diff --git a/eden/scm/lib/conflict/Cargo.toml b/eden/scm/lib/conflict/Cargo.toml
new file mode 100644
index 0000000000000..95d921052c694
--- /dev/null
+++ b/eden/scm/lib/conflict/Cargo.toml
@@ -0,0 +1,9 @@
+# @generated by autocargo from //eden/scm/lib/conflict:conflict
+[package]
+name = "conflict"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+serde = { version = "=1.0.118", features = ["derive", "rc"] }
+types = { path = "../types" }
diff --git a/eden/scm/lib/conflict/src/lib.rs b/eden/scm/lib/conflict/src/lib.rs
new file mode 100644
index 0000000000000..fbaa520dd8a1f
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+++ b/eden/scm/lib/conflict/src/lib.rs
@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+//! Check-in conflict support.
+//!
+//! Main idea comes from [Jujube](https://github.com/martinvonz/jj).
+
+mod model;
+
+pub use model::{CommitConflict, FileConflict, FileContext};
diff --git a/eden/scm/lib/conflict/src/model.rs b/eden/scm/lib/conflict/src/model.rs
new file mode 100644
index 0000000000000..e7cc34e7e5f35
--- /dev/null
+++ b/eden/scm/lib/conflict/src/model.rs
@@ -0,0 +1,150 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+//! Check-in conflict data models.
+
+use serde::Deserialize;
+use serde::Serialize;
+use std::collections::btree_map::Entry;
+use std::collections::BTreeMap;
+use std::hash::Hash;
+use std::ops::Add;
+use std::ops::Sub;
+use types::HgId;
+
+/// Conflict state in a commit.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CommitConflict {
+    // Not using RepoPathBuf to avoid one nested level of serde serialization.
+    #[serde(rename = "files")]
+    pub files: BTreeMap<String, FileConflict>,
+}
+
+/// Conflict state in a file.
+///
+/// Also represent a resolution of a 3-way merge. See [`FileConflict::from_3way`].
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FileConflict {
+    /// Added contexts. Usually they are the commits that have conflicts
+    #[serde(rename = "adds")]
+    pub adds: Vec<FileContext>,
+
+    /// Removed contexts. Usually they are merge bases.
+    #[serde(rename = "removes")]
+    pub removes: Vec<FileContext>,
+}
+
+/// A version of a file (no conflict).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FileContext {
+    /// File content identity. `None` means the file was deleted.
+    #[serde(rename = "id")]
+    pub id: Option<HgId>,
+
+    /// Flags of the file. "x": executable. "l": symlink.
+    ///
+    /// Note: This is not useful if `id` is `None`. But it seems simpler if we
+    /// introduces one less struct/enum.
+    #[serde(rename = "flags", default)]
+    pub flags: String,
+
+    // Not using RepoPathBuf to avoid one nested level of serde serialization.
+    /// Copy-from information.
+    #[serde(rename = "copy_from", default)]
+    pub copy_from: Option<String>,
+
+    /// Commit identity. Useful to find merge base. Or show up in conflict marker.
+    ///
+    /// `None` means the current commit.
+    #[serde(rename = "commit")]
+    pub commit_id: Option<HgId>,
+}
+
+impl PartialEq for FileContext {
+    fn eq(&self, other: &Self) -> bool {
+        self.id == other.id && self.flags == other.flags && self.copy_from == other.copy_from
+    }
+}
+
+impl Hash for FileContext {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        (&self.id, &self.flags, &self.copy_from).hash(state)
+    }
+}
+
+impl FileConflict {
+    /// Test if the conflict is resolved.
+    pub fn is_resolved(&self) -> bool {
+        self.adds.len() == 1 && self.removes.is_empty()
+    }
+
+    /// Create a `FileConflict` that represents a resolved content.
+    pub fn from_file(file: FileContext) -> Self {
+        Self {
+            adds: vec![file],
+            removes: Vec::new(),
+        }
+    }
+
+    /// Create a `FileConflict` that is a conflict of a 3-way merge.
+    pub fn from_3way(base: FileContext, local: FileContext, other: FileContext) -> Self {
+        Self {
+            adds: vec![local, other],
+            removes: vec![base],
+        }
+        .simplify()
+    }
+
+    /// Create a "resolution" that resolves the current conflict.
+    ///
+    /// Can be chained to record a resolution, for example,
+    /// `FileConflict::from_3way(base, local, other).with_resolution(res)`
+    /// (then store it in a re-re-re storage).
+    pub fn with_resolution(self, resolution: FileContext) -> Self {
+        let mut adds = self.removes;
+        let removes = self.adds;
+        adds.push(resolution);
+        Self { adds, removes }.simplify()
+    }
+
+    /// Number of 3-way merge steps needed to resolve the conflict.
+    pub fn complexity(&self) -> usize {
+        self.adds.len() - 1
+    }
+
+    /// Simplify internal. Cancel out same content from adds and removes.
+    fn simplify(mut self) -> Self {
+        for (i, add) in self.adds.clone().into_iter().enumerate().rev() {
+            if let Some(j) = self.removes.iter().position(|r| r == &add) {
+                // The add and remove cancel out.
+                self.removes.remove(j);
+                self.adds.remove(i);
+            }
+        }
+        self
+    }
+}
+
+impl Add<FileConflict> for FileConflict {
+    type Output = FileConflict;
+
+    fn add(mut self, rhs: FileConflict) -> Self::Output {
+        self.adds.extend(rhs.adds);
+        self.removes.extend(rhs.removes);
+        self.simplify()
+    }
+}
+
+impl Sub<FileConflict> for FileConflict {
+    type Output = FileConflict;
+
+    fn sub(mut self, rhs: FileConflict) -> Self::Output {
+        self.adds.extend(rhs.removes);
+        self.removes.extend(rhs.adds);
+        self.simplify()
+    }
+}