refactor: improve docs, cleanup unnecessary types and improve code

crates/chain/src/indexed_tx_graph.rs

@@ -6,7 +6,7 @@ use alloc::vec::Vec;
 use bitcoin::{OutPoint, Transaction, TxOut};
 
 use crate::{
-    keychain,
+    keychain, local_chain,
     tx_graph::{self, TxGraph},
     Anchor, Append,
 };
@@ -225,7 +225,16 @@ impl<A, K> From<keychain::ChangeSet<K>> for ChangeSet<A, keychain::ChangeSet<K>>
     }
 }
 
-/// Represents a structure that can index transaction data.
+impl<A, IA> From<ChangeSet<A, IA>> for (local_chain::ChangeSet, ChangeSet<A, IA>) {
+    fn from(indexed_changeset: ChangeSet<A, IA>) -> Self {
+        (local_chain::ChangeSet::default(), indexed_changeset)
+    }
+}
+
+/// Utilities for indexing transaction data.
+///
+/// Types which implement this trait can be used to construct an [`IndexedTxGraph`].
+/// This trait's methods should rarely be called directly.
 pub trait Indexer {
     /// The resultant "changeset" when new transaction data is indexed.
     type ChangeSet;
@@ -234,17 +243,6 @@ pub trait Indexer {
     fn index_txout(&mut self, outpoint: OutPoint, txout: &TxOut) -> Self::ChangeSet;
 
     /// Scans a transaction for relevant outpoints, which are stored and indexed internally.
-    ///
-    /// If the matched script pubkey is part of the lookahead, the last stored index is updated for
-    /// the script pubkey's keychain and the [`ChangeSet`] returned will reflect the
-    /// change.
-    ///
-    /// Typically, this method is used in two situations:
-    ///
-    /// 1. After loading transaction data from the disk, you may scan over all the txouts to restore all
-    /// your txouts.
-    /// 2. When getting new data from the chain, you usually scan it before incorporating it into
-    /// your chain state.
     fn index_tx(&mut self, tx: &Transaction) -> Self::ChangeSet;
 
     /// Apply changeset to itself.

crates/chain/src/keychain/txout_index.rs

@@ -91,11 +91,10 @@ impl<K: Clone + Ord + Debug> Indexer for KeychainTxOutIndex<K> {
     type ChangeSet = super::ChangeSet<K>;
 
     fn index_txout(&mut self, outpoint: OutPoint, txout: &TxOut) -> Self::ChangeSet {
-        let mut changeset = super::ChangeSet::<K>::default();
-        for (keychain, index) in self.inner.index_txout(outpoint, txout) {
-            changeset.append(self.reveal_to_target(&keychain, index).1);
+        match self.inner.scan_txout(outpoint, txout).cloned() {
+            Some((keychain, index)) => self.reveal_to_target(&keychain, index).1,
+            None => super::ChangeSet::default(),
         }
-        changeset
     }
 
     fn index_tx(&mut self, tx: &bitcoin::Transaction) -> Self::ChangeSet {
@@ -175,8 +174,10 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
 
     /// Set the lookahead count for `keychain`.
     ///
-    /// The lookahead is the number of scripts to cache ahead of the last stored script index. This
-    /// is useful during a scan via [`Indexer::index_tx`] or [`Indexer::index_txout`].
+    /// The lookahead is the number of scripts to cache ahead of the last revealed script index. This
+    /// is useful to find outputs you own when processing block data that lie beyond the last revealed
+    /// index. In certain situations, such as when performing an initial scan of the blockchain during
+    /// wallet import, it may be uncertain or unknown what the last revealed index is.
     ///
     /// # Panics
     ///

