 2) with
      | u when u < 0xDC00 || u > 0xDFFF -> dec_invalid 2 (* retry here *)
      | lo ->
          let u = (((hi land 0x3FF) lsl 10) lor (lo land 0x3FF)) + 0x10000 in
          dec_ret 4 u

let set_utf_16le_uchar b i u =
  let set = unsafe_set_uint16_le in
  let max = length b - 1 in
  if i < 0 || i > max then invalid_arg "index out of bounds" else
  match Stdcompat__uchar.to_int u with
  | u when u < 0 -> assert false
  | u when u <= 0xFFFF ->
      let last = i + 1 in
      if last > max then 0 else (set b i u; 2)
  | u when u <= 0x10FFFF ->
      let last = i + 3 in
      if last > max then 0 else
      let u' = u - 0x10000 in
      let hi = (0xD800 lor (u' lsr 10)) in
      let lo = (0xDC00 lor (u' land 0x3FF)) in
      set b i hi; set b (i + 2) lo; 4
  | _ -> assert false

let is_valid_utf_16le b =
  let rec loop max b i =
    let get = unsafe_get_uint16_le in
    if i > max then true else
    if i = max then false else
    match get b i with
    | u when u < 0xD800 || u > 0xDFFF -> loop max b (i + 2)
    | u when u > 0xDBFF -> false
    | _hi ->
        let last = i + 3 in
        if last > max then false else
        match get b (i + 2) with
        | u when u < 0xDC00 || u > 0xDFFF -> false
        | _lo -> loop max b (i + 4)
  in
  loop (length b - 1) b 0
*)
