diff --git a/src/base/Chunk_stack.ml b/src/base/Chunk_stack.ml
deleted file mode 100644
index d9e3af3b..00000000
--- a/src/base/Chunk_stack.ml
+++ /dev/null
@@ -1,112 +0,0 @@
-
-module Buf = struct
-  type t = {
-    mutable b: bytes;
-    mutable len: int;
-  }
-
-  let create ?(cap=16) () : t =
-    { len=0; b=Bytes.create cap; }
-
-  let ensure_size_ (self:t) (new_len:int) : unit =
-    if new_len > Bytes.length self.b then (
-      let size = min (new_len + new_len / 4 + 5) Sys.max_string_length in
-      if new_len > size then failwith "max buf size exceeded";
-
-      let b2 = Bytes.create size in
-      Bytes.blit self.b 0 b2 0 self.len;
-      self.b <- b2
-    )
-
-  let add_bytes (self:t) (b:bytes) (off:int) (len:int) =
-    ensure_size_ self (self.len + len);
-    Bytes.blit self.b self.len b off len;
-    self.len <- self.len + len
-
-  let[@inline] add_buf (self:t) (other:t) =
-    add_bytes self other.b 0 other.len
-
-  let clear self = self.len <- 0
-
-  let contents self = Bytes.sub_string self.b 0 self.len
-end
-
-module Writer = struct
-  type t = {
-    write: Buf.t -> unit;
-  }
-
-  let nop_ _ = ()
-
-  let dummy : t = { write=nop_; }
-
-  let into_buf (into:Buf.t) : t =
-    let blen = Bytes.create 4 in
-    let write buf =
-      Buf.add_buf into buf;
-      (* add len *)
-      Bytes.set_int32_le blen 0 (Int32.of_int buf.Buf.len);
-      Buf.add_bytes into blen 0 4;
-    in
-    { write; }
-
-  let into_channel (oc:out_channel) : t =
-    let blen = Bytes.create 4 in
-    let write buf =
-      output oc buf.Buf.b 0 buf.Buf.len;
-      (* add len *)
-      Bytes.set_int32_le blen 0 (Int32.of_int buf.Buf.len);
-      output oc blen 0 4
-    in
-    { write; }
-end
-
-module Reader = struct
-  type t = {
-    read: Buf.t -> bool;
-  }
-
-  let[@inline] next (self:t) buf : bool = self.read buf
-
-  let empty : t = { read=fun _ -> false }
-
-  let from_buf (buf:Buf.t) : t =
-    assert false (* TODO *)
-
-  let with_file_backward (filename:string) f =
-    CCIO.with_in ~flags:[Open_binary; Open_rdonly] filename @@ fun ic ->
-
-    let len = in_channel_length ic in
-    seek_in ic len;
-
-    (* read length *)
-    let blen = Bytes.create 4 in
-
-    let read buf : bool =
-      let pos = pos_in ic in
-      if pos > 0 then (
-        (* read length of preceding chunk *)
-        assert (pos>=4);
-        seek_in ic (pos - 4);
-
-        really_input ic blen 0 4;
-        let chunk_len = Int32.to_int (Bytes.get_int32_le blen 0) in
-        Printf.printf "read chunk of len %d\n%!" chunk_len;
-
-        (* now read chunk *)
-        Buf.ensure_size_ buf chunk_len;
-        seek_in ic (pos - 4 - chunk_len);
-        really_input ic buf.Buf.b 0 chunk_len;
-        buf.Buf.len <- chunk_len;
-
-        true
-      ) else (
-        false
-      )
-    in
-    f {read}
-end
-
-(*$T
-    false
-    *)
diff --git a/src/util/Chunk_stack.ml b/src/util/Chunk_stack.ml
new file mode 100644
index 00000000..eef6ff36
--- /dev/null
+++ b/src/util/Chunk_stack.ml
@@ -0,0 +1,146 @@
+
+module Buf = struct
+  type t = {
+    mutable b: bytes;
+    mutable len: int;
+  }
+
+  let create ?(cap=16) () : t =
+    { len=0; b=Bytes.create cap; }
+
+  let resize_ self new_len : unit =
+    let size = min (new_len + new_len / 4 + 5) Sys.max_string_length in
+    if new_len > size then failwith "max buf size exceeded";
+
+    let b2 = Bytes.create size in
+    Bytes.blit self.b 0 b2 0 self.len;
+    self.b <- b2
+
+  let[@inline] ensure_size_ (self:t) (new_len:int) : unit =
+    if new_len > Bytes.length self.b then (
+      resize_ self new_len
+    )
+
+  let add_bytes (self:t) (b:bytes) (off:int) (len:int) =
+    Printf.printf "add_bytes len=%d\n%!" len;
+    ensure_size_ self (self.len + len);
+    Bytes.blit b off self.b self.len len;
+    self.len <- self.len + len
+
+  let[@inline] add_buf (self:t) (other:t) =
+    add_bytes self other.b 0 other.len
+
+  let clear self = self.len <- 0
+
+  let contents self = Bytes.sub_string self.b 0 self.len
+end
+
+module Writer = struct
+  type t = {