crates/chain/src/lib.rs

@@ -100,11 +100,3 @@ pub mod collections {
 
 /// How many confirmations are needed f or a coinbase output to be spent.
 pub const COINBASE_MATURITY: u32 = 100;
-
-impl<A, IA> From<indexed_tx_graph::ChangeSet<A, IA>>
-    for (local_chain::ChangeSet, indexed_tx_graph::ChangeSet<A, IA>)
-{
-    fn from(indexed_changeset: indexed_tx_graph::ChangeSet<A, IA>) -> Self {
-        (local_chain::ChangeSet::default(), indexed_changeset)
-    }
-}

crates/chain/src/spk_txout_index.rs

@@ -53,35 +53,19 @@ impl<I> Default for SpkTxOutIndex<I> {
 }
 
 impl<I: Clone + Ord> Indexer for SpkTxOutIndex<I> {
-    type ChangeSet = BTreeSet<I>;
+    type ChangeSet = ();
 
     fn index_txout(&mut self, outpoint: OutPoint, txout: &TxOut) -> Self::ChangeSet {
-        let spk_i = self.spk_indices.get(&txout.script_pubkey);
-        let mut scanned_indices = BTreeSet::new();
-        if let Some(spk_i) = spk_i {
-            self.txouts.insert(outpoint, (spk_i.clone(), txout.clone()));
-            self.spk_txouts.insert((spk_i.clone(), outpoint));
-            self.unused.remove(spk_i);
-            scanned_indices.insert(spk_i.clone());
-        }
-        scanned_indices
+        self.scan_txout(outpoint, txout);
+        Default::default()
     }
 
     fn index_tx(&mut self, tx: &Transaction) -> Self::ChangeSet {
-        let mut scanned_indices = BTreeSet::new();
-
-        for (i, txout) in tx.output.iter().enumerate() {
-            let op = OutPoint::new(tx.txid(), i as u32);
-            let mut txout_indices = self.index_txout(op, txout);
-            scanned_indices.append(&mut txout_indices);
-        }
-
-        scanned_indices
+        self.scan(tx);
+        Default::default()
     }
 
-    fn initial_changeset(&self) -> Self::ChangeSet {
-        self.spks.keys().cloned().collect()
-    }
+    fn initial_changeset(&self) -> Self::ChangeSet {}
 
     fn apply_changeset(&mut self, _changeset: Self::ChangeSet) {
         // This applies nothing.
@@ -93,6 +77,38 @@ impl<I: Clone + Ord> Indexer for SpkTxOutIndex<I> {
 }
 
 impl<I: Clone + Ord> SpkTxOutIndex<I> {
+    /// Scans a transaction's outputs for matching script pubkeys.
+    ///
+    /// Typically, this is used in two situations:
+    ///
+    /// 1. After loading transaction data from the disk, you may scan over all the txouts to restore all
+    /// your txouts.
+    /// 2. When getting new data from the chain, you usually scan it before incorporating it into your chain state.
+    pub fn scan(&mut self, tx: &Transaction) -> BTreeSet<I> {
+        let mut scanned_indices = BTreeSet::new();
+        let txid = tx.txid();
+        for (i, txout) in tx.output.iter().enumerate() {
+            let op = OutPoint::new(txid, i as u32);
+            if let Some(spk_i) = self.scan_txout(op, txout) {
+                scanned_indices.insert(spk_i.clone());
+            }
+        }
+
+        scanned_indices
+    }
+
+    /// Scan a single `TxOut` for a matching script pubkey and returns the index that matches the
+    /// script pubkey (if any).
+    pub fn scan_txout(&mut self, op: OutPoint, txout: &TxOut) -> Option<&I> {
+        let spk_i = self.spk_indices.get(&txout.script_pubkey);
+        if let Some(spk_i) = spk_i {
+            self.txouts.insert(op, (spk_i.clone(), txout.clone()));
+            self.spk_txouts.insert((spk_i.clone(), op));
+            self.unused.remove(spk_i);
+        }
+        spk_i
+    }
+
     /// Get a reference to the set of indexed outpoints.
     pub fn outpoints(&self) -> &BTreeSet<(I, OutPoint)> {
         &self.spk_txouts