change address doesn't keep generating gaps

app/btc-wallet-hook.hoon

@@ -335,6 +335,7 @@
     ::   if no peta (payer/value), just prints address
     ::
       %generate-address
+    :: TODO: find a way to not print the below for a change address
     ?~  peta.upd  ~&(> "wallet-hook: %generate-address: {<address.upd>}" `state)
     =/  [payer=ship value=sats]  u.peta.upd
     :_  state(ps.piym (~(put by ps.piym) payer [xpub.upd address.upd payer value]))

app/btc-wallet-store.hoon

@@ -136,8 +136,8 @@
     ::    - generate new change address
     ::    - add that address+change value to the txbu
     ::    - send txbu update
-    ::    - send address update
     ::    - send a request for info on the address (watch it)
+    ::    - DON'T send an address update for the address, since it's change
     ::
     ::  TODO: end to end tests
       %generate-txbu
@@ -155,11 +155,10 @@
     ?~  chng
       [~[(send-update [%generate-txbu xpub.act u.tb])] state]
     =/  [addr=address:btc =idx w=walt]
-      ~(gen-address wad u.uw %1)
+      ~(nixt-address wad u.uw %1)
     =+  new-txbu=(~(add-output txb u.tb) addr u.chng `[fprint.w bipt.w %1 idx])
     :_  state(walts (~(put by walts) xpub.act w))
     :~  (send-update [%generate-txbu xpub.act new-txbu])
-        (send-update [%generate-address xpub.act addr ~])
         %+  send-request  ~[requests-path]
           :*  %address-info  last-block
               addr  xpub.act  %1  idx

lib/btc-wallet-store.hoon

@@ -80,14 +80,31 @@
     ?:  ?=(%bip84 bipt.w)
       (need (encode-pubkey:bech32:btc %main dat:(pubkey idx)))
     ~|("legacy addresses not supported yet " !!)
-  ::  generates and watches the next available address
+  ::  +nixt-address: used to get change addresses
+  ::   - gets the current next available address
+  ::   - doesn't bump nixt-address if it's unused
+  ::   - if used, fall back to gen-address and make a new one
+  ::
+  ++  nixt-address
+    ^-  (trel address:btc idx:btc walt)
+    =/  addr  (mk-address nixt-idx)
+    ~|  "lib/btc-wallet-store: get-next-address: nixt shouldn't be blank"
+    =/  =addi  (~(got by wach.w) addr)
+    ?.  used.addi
+      [addr nixt-idx w]
+    gen-address
+  ::
+  ::  +gen-address:
+  ::   - generates the next available address
+  ::   - watches it (using update address)
   ::
   ++  gen-address
     ^-  (trel address:btc idx:btc walt)
     =/  addr  (mk-address nixt-idx)
     :*  addr
         nixt-idx
-        (update-address addr [%.n chyg nixt-idx *(set utxo:btc)])
+        %+  update-address  addr
+          [%.n chyg nixt-idx *(set utxo:btc)]
     ==
   ::  +update-address
   ::   - insert a new address
@@ -112,7 +129,7 @@
     ?:(?=(%0 chyg) p.nixt.w q.nixt.w)
   ::  +bump-nixt: return wallet with bumped nixt
   ::   - find next unused address
-  ::   - add that address to wach
+  ::   - watches that address
   ::   - crashes if max-index is passed
   ::
   ++  bump-nixt