1 files changed, 60 insertions, 0 deletions
diff --git a/lib/management/diff.ml b/lib/management/diff.ml
new file mode 100644
index 0000000..865c757
--- /dev/null
+++ b/lib/management/diff.ml
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+(** Structural delta between two Database.t states.
+    Computes what changed from ancestor → target at the relation level. *)
+
+type relation_diff =
+  | RelationAdded   of Relation.t
+  | RelationRemoved of string
+  | RelationModified of {
+      name           : string;
+      added_tuples   : Conventions.Hash.t list;
+      removed_tuples : Conventions.Hash.t list;
+      schema_changed : bool;
+    }
+
+type t = {
+  ancestor       : Conventions.Hash.t;
+  left           : Conventions.Hash.t;
+  right          : Conventions.Hash.t;
+  relation_diffs : relation_diff list;
+}
+
+module StringSet = Set.Make(String)
+
+let relation_keys rel =
+  match rel.Relation.tree with
+  | None -> StringSet.empty
+  | Some tree -> StringSet.of_list (Merkle.keys tree)
+
+(** Compute relation-level diffs from ancestor → target. *)
+let diff ~ancestor ~(target : Database.t) =
+  let ancestor_names =
+    Database.get_relation_names ancestor |> StringSet.of_list
+  in
+  let target_names =
+    Database.get_relation_names target |> StringSet.of_list
+  in
+  let all_names = StringSet.union ancestor_names target_names in
+  StringSet.fold (fun name acc ->
+    let in_ancestor = StringSet.mem name ancestor_names in
+    let in_target   = StringSet.mem name target_names   in
+    match in_ancestor, in_target with
+    | false, true ->
+      let rel = Option.get (Database.get_relation target name) in
+      RelationAdded rel :: acc
+    | true, false ->
+      RelationRemoved name :: acc
+    | true, true ->
+      let anc_rel = Option.get (Database.get_relation ancestor name) in
+      let tgt_rel = Option.get (Database.get_relation target  name) in
+      (* Compare hashes — if identical, no diff *)
+      if anc_rel.Relation.hash = tgt_rel.Relation.hash then acc
+      else begin
+        let anc_keys = relation_keys anc_rel in
+        let tgt_keys = relation_keys tgt_rel in
+        let added_tuples   = StringSet.diff tgt_keys anc_keys |> StringSet.elements in
+        let removed_tuples = StringSet.diff anc_keys tgt_keys |> StringSet.elements in
+        let schema_changed = anc_rel.Relation.schema <> tgt_rel.Relation.schema in
+        RelationModified { name; added_tuples; removed_tuples; schema_changed } :: acc
+      end
+    | false, false -> acc (* impossible *)
+  ) all_names []