diff --git a/src/3d/hashchk/Hashing_Op.fs b/src/3d/hashchk/Hashing_Op.fs
index 911b65d34..5d48e6299 100755
--- a/src/3d/hashchk/Hashing_Op.fs
+++ b/src/3d/hashchk/Hashing_Op.fs
@@ -1,8 +1,11 @@
 module Hashing_Op
 
+let debug_hash = false
+
 type byte_array(ap: System.Buffers.ArrayPool<byte>, len: int32) =
     let ar : byte[] = ap.Rent(len)
-    member val array = ar
+    member this.rw = System.Span(ar, 0, len)
+    member this.ro = System.ReadOnlySpan(ar, 0, len)
     interface System.IDisposable with
       member this.Dispose() = ap.Return(ar, false)
 
@@ -11,14 +14,14 @@ let hex_digits = [| "0"; "1"; "2"; "3"; "4"; "5"; "6"; "7"; "8"; "9"; "a"; "b";
 let byte_to_hex (b: byte) =
     System.String.Concat (hex_digits.[int b / 16], hex_digits.[int b % 16])
 
-let rec bytes_to_hex0 (b: byte[]) (i: int) : string =
+let rec bytes_to_hex0 (b: System.ReadOnlySpan<byte>) (i: int) : string =
     if i <= 0
     then ""
     else
         let j = i - 1
         System.String.Concat (bytes_to_hex0 b j, byte_to_hex b.[j])
 
-let bytes_to_hex (b: byte[]) : string =
+let bytes_to_hex (b: System.ReadOnlySpan<byte>) : string =
     bytes_to_hex0 b b.Length
 
 type hash_state() =
@@ -28,15 +31,25 @@ type hash_state() =
         System.Security.Cryptography.IncrementalHash.CreateHash(System.Security.Cryptography.HashAlgorithmName.SHA256)
     member this.encoding = enc
     member this.AppendData(ar: byte_array) =
-        h.AppendData(ar.array)
+        if debug_hash then begin
+          let s = bytes_to_hex ar.ro
+          System.Console.WriteLine ("Trying to update with: " ^ s)
+        end
+        h.AppendData(ar.ro)
+        if debug_hash then begin
+          let len = h.HashLengthInBytes
+          use arr = this.new_array(len)
+          ignore (h.GetCurrentHash(arr.rw))
+          let s = bytes_to_hex arr.ro
+          System.Console.WriteLine ("Update: " ^ s)
+        end
     member this.new_array(len: int32) : byte_array =
         new byte_array(ap, len)
     member this.finish() : string =
         let len = h.HashLengthInBytes
         use arr = this.new_array(len)
-        let dest = System.Span(arr.array)
-        ignore (h.GetHashAndReset(dest))
-        bytes_to_hex arr.array
+        ignore (h.GetHashAndReset(arr.rw))
+        bytes_to_hex arr.ro
     interface System.IDisposable with
       member this.Dispose() =
         h.Dispose()
@@ -47,13 +60,12 @@ let hash_init () : hash_state =
 let hash_bool (h: hash_state) (b: bool) : unit =
     let byte : byte = System.Convert.ToByte (if b then 1 else 0)
     use arr = h.new_array(1)
-    arr.array.[0] <- byte
+    arr.rw.[0] <- byte
     h.AppendData arr
 
 let hash_int (h: hash_state) (i: int32) : unit =
     use arr = h.new_array(4)
-    let sp = System.Span(arr.array)
-    System.Buffers.Binary.BinaryPrimitives.WriteInt32BigEndian (sp, i)
+    System.Buffers.Binary.BinaryPrimitives.WriteInt32BigEndian (arr.rw, i)
     h.AppendData arr
 
 let hash_file (h: hash_state) (f: string) : unit =
@@ -61,7 +73,7 @@ let hash_file (h: hash_state) (f: string) : unit =
     let len = int32 file.Length
     hash_int h len
     use arr = h.new_array(len)
-    let read = file.Read(arr.array, 0, len)
+    let read = file.Read(arr.rw)
     assert (read = len)
     h.AppendData arr
 
@@ -71,10 +83,13 @@ let hash_file_option (h: hash_state) (s: FStar_Pervasives_Native.option<string>)
     | FStar_Pervasives_Native.Some f -> hash_bool h true; hash_file h f
 
 let hash_string (h: hash_state) (s: string) : unit =
+    if debug_hash then begin
+      System.Console.WriteLine("hash_string: \"" ^ s ^ "\"")
+    end
     let len = int32 (h.encoding.GetByteCount(s))
     hash_int h len
     use arr = h.new_array(len)
-    let encoded = h.encoding.GetBytes(s, 0, s.Length, arr.array, 0)
+    let encoded = h.encoding.GetBytes(System.String.op_Implicit(s), arr.rw)
     assert (encoded = len)
     h.AppendData arr
 
diff --git a/src/3d/ocaml/Hashing_Op.ml b/src/3d/ocaml/Hashing_Op.ml
index f29eb7753..7416a8d00 100644
--- a/src/3d/ocaml/Hashing_Op.ml
+++ b/src/3d/ocaml/Hashing_Op.ml
@@ -1,5 +1,7 @@
 type t = HashingBase.t
 
+let debug_hash = false
+
 (* Hash a boolean *)
 
 let hash_bool h b =
@@ -32,6 +34,9 @@ let hash_file_option h = function
 (* Hash a string *)
 
 let hash_string h s =
+  if debug_hash then begin
+      print_endline ("hash_string: \"" ^ s ^ "\"")
+  end;
   hash_int h (String.length s);
   HashingBase.update h (Bytes.of_string s)
 
diff --git a/src/3d/ocaml/sha/HashingBase.ml b/src/3d/ocaml/sha/HashingBase.ml
index 05a49cb09..651559519 100644
--- a/src/3d/ocaml/sha/HashingBase.ml
+++ b/src/3d/ocaml/sha/HashingBase.ml
@@ -1,7 +1,38 @@
 type t = Sha256.ctx
 
-let init () = Sha256.init ()
+let debug_hash = false
 
-let update h b = Sha256.update_string h (Bytes.to_string b)
+let init () = Sha256.init ()
 
 let finish h = Bytes.of_string (Sha256.to_bin (Sha256.finalize h))
+
+let char_of_int4 x =
+  assert (0 <= x && x < 16);
+  if x < 10
+  then char_of_int (int_of_char '0' + x)
+  else char_of_int (int_of_char 'a' + x - 10)
+
+let hex_of_char c =
+  let i = int_of_char c in
+  assert (0 <= i && i < 256);
+  char_of_int4 (i / 16), char_of_int4 (i mod 16)
+
+let hex_of_bytes buf =
+  let hex = Bytes.create (Bytes.length buf * 2) in
+  Bytes.iteri (fun idx c ->
+      let hi, lo = hex_of_char c in
+      Bytes.set hex (2 * idx) hi;
+      Bytes.set hex (2 * idx + 1) lo
+    ) buf;
+  Bytes.to_string hex
+
+let update h b =
+  if debug_hash then begin
+      print_endline ("Trying to update with: " ^ hex_of_bytes b);
+  end;
+  Sha256.update_string h (Bytes.to_string b);
+  if debug_hash then begin
+      let h' = Sha256.copy h in
+      let s = finish h' in
+      print_endline ("Update: " ^ hex_of_bytes s)
+  end