diff --git a/crates/loro-core/src/dag.rs b/crates/loro-core/src/dag.rs
new file mode 100644
index 00000000..e69de29b
diff --git a/crates/loro-core/src/lib.rs b/crates/loro-core/src/lib.rs
index a8ff0038..b8c91195 100644
--- a/crates/loro-core/src/lib.rs
+++ b/crates/loro-core/src/lib.rs
@@ -6,16 +6,17 @@
 pub mod change;
 pub mod configure;
 pub mod container;
+pub mod dag;
 pub mod id;
 pub mod op;
 pub mod version;
 
 mod error;
-mod id_span;
 mod log_store;
 mod loro;
 mod smstring;
 mod snapshot;
+mod span;
 mod tests;
 mod value;
 
diff --git a/crates/loro-core/src/log_store.rs b/crates/loro-core/src/log_store.rs
index e6bf1039..453e9388 100644
--- a/crates/loro-core/src/log_store.rs
+++ b/crates/loro-core/src/log_store.rs
@@ -14,8 +14,8 @@ use crate::{
     configure::Configure,
     container::{Container, ContainerID, ContainerManager},
     id::{ClientID, Counter},
-    id_span::IdSpan,
     op::OpProxy,
+    span::IdSpan,
     version::TotalOrderStamp,
     Lamport, Op, Timestamp, ID,
 };
diff --git a/crates/loro-core/src/op.rs b/crates/loro-core/src/op.rs
index a5dac56c..7d6712f0 100644
--- a/crates/loro-core/src/op.rs
+++ b/crates/loro-core/src/op.rs
@@ -1,4 +1,8 @@
-use crate::{container::ContainerID, id::ID, id_span::IdSpan};
+use crate::{
+    container::ContainerID,
+    id::ID,
+    span::{CounterSpan, IdSpan},
+};
 use rle::{HasLength, Mergable, RleVec, Sliceable};
 mod insert_content;
 mod op_content;
@@ -47,7 +51,7 @@ impl Op {
     }
 
     #[inline]
-    pub fn new_delete_op(id: ID, container: ContainerID, target: RleVec<IdSpan>) -> Self {
+    pub fn new_delete_op(id: ID, container: ContainerID, target: RleVec<CounterSpan>) -> Self {
         Op::new(id, OpContent::Undo { target }, container)
     }
 
diff --git a/crates/loro-core/src/op/insert_content.rs b/crates/loro-core/src/op/insert_content.rs
index e46151a6..7fb08a59 100644
--- a/crates/loro-core/src/op/insert_content.rs
+++ b/crates/loro-core/src/op/insert_content.rs
@@ -32,6 +32,7 @@ pub trait InsertContent:
     HasLength + std::fmt::Debug + Any + MergeableContent + SliceableContent + CloneContent
 {
     fn id(&self) -> ContentType;
+    // TODO: provide an encoding method
 }
 
 impl<T: Sliceable + InsertContent> SliceableContent for T {
diff --git a/crates/loro-core/src/op/op_content.rs b/crates/loro-core/src/op/op_content.rs
index cdc3e66c..77372483 100644
--- a/crates/loro-core/src/op/op_content.rs
+++ b/crates/loro-core/src/op/op_content.rs
@@ -1,14 +1,19 @@
 use rle::{HasLength, Mergable, RleVec, Sliceable};
 
-use crate::{container::ContainerID, id::ID, id_span::IdSpan, OpType};
+use crate::{
+    container::ContainerID,
+    id::ID,
+    span::{CounterSpan, IdSpan},
+    OpType,
+};
 
 use super::{InsertContent, MergeableContent};
 
 #[derive(Debug)]
 pub enum OpContent {
     Normal { content: Box<dyn InsertContent> },
-    Undo { target: RleVec<IdSpan> },
-    Redo { target: RleVec<IdSpan> },
+    Undo { target: RleVec<CounterSpan> },
+    Redo { target: RleVec<CounterSpan> },
 }
 
 impl OpContent {
diff --git a/crates/loro-core/src/id_span.rs b/crates/loro-core/src/span.rs
similarity index 60%
rename from crates/loro-core/src/id_span.rs
rename to crates/loro-core/src/span.rs
index 23d2e954..966bf20d 100644
--- a/crates/loro-core/src/id_span.rs
+++ b/crates/loro-core/src/span.rs
@@ -1,16 +1,20 @@
-use crate::id::{ClientID, ID};
+use crate::id::{ClientID, Counter, ID};
 use rle::{HasLength, Mergable, Slice, Sliceable};
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
-pub struct IdSpan {
-    pub client_id: ClientID,
-    pub from: usize,
-    pub to: usize,
+pub struct CounterSpan {
+    pub from: Counter,
+    pub to: Counter,
 }
 
