1 files changed, 88 insertions, 0 deletions
diff --git a/src/swarm/changes.rs b/src/swarm/changes.rs
new file mode 100644
index 0000000..10a3bfe
--- /dev/null
+++ b/src/swarm/changes.rs
@@ -0,0 +1,88 @@
+//! The change publisher: after document changes, which store paths they touched
+//! (`AppState::change_events`, for the API's streams, the dashboard and remakes), and
+//! a nudge to topology when the member list changed.
+
+use std::sync::Arc;
+
+use tracing::{info, warn};
+
+use crate::document::store::ChangeSet;
+use crate::state::AppState;
+
+fn patch_store_path(patch: &automerge::Patch) -> Vec<String> {
+    let mut path: Vec<String> = patch
+        .path
+        .iter()
+        .map(|(_, prop)| match prop {
+            automerge::Prop::Map(key) => key.clone(),
+            automerge::Prop::Seq(index) => index.to_string(),
+        })
+        .collect();
+
+    match &patch.action {
+        automerge::PatchAction::PutMap { key, .. }
+        | automerge::PatchAction::DeleteMap { key, .. } => path.push(key.clone()),
+        // List, text and counter changes: the containing object changed.
+        _ => {}
+    }
+    path
+}
+
+pub(crate) async fn run_change_publisher(state: AppState) {
+    let mut changed = state.change_tx.subscribe();
+    let mut last_heads = state.write_doc(|doc| doc.get_heads()).await;
+
+    info!("Starting change publisher");
+
+    while changed.changed().await.is_ok() {
+        let before = last_heads.clone();
+
+        let (heads, paths, conflicts) = state
+            .write_doc(move |doc| {
+                let heads = doc.get_heads();
+                if heads == before {
+                    return (heads, Vec::new(), Vec::new());
+                }
+
+                // After an epoch switch the previous heads belong to the old document:
+                // report the root as changed, which every stream is related to.
+                if !before.iter().all(|h| doc.get_change_by_hash(h).is_some()) {
+                    return (heads, vec![Vec::new()], Vec::new());
+                }
+
+                let mut paths = Vec::new();
+                let mut conflicts = Vec::new();
+                for patch in doc.diff(&before, &heads) {
+                    let path = patch_store_path(&patch);
+                    if matches!(
+                        patch.action,
+                        automerge::PatchAction::PutMap { conflict: true, .. }
+                    ) {
+                        conflicts.push(path.clone());
+                    }
+                    paths.push(path);
+                }
+                (heads, paths, conflicts)
+            })
+            .await;
+
+        if heads == last_heads {
+            continue;
+        }
+        last_heads = heads;
+
+        for path in &conflicts {
+            warn!(path = ?path, "Concurrent writes produced a conflict");
+        }
+
+        if paths
+            .iter()
+            .any(|path| crate::swarm::topology::affects_neighbours(path))
+        {
+            state.topology_changed.notify_one();
+        }
+
+        // Fails only when nobody is subscribed.
+        let _ = state.change_events.send(Arc::new(ChangeSet { paths }));
+    }
+}