prioritise unused change addresses for split

Author: nickfarrow

src/cmd/wallet.rs

@@ -595,9 +595,49 @@ pub fn run_split_cmd(wallet: &GunWallet, opt: SplitOpt) -> anyhow::Result<CmdOut
 
     builder.unspendable(already_correct.map(|utxo| utxo.outpoint).collect());
 
+    let mut map: HashMap<Script, Vec<Transaction>> = HashMap::new();
+    for txn in bdk_wallet
+        .database()
+        .iter_txs(true)?
+        .iter()
+        .flat_map(|tx_details| tx_details.transaction.as_ref())
+    {
+        for out in &txn.output {
+            map.entry(out.script_pubkey.clone())
+                .and_modify(|v| v.push(txn.clone()))
+                .or_insert(vec![txn.clone()]);
+        }
+    }
+
     match n {
         Some(n) => {
-            for _ in 0..n {
+            let last_unused_index = bdk_wallet
+                .get_change_address(AddressIndex::LastUnused)?
+                .index;
+            let mut added_recipients = 0;
+            // Check for unused change addresses up to most recent
+            for check_used_index in 0..(last_unused_index + 1) {
+                let check_script_pk = bdk_wallet
+                    .get_change_address(AddressIndex::Peek(check_used_index as u32))?
+                    .address
+                    .script_pubkey();
+
+                let used = match map.get(&check_script_pk) {
+                    Some(_) => true,
+                    None => false,
+                };
+
+                // Keep adding unused recipients until we have n
+                if !used {
+                    builder.add_recipient(check_script_pk, output_size.as_sat());
+                    added_recipients += 1;
+                    if added_recipients == n {
+                        break;
+                    };
+                }
+            }
+            // If we don't have enough unused addresses, fill remaining n-added with new
+            for _ in 0..(n - added_recipients) {
                 builder.add_recipient(
                     bdk_wallet
                         .get_change_address(AddressIndex::New)?
@@ -613,7 +653,7 @@ pub fn run_split_cmd(wallet: &GunWallet, opt: SplitOpt) -> anyhow::Result<CmdOut
                 // add one recipient so we at least get one split utxo of the correct size.
                 .add_recipient(
                     bdk_wallet
-                        .get_change_address(AddressIndex::New)?
+                        .get_change_address(AddressIndex::LastUnused)?
                         .address
                         .script_pubkey(),
                     output_size.as_sat(),