diff --git a/src/util/Bitvec.ml b/src/util/Bitvec.ml
index 0c043858..74024e4f 100644
--- a/src/util/Bitvec.ml
+++ b/src/util/Bitvec.ml
@@ -1,55 +1,45 @@
 
-type t = {
-  mutable chunks: bytes; (* TODO: use a in32vec with bigarray *)
-}
+type t = VecI32.t
 
-let create () : t = {
-  chunks = Bytes.make 32 '\x00';
-}
-
-let i2c = Char.chr
-let c2i = Char.code
+let create () : t = VecI32.create ~cap:16 ()
 
 (* from index to offset in bytes *)
-let[@inline] idx_bytes_ (i:int) : int = i lsr 3
+let[@inline] idx_in_vec_ (i:int) : int = i lsr 5
 
-(* from index to offset in a single char *)
-let mask_ = 0b111
+(* from index to offset in a single chunk *)
+let mask_ = 0b11111
 
 (* number of bytes *)
-let[@inline] len_ (self:t) : int = Bytes.length self.chunks
+let[@inline] len_ (self:t) : int = VecI32.size self
 
 let[@inline never] resize_ self idx : unit =
   let new_size =
     min Sys.max_string_length
       (max (idx+2) (let l = len_ self in l + 10 + l / 2))
   in
-  let new_chunks = Bytes.make new_size (i2c 0) in
-  Bytes.blit self.chunks 0 new_chunks 0 (len_ self);
-  self.chunks <- new_chunks;
+  VecI32.ensure_size self new_size;
   assert (len_ self > idx)
 
 let[@inline] ensure_size self i =
-  let idx = idx_bytes_ i in
+  let idx = idx_in_vec_ i in
   if idx >= len_ self then (
     resize_ self idx
   )
 
 let[@inline] get self i : bool =
-  let idx = idx_bytes_ i in
-  let c = c2i (Bytes.get self.chunks idx) in
+  let idx = idx_in_vec_ i in
+  let c = VecI32.get self idx in
   (c land (1 lsl (i land mask_))) <> 0
 
 let set self i b : unit =
-  let idx = idx_bytes_ i in
-  let c = c2i (Bytes.get self.chunks idx) in
+  let idx = idx_in_vec_ i in
+  let c = VecI32.get self idx in
   let c =
     if b
     then c lor (1 lsl (i land mask_))
     else c land (lnot (1 lsl (i land mask_)))
   in
-  Bytes.set self.chunks idx (i2c c)
+  VecI32.set self idx c
 
-let clear_all self =
-  Bytes.fill self.chunks 0 (len_ self) '\x00'
+let clear_all self = VecI32.fill self 0
 
diff --git a/src/util/VecI32.ml b/src/util/VecI32.ml
index e595d30d..3be2aacc 100644
--- a/src/util/VecI32.ml
+++ b/src/util/VecI32.ml
@@ -18,6 +18,8 @@ let[@inline] shrink self n = if n < self.sz then self.sz <- n
 let[@inline] size self = self.sz
 let[@inline] is_empty self = self.sz = 0
 
+let[@inline] fill self x = A.fill self.data (Int32.of_int x)
+
 (* ensure capacity is [new_cap] *)
 let resize_cap_ self new_cap =
   assert (A.dim self.data < new_cap);
@@ -34,7 +36,10 @@ let ensure_cap self (n:int) =
 let ensure_size self n =
   if n > self.sz then (
     ensure_cap self n;
-    self.sz <- n
+    for i=self.sz to n-1 do
+      A.set self.data i 0l;
+    done;
+    self.sz <- n;
   )
 
 let[@inline] push (self:t) i : unit =
diff --git a/src/util/VecI32.mli b/src/util/VecI32.mli
index d0f733ac..af50d61b 100644
--- a/src/util/VecI32.mli
+++ b/src/util/VecI32.mli
@@ -11,6 +11,8 @@ val ensure_size : t -> int -> unit
 
 val size : t -> int
 
+val fill : t -> int -> unit
+
 val clear : t -> unit
 
 val is_empty : t -> bool