Prepare semantic commands before atomic source adoption

crates/oa-core/src/session.rs

@@ -85,6 +85,31 @@ pub struct Session {
     saved: Project,
     revision: u64,
 }
+
+/// One validated command result, ready for an owning editor or service to adopt.
+/// Preparation never mutates the input and allocates creation identities once.
+/// A source editor can validate its corresponding document edits before either
+/// representation becomes visible.
+pub struct PreparedCommand {
+    pub project: Project,
+    pub selected: Option<Id>,
+}
+
+pub fn prepare_command(
+    project: &Project,
+    command: &Command,
+) -> Result<PreparedCommand, ModelError> {
+    let mut candidate = project.clone();
+    let mut pending_deletions = BTreeSet::new();
+    let selected = apply_command(&mut candidate, command, 0, &mut pending_deletions)?;
+    crate::references::finish_deletions(&mut candidate, &pending_deletions);
+    validate_candidate(&candidate)?;
+    Ok(PreparedCommand {
+        project: candidate,
+        selected,
+    })
+}
+
 impl Session {
     pub fn try_new(project: Project) -> Result<Self, ModelError> {
         validate_candidate(&project)?;
@@ -143,11 +168,10 @@ impl Session {
         Ok(())
     }
     pub fn execute(&mut self, command: Command) -> Result<Option<Id>, ModelError> {
-        let mut candidate = self.project.clone();
-        let mut pending_deletions = BTreeSet::new();
-        let selected = apply_command(&mut candidate, &command, 0, &mut pending_deletions)?;
-        crate::references::finish_deletions(&mut candidate, &pending_deletions);
-        validate_candidate(&candidate)?;
+        let PreparedCommand {
+            project: candidate,
+            selected,
+        } = prepare_command(&self.project, &command)?;
         if candidate != self.project {
             self.past
                 .push(std::mem::replace(&mut self.project, candidate));