From a146a0babc25547f269c784e090e308fa831ab32 Mon Sep 17 00:00:00 2001
From: Quentin Dufour <quentin@deuxfleurs.fr>
Date: Wed, 27 Mar 2024 15:09:18 +0100
Subject: [PATCH] Sync algorithm

---
 aero-collections/src/davdag.rs | 82 +++++++++++++++++++++++++++++++---
 1 file changed, 77 insertions(+), 5 deletions(-)

diff --git a/aero-collections/src/davdag.rs b/aero-collections/src/davdag.rs
index 696b985..59dcc7b 100644
--- a/aero-collections/src/davdag.rs
+++ b/aero-collections/src/davdag.rs
@@ -1,3 +1,4 @@
+use anyhow::{bail, Result};
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use im::{OrdMap, OrdSet, ordset};
 
@@ -23,9 +24,14 @@ pub struct DavDag {
     /// Partial synchronization graph
     /// parent -> direct children
     pub successors: OrdMap<UniqueIdent, OrdSet<UniqueIdent>>,
-    
+    pub ancestors: OrdMap<UniqueIdent, OrdSet<UniqueIdent>>,
+
+    /// All nodes
+    pub all_nodes: OrdSet<UniqueIdent>,
     /// Head nodes
     pub heads: OrdSet<UniqueIdent>,
+    /// Origin nodes
+    pub origins: OrdSet<UniqueIdent>,
 }
 
 #[derive(Clone, Serialize, Deserialize, Debug)]
@@ -54,11 +60,57 @@ impl DavDag {
     }
 
     // HELPER functions
+
+    /// All HEAD events
     pub fn heads_vec(&self) -> Vec<UniqueIdent> {
         self.heads.clone().into_iter().collect()
     }
 
+    /// Resolve a sync token
+    pub fn resolve(&self, known: UniqueIdent) -> Result<OrdSet<UniqueIdent>> {
+        let already_known = self.all_ancestors(known);
+
+        // We can't capture all missing events if we are not connected
+        // to all sinks of the graph, ie. if we don't already know all the sinks.
+        if !self.origins.is_subset(already_known.clone()) {
+            bail!("Not enough history to produce a correct diff, a full resync is needed");
+        }
+
+        // Missing items are all existing graph items from which
+        // we removed all items known by the given node.
+        // In other words, all values in all_nodes that are not in already_known.
+        Ok(self.all_nodes.clone().relative_complement(already_known))
+    }
+
+    /// Find all ancestors of a given 
+    fn all_ancestors(&self, known: UniqueIdent) -> OrdSet<UniqueIdent> {
+        let mut all_known: OrdSet<UniqueIdent> = OrdSet::new();
+        let mut to_collect = vec![known];
+        loop {
+            let cursor = match to_collect.pop() {
+                // Loop stops here
+                None => break,
+                Some(v) => v,
+            };
+
+            if all_known.insert(cursor).is_some() {
+                // Item already processed
+                continue
+            }
+
+            // Collect parents
+            let parents = match self.ancestors.get(&cursor) {
+                None => continue,
+                Some(c) => c,
+            };
+            to_collect.extend(parents.iter());
+        }
+        all_known
+    }
+
     // INTERNAL functions
+
+    /// Register a WebDAV item (put, copy, move)
     fn register(&mut self, ident: UniqueIdent, entry: IndexEntry) {
         // Insert item in the source of trust
         self.table.insert(ident, entry.clone());
@@ -68,6 +120,7 @@ impl DavDag {
         self.idx_by_filename.insert(filename, ident);
     }
 
+    /// Unregister a WebDAV item (delete, move)
     fn unregister(&mut self, ident: &UniqueIdent) {
         // Query the source of truth to get the information we
         // need to clean the indexes
@@ -84,8 +137,11 @@ impl DavDag {
     // @FIXME: maybe in case of error we could simply disable the sync graph
     // and ask the client to rely on manual sync. For now, we are skipping the event
     // which is midly satisfying.
+
+    /// When an event is processed, update the synchronization DAG
     fn sync_dag(&mut self, child: &UniqueIdent, parents: &[UniqueIdent]) -> bool {
-        // All parents must exist in successors otherwise we can't accept item
+        // --- Update SUCCESSORS
+        // All parents must exist in successors otherwise we can't accept item:
         // do the check + update successors
         let mut try_successors = self.successors.clone();
         for par in parents.iter() {
@@ -99,15 +155,29 @@ impl DavDag {
         }
         self.successors = try_successors;
 
+        // This event is also a future successor
+        self.successors.insert(*child, ordset![]);
+
+        // --- Update ANCESTORS
+        // We register ancestors as it is required for the sync algorithm
+        self.ancestors.insert(*child, parents.iter().fold(ordset![], |mut acc, p| {
+            acc.insert(*p);
+            acc
+        }));
+
+        // --- Update ORIGINS
+        // If this event has no parents, it's an origin
+        if parents.is_empty() {
+            self.origins.insert(*child);
+        }
+
+        // --- Update HEADS
         // Remove from HEADS this event's parents
         parents.iter().for_each(|par| { self.heads.remove(par); });
 
         // This event becomes a new HEAD in turn
         self.heads.insert(*child);
 
-        // This event is also a future successor
-        self.successors.insert(*child, ordset![]);
-
         true
     }
 }
@@ -160,6 +230,8 @@ impl<'de> Deserialize<'de> for DavDag {
         val.heads.into_iter().for_each(|ident| {
             davdag.successors.insert(ident, ordset![]);
             davdag.heads.insert(ident);
+            davdag.origins.insert(ident);
+            davdag.all_nodes.insert(ident);
         });
 
         Ok(davdag)