cob: Compute implicit monotonic clock on changes

Signed-off-by: Alexis Sellier <alexis@radicle.xyz>

radicle-cob/src/change_graph.rs

@@ -113,11 +113,12 @@ impl ChangeGraph {
             let child_commits = outgoing_edges
                 .map(|e| *self.graph[e.target()].id())
                 .collect::<Vec<_>>();
-            (node, child_commits)
+
+            (node, idx, child_commits)
         });
         let history = {
             let root_change = &self.graph[*root];
-            evaluate(*root_change.id(), items)
+            evaluate(*root_change.id(), &self.graph, items)
         };
         CollaborativeObject {
             typename,

radicle-cob/src/change_graph/evaluation.rs

@@ -6,7 +6,9 @@
 use std::{collections::HashMap, ops::ControlFlow};
 
 use git_ext::Oid;
+use petgraph::{visit::EdgeRef, EdgeDirection};
 
+use crate::history::entry::{EntryId, EntryWithClock};
 use crate::{change::Change, history, pruning_fold};
 
 /// # Panics
@@ -14,11 +16,13 @@ use crate::{change::Change, history, pruning_fold};
 /// If the change corresponding to the root OID is not in `items`
 pub fn evaluate<'b>(
     root: Oid,
-    items: impl Iterator<Item = (&'b Change, Vec<Oid>)>,
+    graph: &petgraph::Graph<Change, ()>,
+    items: impl Iterator<Item = (&'b Change, petgraph::graph::NodeIndex<u32>, Vec<Oid>)>,
 ) -> history::History {
     let entries = pruning_fold::pruning_fold(
-        HashMap::new(),
-        items.map(|(change, children)| ChangeWithChildren {
+        HashMap::<EntryId, EntryWithClock>::new(),
+        items.map(|(change, idx, children)| ChangeWithChildren {
+            idx,
             change,
             child_commits: children,
         }),
@@ -31,8 +35,18 @@ pub fn evaluate<'b>(
                 ControlFlow::Break(entries)
             }
             Ok(entry) => {
+                // Get parent commits and calculate this node's clock based on theirs.
+                let incoming = graph.edges_directed(c.idx, EdgeDirection::Incoming);
+                let clock = incoming
+                    .into_iter()
+                    .map(|e| entries[&graph[e.source()].id.into()].clock())
+                    .max()
+                    .map(|n| n + 1)
+                    .unwrap_or_default();
                 log::trace!("change '{}' accepted", c.change.id());
-                entries.insert((*c.change.id()).into(), entry);
+
+                entries.insert(*entry.id(), EntryWithClock { entry, clock });
+
                 ControlFlow::Continue(entries)
             }
         },
@@ -60,6 +74,7 @@ fn evaluate_change(
 }
 
 struct ChangeWithChildren<'a> {
+    idx: petgraph::graph::NodeIndex<u32>,
     change: &'a Change,
     child_commits: Vec<Oid>,
 }

radicle-cob/src/history.rs

@@ -14,7 +14,7 @@ use petgraph::visit::Walker as _;
 use crate::pruning_fold;
 
 pub mod entry;
-pub use entry::{Contents, Entry, EntryId};
+pub use entry::{Clock, Contents, Entry, EntryId, EntryWithClock};
 
 #[derive(
     Clone, Copy, Debug, Default, PartialEq, Eq, Hash, serde::Serialize, serde::Deserialize,
@@ -29,7 +29,7 @@ pub enum HistoryType {
 /// The DAG of changes making up the history of a collaborative object.
 #[derive(Clone, Debug)]
 pub struct History {
-    graph: petgraph::Graph<Entry, (), petgraph::Directed, u32>,
+    graph: petgraph::Graph<EntryWithClock, (), petgraph::Directed, u32>,
     indices: HashMap<EntryId, petgraph::graph::NodeIndex<u32>>,
 }
 
@@ -66,12 +66,12 @@ impl History {
             contents,
         };
         let mut entries = HashMap::new();
-        entries.insert(id, root_entry.clone());
-        let NewGraph { graph, indices } = create_petgraph(&root_entry.id, &entries);
+        entries.insert(id, EntryWithClock::from(root_entry));
+        let NewGraph { graph, indices } = create_petgraph(&id, &entries);
         Self { graph, indices }
     }
 
-    pub fn new<Id>(root: Id, entries: HashMap<EntryId, Entry>) -> Result<Self, CreateError>
+    pub fn new<Id>(root: Id, entries: HashMap<EntryId, EntryWithClock>) -> Result<Self, CreateError>
     where
         Id: Into<EntryId>,
     {
@@ -84,6 +84,16 @@ impl History {
         }
     }
 