-impl IdSpan {
+impl CounterSpan {
     #[inline]
-    pub fn min(&self) -> usize {
+    pub fn new(from: Counter, to: Counter) -> Self {
+        CounterSpan { from, to }
+    }
+
+    #[inline]
+    pub fn min(&self) -> Counter {
         if self.from < self.to {
             self.from
         } else {
@@ -19,7 +23,7 @@ impl IdSpan {
     }
 
     #[inline]
-    pub fn max(&self) -> usize {
+    pub fn max(&self) -> Counter {
         if self.from > self.to {
             self.from
         } else {
@@ -28,44 +32,90 @@ impl IdSpan {
     }
 }
 
-impl HasLength for IdSpan {
+impl HasLength for CounterSpan {
+    #[inline]
     fn len(&self) -> usize {
         if self.to > self.from {
-            self.to - self.from
+            (self.to - self.from) as usize
         } else {
-            self.from - self.to
+            (self.from - self.to) as usize
         }
     }
 }
 
-impl Sliceable for IdSpan {
+impl Sliceable for CounterSpan {
     fn slice(&self, from: usize, to: usize) -> Self {
         assert!(from <= to);
         let len = to - from;
         assert!(len <= self.len());
         if self.from < self.to {
-            IdSpan {
-                client_id: self.client_id,
-                from: self.from + from,
-                to: self.from + to,
+            CounterSpan {
+                from: self.from + from as Counter,
+                to: self.from + to as Counter,
             }
         } else {
-            IdSpan {
-                client_id: self.client_id,
-                from: self.from - from,
-                to: self.from - to,
+            CounterSpan {
+                from: self.from - from as Counter,
+                to: self.from - to as Counter,
             }
         }
     }
 }
 
+impl Mergable for CounterSpan {
+    #[inline]
+    fn is_mergable(&self, other: &Self, _: &()) -> bool {
+        self.to == other.from
+    }
+
+    #[inline]
+    fn merge(&mut self, other: &Self, _: &()) {
+        self.to = other.to;
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub struct IdSpan {
+    pub client_id: ClientID,
+    pub counter: CounterSpan,
+}
+
+impl IdSpan {
+    #[inline]
+    pub fn min(&self) -> Counter {
+        self.counter.min()
+    }
+
+    #[inline]
+    pub fn max(&self) -> Counter {
+        self.counter.max()
+    }
+}
+
+impl HasLength for IdSpan {
+    #[inline]
+    fn len(&self) -> usize {
+        self.counter.len()
+    }
+}
+
+impl Sliceable for IdSpan {
+    #[inline]
+    fn slice(&self, from: usize, to: usize) -> Self {
+        IdSpan {
+            client_id: self.client_id,
+            counter: self.counter.slice(from, to),
+        }
+    }
+}
+
 impl Mergable for IdSpan {
     fn is_mergable(&self, other: &Self, _: &()) -> bool {
-        self.client_id == other.client_id && self.to == other.from
+        self.client_id == other.client_id && self.counter.is_mergable(&other.counter, &())
     }
 
     fn merge(&mut self, other: &Self, _: &()) {
-        self.to = other.to;
+        self.counter.merge(&other.counter, &())
     }
 }
 
@@ -82,8 +132,7 @@ mod test_id_span {
                 $(
                     id_spans.push(IdSpan {
                         client_id: $client_id,
-                        from: $from,
-                        to: $to,
+                        counter: CounterSpan::new($from, $to),
                     });
                 )*
                 id_spans
@@ -96,22 +145,19 @@ mod test_id_span {
         let mut id_span_vec = RleVec::new();
         id_span_vec.push(IdSpan {
             client_id: 0,
-            from: 0,
-            to: 2,
+            counter: CounterSpan::new(0, 2),
         });
         assert_eq!(id_span_vec.merged_len(), 1);
         assert_eq!(id_span_vec.len(), 2);
         id_span_vec.push(IdSpan {
             client_id: 0,
-            from: 2,
-            to: 4,
+            counter: CounterSpan::new(2, 4),
         });
         assert_eq!(id_span_vec.merged_len(), 1);
         assert_eq!(id_span_vec.len(), 4);
         id_span_vec.push(IdSpan {
             client_id: 2,
-            from: 2,
-            to: 4,
+            counter: CounterSpan::new(2, 4),
         });
         assert_eq!(id_span_vec.merged_len(), 2);
         assert_eq!(id_span_vec.len(), 6);