+    write: bytes -> int -> int -> unit;
+  }
+
+  let nop_ _ = ()
+
+  let dummy : t = { write=fun _ _ _ -> (); }
+
+  let into_buf (into:Buf.t) : t =
+    let blen = Bytes.create 4 in
+    let write b i len =
+      Buf.add_bytes into b i len;
+      (* add len *)
+      Bytes.set_int32_le blen 0 (Int32.of_int len);
+      Buf.add_bytes into blen 0 4;
+    in
+    { write; }
+
+  let into_channel (oc:out_channel) : t =
+    let blen = Bytes.create 4 in
+    let write b i len =
+      output oc b i len;
+      (* add len *)
+      Bytes.set_int32_le blen 0 (Int32.of_int len);
+      output oc blen 0 4
+    in
+    { write; }
+
+  let[@inline] add_buf self buf = self.write buf.Buf.b 0 buf.Buf.len
+  let[@inline] add_bytes self b i len = self.write b i len
+  let[@inline] add_string self s =
+    add_bytes self (Bytes.unsafe_of_string s) 0 (String.length s)
+end
+
+module Reader = struct
+  type t = {
+    read: 'a. yield:(bytes -> int -> int -> 'a) -> finish:(unit -> 'a) -> 'a;
+  } [@@unboxed]
+
+  let[@inline] next (self:t) f : bool =
+    self.read
+      ~yield:(fun b i len -> f b i len; true)
+      ~finish:(fun () -> false)
+
+  let next_string (self:t) : string option =
+    self.read
+      ~yield:(fun b i len -> Some (Bytes.sub_string b i len))
+      ~finish:(fun () -> None)
+
+  let empty : t = { read=fun ~yield:_ ~finish -> finish() }
+
+  let from_buf (ibuf:Buf.t) : t =
+    let i = ref ibuf.Buf.len in
+
+    (* read next record *)
+    let read ~yield ~finish =
+      if !i > 0 then (
+        Printf.printf "reading (!i = %d)\n%!" !i;
+        assert (!i >= 4);
+
+        i := !i - 4;
+        let chunk_size = Int32.to_int (Bytes.get_int32_le ibuf.Buf.b !i) in
+        Printf.printf "chunk size is %d\n%!" chunk_size;
+
+        i := !i - chunk_size;
+        yield ibuf.Buf.b !i chunk_size
+      ) else (
+        finish()
+      )
+    in
+    { read; }
+
+  let with_file_backward (filename:string) f =
+    CCIO.with_in ~flags:[Open_binary; Open_rdonly] filename @@ fun ic ->
+
+    let len = in_channel_length ic in
+    seek_in ic len;
+
+    let blen = Bytes.create 4 in (* to read length *)
+    let buf = Buf.create() in (* local buffer *)
+
+    let read ~yield ~finish =
+      let pos = pos_in ic in
+      if pos > 0 then (
+        (* read length of preceding chunk *)
+        assert (pos>=4);
+        seek_in ic (pos - 4);
+
+        really_input ic blen 0 4;
+        let chunk_len = Int32.to_int (Bytes.get_int32_le blen 0) in
+
+        (* now read chunk *)
+        Buf.ensure_size_ buf chunk_len;
+        seek_in ic (pos - 4 - chunk_len);
+        really_input ic buf.Buf.b 0 chunk_len;
+        buf.Buf.len <- chunk_len;
+
+        yield buf.Buf.b 0 buf.Buf.len
+      ) else (
+        finish()
+      )
+    in
+    f {read}
+end
+
+(*$T
+    false
+    *)
diff --git a/src/base/Chunk_stack.mli b/src/util/Chunk_stack.mli
similarity index 80%
rename from src/base/Chunk_stack.mli
rename to src/util/Chunk_stack.mli
index e508e673..3ee6ab6b 100644
--- a/src/base/Chunk_stack.mli
+++ b/src/util/Chunk_stack.mli
@@ -35,15 +35,24 @@ module Writer : sig
   val into_buf : Buf.t -> t
 
   val into_channel: out_channel -> t
+
+  val add_buf : t -> Buf.t -> unit
+
+  val add_bytes : t -> bytes -> int -> int -> unit
+
+  val add_string : t -> string -> unit
 end
 
 module Reader : sig
   type t
 
-  val next : t -> Buf.t -> bool
-  (** Read next chunk into buf.
+  val next : t -> (bytes -> int -> int -> unit) -> bool
+  (** Read next chunk, call the function with a slice of bytes.
       Returns [true] if a chunk was read, [false] if no more chunks are there.  *)
 
+  val next_string : t -> string option
+  (** Read next chunk as a string *)
+
   val empty : t
 
   val from_buf : Buf.t -> t
diff --git a/src/util/Sidekick_util.ml b/src/util/Sidekick_util.ml
index 3e70f3da..b49f6c31 100644
--- a/src/util/Sidekick_util.ml
+++ b/src/util/Sidekick_util.ml
@@ -28,5 +28,6 @@ module Bag = Bag
 module Stat = Stat
 module Hash = Hash
 module Profile = Profile
+module Chunk_stack = Chunk_stack
 
 module Intf = Sidekick_sigs