+    /// Get the current value of the logical clock.
+    /// This is the maximum value of all tips.
+    pub fn clock(&self) -> Clock {
+        self.graph
+            .externals(petgraph::Direction::Outgoing)
+            .map(|n| self.graph[n].clock)
+            .max()
+            .unwrap_or_default()
+    }
+
     /// A topological (parents before children) traversal of the dependency
     /// graph of this history. This is analagous to
     /// [`std::iter::Iterator::fold`] in that it folds every change into an
@@ -92,13 +102,13 @@ impl History {
     /// `ControlFlow::Break`.
     pub fn traverse<F, A>(&self, init: A, f: F) -> A
     where
-        F: for<'r> FnMut(A, &'r Entry) -> ControlFlow<A, A>,
+        F: for<'r> FnMut(A, &'r EntryWithClock) -> ControlFlow<A, A>,
     {
         let topo = petgraph::visit::Topo::new(&self.graph);
         #[allow(clippy::let_and_return)]
         let items = topo.iter(&self.graph).map(|idx| {
-            let node = &self.graph[idx];
-            node
+            let entry = &self.graph[idx];
+            entry
         });
         pruning_fold::pruning_fold(init, items, f)
     }
@@ -131,7 +141,10 @@ impl History {
             std::iter::empty::<git2::Oid>(),
             new_contents,
         );
-        let new_ix = self.graph.add_node(new_entry);
+        let new_ix = self.graph.add_node(EntryWithClock {
+            entry: new_entry,
+            clock: self.clock() + 1,
+        });
         for tip in tips {
             let tip_ix = self.indices.get(&tip.into()).unwrap();
             self.graph.update_edge(*tip_ix, new_ix, ());
@@ -140,11 +153,14 @@ impl History {
 }
 
 struct NewGraph {
-    graph: petgraph::Graph<Entry, (), petgraph::Directed, u32>,
+    graph: petgraph::Graph<EntryWithClock, (), petgraph::Directed, u32>,
     indices: HashMap<EntryId, petgraph::graph::NodeIndex<u32>>,
 }
 
-fn create_petgraph<'a>(root: &'a EntryId, entries: &'a HashMap<EntryId, Entry>) -> NewGraph {
+fn create_petgraph<'a>(
+    root: &'a EntryId,
+    entries: &'a HashMap<EntryId, EntryWithClock>,
+) -> NewGraph {
     let mut graph = petgraph::Graph::new();
     let mut indices = HashMap::<EntryId, petgraph::graph::NodeIndex<u32>>::new();
     let root = entries.get(root).unwrap().clone();
@@ -153,8 +169,8 @@ fn create_petgraph<'a>(root: &'a EntryId, entries: &'a HashMap<EntryId, Entry>)
     let mut to_process = vec![root];
     while let Some(entry) = to_process.pop() {
         let entry_ix = indices[&entry.id];
-        for child_id in entry.children {
-            let child = entries[&child_id].clone();
+        for child_id in entry.children() {
+            let child = entries[child_id].clone();
             let child_ix = graph.add_node(child.clone());
             indices.insert(child.id, child_ix);
             graph.update_edge(entry_ix, child_ix, ());

radicle-cob/src/history/entry.rs

@@ -11,6 +11,9 @@ use crate::pruning_fold;
 /// This is the change payload.
 pub type Contents = Vec<u8>;
 
+/// Logical clock used to track causality in change graph.
+pub type Clock = u64;
+
 /// A unique identifier for a history entry.
 #[derive(Clone, Copy, Debug, PartialEq, Hash, Eq, PartialOrd, Ord)]
 pub struct EntryId(Oid);
@@ -107,3 +110,46 @@ impl pruning_fold::GraphNode for Entry {
         &self.children
     }
 }
+
+/// Wraps an [`Entry`], adding a logical clock to it.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct EntryWithClock {
+    pub entry: Entry,
+    pub clock: Clock,
+}
+
+impl From<Entry> for EntryWithClock {
+    fn from(entry: Entry) -> Self {
+        Self {
+            entry,
+            clock: Clock::default(),
+        }
+    }
+}
+
+impl EntryWithClock {
+    /// Get the clock value.
+    pub fn clock(&self) -> Clock {
+        self.clock
+    }
+}
+
+impl pruning_fold::GraphNode for EntryWithClock {
+    type Id = EntryId;
+
+    fn id(&self) -> &Self::Id {
+        &self.entry.id
+    }
+
+    fn child_ids(&self) -> &[Self::Id] {
+        &self.entry.children
+    }
+}
+
+impl std::ops::Deref for EntryWithClock {
+    type Target = Entry;
+
+    fn deref(&self) -> &Self::Target {
+        &self.entry
+    }
+}