working HD path for PSBT

PSBT.md

@@ -25,6 +25,18 @@ bitcoin-cli -rpcuser=__cookie__ -rpcpassword=9e0ca21c70d6b7307b750c8012d12df04ef
 (parse:^psbt:btc psbt)
 ```
 
+## Make an HD Path
+```
+=btc -build-file %/lib/btc/hoon
+=print [4 0x9ca7.9d5b]
+=pubkey [33 0x3.6093.e9e0.6a5f.736e.751f.5486.c1c6.7647.c258.b946.b314.d2d3.f03a.33c2.b5cf.b9ab]
+(hd-path:^psbt:btc print pubkey %output %bip84 %1 11)
+
+:: GIVES
+[ key=[wid=34 dat=0x203.6093.e9e0.6a5f.736e.751f.5486.c1c6.7647.c258.b946.b314.d2d3.f03a.33c2.b5cf.b9ab]
+  val=[wid=21 dat=0x9c.a79d.5b54.0000.8000.0000.8000.0000.8001.0000.000b]]
+```
+
 ## Making Derivation Paths
 Below is hex for `m/84'/0'/0'/0/0`
 `*80 is the ending for hardened paths*

lib/btc.hoon

@@ -68,6 +68,37 @@
 ::
 ++  psbt
   |%
+  ++  hd-path
+  |=  [print=btc-byts pubkey=btc-byts =target:^psbt =bipt =chyg idx=@ud]
+    ^-  keyval:^psbt
+    =/  k=btc-byts
+      %-  to-byts:buffer
+      ?-  target
+          %input
+        [0x6 (from-byts:buffer pubkey)]
+          ::
+          %output
+        [0x2 (from-byts:buffer pubkey)]
+      ==
+    =/  bip  ?-  bipt
+               %bip84  0x54
+               %bip49  0x31
+               %bip44  0x2c
+             ==
+    =/  hdpath=^buffer
+      %+  weld
+        :~   bip  0x0  0x0  0x80
+             0x0  0x0  0x0  0x80
+             0x0  0x0  0x0  0x80
+             `@ux`chyg  0x0  0x0  0x0
+        ==
+      (from-atom-le:buffer (met 3 idx) idx)
+    :-  k
+    %-  concat-as-byts:buffer
+    :~  (from-byts:buffer print)
+        hdpath
+    ==
+  ::
   ++  parse
     |=  psbt-base64=cord
     ^-  (list map:^psbt)
@@ -136,6 +167,7 @@
     =/  bigend=@ux  (swp 3 q.u.p)
     (from-byts:buffer [(met 3 bigend) bigend])
   --
+::  buffer: byte buffer utilities
 ::  list of @ux that is big endian for hashing purposes
 ::  used to preserve 0s when concatenating byte sequences
 ::

sur/btc.hoon

@@ -5,6 +5,7 @@
 +$  bech32-address  $%([%bech32 cord])
 +$  address  ?(legacy-address bech32-address)
 +$  bipt  $?(%bip44 %bip49 %bip84)
++$  chyg  $?(%0 %1)
 +$  sats  @ud
 +$  vbytes  @ud
 +$  btc-byts  [wid=@ dat=@ux]
@@ -14,7 +15,7 @@
 +$  txid  hash256
 +$  rawtx  btc-byts
 +$  buffer  (list @ux)
-+$  utxo  [pos=@ =txid height=@ value=sats recvd=(unit @da)]
++$  utxo  [pos=@ =txid height=@ value=sats recvd=(unit @da)] 
 ++  address-info
   $:  =address
       confirmed-value=sats
@@ -55,6 +56,7 @@
   --
 ++  psbt
   |%
+  +$  target  $?(%input %output)
   +$  keyval  [key=btc-byts val=btc-byts]
   +$  map  (list keyval)
   --