type t = { length : int;
           bits : string; }

let create ?(init=false) size =
  let i = if init then '\xff' else '\x00' in
  { length = size;
    bits = String.make (1 + size / 8) i
  }

let length v = v.length

let unsafe_get v n =
  let i = n / 8
  and j = n mod 8 in
  (((Char.code (String.unsafe_get v.bits i)) lsr j) land 1) == 1


let unsafe_set v n (b:bool) =
  let x : int = Obj.magic b in
  let i = n / 8
  and j = n mod 8 in
  let m = 1 lsl j in
  let w = Char.code (String.unsafe_get v.bits i) in
  let w = (w land lnot m) lor (~-x land m) in
  String.unsafe_set v.bits i (Char.unsafe_chr (w land 0xff))
;;

let get v n =
  if n < 0 || n >= v.length then failwith "Bitvector.get"
  else unsafe_get v n
;;

let set v n b =
  if n < 0 || n >= v.length then failwith "Bitvector.set"
  else unsafe_set v n b
;;