From 88f8bf707898701b5a1a14068038fac6263fb540 Mon Sep 17 00:00:00 2001
From: Rudi Grinberg <me@rgrinberg.com>
Date: Sat, 30 Mar 2024 16:44:08 +0000
Subject: [PATCH] refactor: re-implement bit bector properly

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
---
 lib/automata.ml    |  4 +---
 lib/bit_vector.ml  | 49 ++++++++++++++++++++++++++++++++++++++--------
 lib/bit_vector.mli |  2 +-
 3 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/lib/automata.ml b/lib/automata.ml
index 44f1f4ff..eefe14d6 100644
--- a/lib/automata.ml
+++ b/lib/automata.ml
@@ -330,8 +330,6 @@ let create_working_area () = ref (Bit_vector.singleton false)
 
 let index_count w = Bit_vector.length !w
 
-let reset_table a = Bit_vector.set_all a false
-
 let rec mark_used_indices tbl =
   List.iter (function
       | E.TSeq (l, _, _) -> mark_used_indices tbl l
@@ -345,7 +343,7 @@ let rec find_free tbl idx len =
 
 let free_index tbl_ref l =
   let tbl = !tbl_ref in
-  reset_table tbl;
+  Bit_vector.reset_zero tbl;
   mark_used_indices tbl l;
   let len = Bit_vector.length tbl in
   let idx = find_free tbl 0 len in
diff --git a/lib/bit_vector.ml b/lib/bit_vector.ml
index f275dede..a731f1e8 100644
--- a/lib/bit_vector.ml
+++ b/lib/bit_vector.ml
@@ -1,14 +1,47 @@
-type t = bool array
+type t =
+  { len : int
+  ; bits : Bytes.t
+  }
+let byte s i = Char.code (Bytes.unsafe_get s i)
 
-let singleton v = [| v |]
+let low_mask = Array.init 9 (fun i -> (1 lsl i) - 1)
+let set_byte s i x = Bytes.unsafe_set s i (Char.chr x)
 
-let length t = Array.length t
+let create len b =
+  let initv = if b then 255 else 0 in
+  let q = len lsr 3 in
+  let r = len land 7 in
+  if r = 0 then
+    { len ; bits = Bytes.make q (Char.chr initv) }
+  else begin
+    let s = Bytes.make (q + 1) (Char.chr initv) in
+    set_byte s q (initv land low_mask.(r));
+    { len ; bits = s }
+  end
 
-let set t i a = t.(i) <- a
+let singleton v = create 1 v
 
-let get t i = t.(i)
+let length t = t.len
 
-let create len v = Array.make len v
+let unsafe_set v n b =
+  let i = n lsr 3 in
+  let c = byte v.bits i in
+  let mask = 1 lsl (n land 7) in
+  set_byte v.bits i (if b then c lor mask else c land (lnot mask))
 
-let set_all t v =
-  Array.fill t 0 (length t) v
+let set v n b =
+  if n < 0 || n >= v.len then invalid_arg "Bitv.set";
+  unsafe_set v n b
+
+let unsafe_get v n =
+  let i = n lsr 3 in
+  (byte v.bits i) land (1 lsl (n land 7)) > 0
+
+let get v n =
+  if n < 0 || n >= v.len then invalid_arg "Bitv.get";
+  unsafe_get v n
+
+let reset_zero t =
+  for i = 0 to Bytes.length t.bits - 1 do
+    Bytes.set t.bits i '\000'
+  done
diff --git a/lib/bit_vector.mli b/lib/bit_vector.mli
index 5155b312..c9420a1e 100644
--- a/lib/bit_vector.mli
+++ b/lib/bit_vector.mli
@@ -10,4 +10,4 @@ val create : int -> bool -> t
 
 val get : t -> int -> bool
 
-val set_all : t -> bool -> unit
+val reset_zero : t -> unit