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type t = { s : string; mutable pos : int }
let of_string (s : string) : t = { s; pos = 0 }
let bit (b : t) (i : int) : int =
let byte = i lsr 3 in
if byte >= String.length b.s then 0 else (Char.code b.s.[byte] lsr (7 - (i land 7))) land 1
let peek (b : t) (n : int) : int =
let v = ref 0 in
for i = 0 to n - 1 do v := (!v lsl 1) lor bit b (b.pos + i) done;
!v
let skip (b : t) (n : int) : unit = b.pos <- b.pos + n
let read (b : t) (n : int) : int =
let v = peek b n in
skip b n;
v
let position (b : t) : int = b.pos
let seek (b : t) (p : int) : unit = b.pos <- p
let at_end (b : t) : bool = b.pos >= 8 * String.length b.s
let next_start_code (b : t) : int option =
let byte = ref ((b.pos + 7) lsr 3) and found = ref None in
let s = b.s in
while !found = None && !byte + 3 < String.length s do
if s.[!byte] = '\000' && s.[!byte + 1] = '\000' && s.[!byte + 2] = '\001' then found := Some (Char.code s.[!byte + 3])
else incr byte
done;
match !found with
| Some code ->
b.pos <- 8 * (!byte + 4);
Some code
| None ->
b.pos <- 8 * String.length s;
None