feat(chain): Add initial_changeset to graphs

Add `initial_changeset` to TxGraph and IndexedTxGraph

crates/chain/src/indexed_tx_graph.rs

@@ -59,6 +59,13 @@ impl<A: Anchor, I: Indexer> IndexedTxGraph<A, I> {
 
         self.graph.apply_changeset(changeset.graph);
     }
+
+    /// Determines the [`ChangeSet`] between `self` and an empty [`IndexedTxGraph`].
+    pub fn initial_changeset(&self) -> ChangeSet<A, I::ChangeSet> {
+        let graph = self.graph.initial_changeset();
+        let indexer = self.index.initial_changeset();
+        ChangeSet { graph, indexer }
+    }
 }
 
 impl<A: Anchor, I: Indexer> IndexedTxGraph<A, I>
@@ -232,6 +239,9 @@ pub trait Indexer {
     /// Apply changeset to itself.
     fn apply_changeset(&mut self, changeset: Self::ChangeSet);
 
+    /// Determines the [`ChangeSet`] between `self` and an empty [`Indexer`].
+    fn initial_changeset(&self) -> Self::ChangeSet;
+
     /// Determines whether the transaction should be included in the index.
     fn is_tx_relevant(&self, tx: &Transaction) -> bool;
 }

crates/chain/src/keychain.rs

@@ -20,6 +20,7 @@ mod txout_index;
 pub use txout_index::*;
 
 /// Represents updates to the derivation index of a [`KeychainTxOutIndex`].
+/// It maps each keychain `K` to its last revealed index.
 ///
 /// It can be applied to [`KeychainTxOutIndex`] with [`apply_changeset`]. [`ChangeSet] are
 /// monotone in that they will never decrease the revealed derivation index.

crates/chain/src/keychain/txout_index.rs

@@ -98,6 +98,10 @@ impl<K: Clone + Ord + Debug> Indexer for KeychainTxOutIndex<K> {
         self.scan(tx)
     }
 
+    fn initial_changeset(&self) -> Self::ChangeSet {
+        super::ChangeSet(self.last_revealed.clone())
+    }
+
     fn apply_changeset(&mut self, changeset: Self::ChangeSet) {
         self.apply_changeset(changeset)
     }

crates/chain/src/spk_txout_index.rs

@@ -66,6 +66,8 @@ impl<I: Clone + Ord> Indexer for SpkTxOutIndex<I> {
         Default::default()
     }
 
+    fn initial_changeset(&self) -> Self::ChangeSet {}
+
     fn apply_changeset(&mut self, _changeset: Self::ChangeSet) {
         // This applies nothing.
     }

crates/chain/src/tx_graph.rs

@@ -438,6 +438,11 @@ impl<A: Clone + Ord> TxGraph<A> {
         changeset
     }
 
+    /// Determines the [`ChangeSet`] between `self` and an empty [`TxGraph`].
+    pub fn initial_changeset(&self) -> ChangeSet<A> {
+        Self::default().determine_changeset(self.clone())
+    }
+
     /// Applies [`ChangeSet`] to [`TxGraph`].
     pub fn apply_changeset(&mut self, changeset: ChangeSet<A>) {
         for tx in changeset.txs {