1 files changed, 67 insertions, 0 deletions
diff --git a/src/document/inspect.rs b/src/document/inspect.rs
index d395008..c42de82 100644
--- a/src/document/inspect.rs
+++ b/src/document/inspect.rs
@@ -86,6 +86,33 @@ impl Place {
     }
 }
 
+/// Where a write at `path` in `doc` is, as rules see it (`Inspector`): placed by the
+/// same rule (`Place::child`), so cut after the first list or text on the way, which
+/// what is inside one touches. Where `doc` holds nothing (or a scalar) on the way, the
+/// rest is as asked: the write makes maps there. For the API's checks, so they and the
+/// rules judge the same path.
+pub(crate) fn placed(doc: &Document, path: &[String]) -> Path {
+    let mut place = Place::root();
+    let mut object = automerge::ROOT;
+    for (index, key) in path.iter().enumerate() {
+        if place.in_list {
+            break;
+        }
+        place = place.child(&Key::Map(key.as_str().into()));
+        match doc.get(&object, key.as_str()) {
+            Ok(Some((automerge::Value::Object(kind), id))) => {
+                place.in_list = matches!(kind, automerge::ObjType::List | automerge::ObjType::Text);
+                object = id;
+            }
+            _ => {
+                place.path.extend(path[index + 1..].iter().cloned());
+                break;
+            }
+        }
+    }
+    place.path
+}
+
 /// Finds the paths changes touch in `doc`, with the places of the objects it looked up
 /// kept for the next changes.
 pub(crate) struct Inspector<'a> {
@@ -296,6 +323,46 @@ mod tests {
     }
 
     /// Inside a list or text, a path stops at it: items move.
+    /// A write's place in the document, as the rules see the change it makes: cut after
+    /// a list or text; as asked where nothing is yet.
+    #[test]
+    fn writes_placed_as_rules_see_them() {
+        let mut doc = Document::new();
+        change(&mut doc, |doc| {
+            store::write_path(
+                doc,
+                &path(&["servers", "list"]),
+                &serde_json::json!([{"name": "a"}]),
+            )
+            .unwrap();
+            store::write_path(doc, &path(&["config", "mode"]), &serde_json::json!("fast")).unwrap();
+        });
+        let placed = |at: &[&str]| super::placed(&doc, &path(at));
+        assert_eq!(
+            placed(&["servers", "list", "0", "name"]),
+            path(&["servers", "list"])
+        );
+        assert_eq!(placed(&["servers", "list"]), path(&["servers", "list"]));
+        assert_eq!(placed(&["config", "mode"]), path(&["config", "mode"]));
+        assert_eq!(
+            placed(&["config", "new", "deep"]),
+            path(&["config", "new", "deep"])
+        );
+        assert_eq!(placed(&["nothing", "here"]), path(&["nothing", "here"]));
+        // As the change of such a write touches it.
+        let before = doc.get_heads();
+        change(&mut doc, |doc| {
+            store::write_path(
+                doc,
+                &path(&["servers", "list", "0", "name"]),
+                &serde_json::json!("b"),
+            )
+            .unwrap();
+        });
+        let touched = Inspector::new(&doc.fork_at(&before).unwrap()).paths(&decoded(&doc, &before));
+        assert_eq!(touched, vec![[path(&["servers", "list"])].into()]);
+    }
+
     #[test]
     fn lists_and_text_stop_paths() {
         let mut doc = Document::new();