Add support for combined descriptor

As proposed by achow101 on the mailing list, see
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-July/020791.html

Also fix a few compiler warnings.

Author: t-bast

src/commonMain/kotlin/fr/acinq/bitcoin/Descriptor.kt

@@ -4,6 +4,9 @@ import fr.acinq.bitcoin.DeterministicWallet.derivePrivateKey
 import fr.acinq.bitcoin.DeterministicWallet.publicKey
 import kotlin.jvm.JvmStatic
 
+/**
+ * Output Script Descriptors: see https://github.com/bitcoin/bips/blob/master/bip-0380.mediawiki
+ */
 public object Descriptor {
     private fun polyMod(cc: Long, value: Int): Long {
         var c = cc
@@ -17,35 +20,21 @@ public object Descriptor {
         return c
     }
 
+    // Taken from: https://github.com/bitcoin/bitcoin/blob/207a22877330709e4462e6092c265ab55c8653ac/src/script/descriptor.cpp
     @JvmStatic
     public fun checksum(span: String): String {
-        /** A character set designed such that:
-         *  - The most common 'unprotected' descriptor characters (hex, keypaths) are in the first group of 32.
-         *  - Case errors cause an offset that's a multiple of 32.
-         *  - As many alphabetic characters are in the same group (while following the above restrictions).
-         *
-         * If p(x) gives the position of a character c in this character set, every group of 3 characters
-         * (a,b,c) is encoded as the 4 symbols (p(a) & 31, p(b) & 31, p(c) & 31, (p(a) / 32) + 3 * (p(b) / 32) + 9 * (p(c) / 32).
-         * This means that changes that only affect the lower 5 bits of the position, or only the higher 2 bits, will just
-         * affect a single symbol.
-         *
-         * As a result, within-group-of-32 errors count as 1 symbol, as do cross-group errors that don't affect
-         * the position within the groups.
-         */
         val INPUT_CHARSET = "0123456789()[],'/*abcdefgh@:$%{}" + "IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~" + "ijklmnopqrstuvwxyzABCDEFGH`#\"\\ "
-
-        /** The character set for the checksum itself (same as bech32). */
-        val CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+        val CHECKSUM_CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l"
 
         var c = 1L
         var cls = 0
         var clscount = 0
         span.forEach { ch ->
-            val pos = INPUT_CHARSET.indexOf(ch);
-            if (pos == -1) return "";
-            c = polyMod(c, pos and 31); // Emit a symbol for the position inside the group, for every character.
-            cls = cls * 3 + (pos shr 5); // Accumulate the group numbers
-            clscount = clscount + 1
+            val pos = INPUT_CHARSET.indexOf(ch)
+            if (pos == -1) return ""
+            c = polyMod(c, pos and 31) // Emit a symbol for the position inside the group, for every character.
+            cls = cls * 3 + (pos shr 5) // Accumulate the group numbers
+            clscount += 1
             if (clscount == 3) {
                 // Emit an extra symbol representing the group numbers, for every 3 characters.
                 c = polyMod(c, cls)
@@ -57,11 +46,10 @@ public object Descriptor {
         for (j in 0 until 8) c = polyMod(c, 0) // Shift further to determine the checksum.
         c = c xor 1 // Prevent appending zeroes from not affecting the checksum.
 
-        var ret = StringBuilder("        ")
+        val ret = StringBuilder("        ")
         for (j in 0 until 8) {
             val pos1 = (c shr (5 * (7 - j))) and 31
-            val char = CHECKSUM_CHARSET.get(pos1.toInt())
-            ret.set(j, char)
+            ret[j] = CHECKSUM_CHARSET[pos1.toInt()]
         }
         return ret.toString()
     }
@@ -73,21 +61,17 @@ public object Descriptor {
     }
 
     @JvmStatic
-    public fun BIP84Descriptors(chainHash: ByteVector32, master: DeterministicWallet.ExtendedPrivateKey): Pair<String, String> {
+    public fun createBIP84Descriptor(chainHash: ByteVector32, master: DeterministicWallet.ExtendedPrivateKey): String {
         val (keyPath, _) = getKeyPath(chainHash)
         val accountPub = publicKey(derivePrivateKey(master, KeyPath(keyPath)))
         val fingerprint = DeterministicWallet.fingerprint(master) and 0xFFFFFFFFL
-        return BIP84Descriptors(chainHash, fingerprint, accountPub)
+        return createBIP84Descriptor(chainHash, fingerprint, accountPub)
     }
 
     @JvmStatic
-    public fun BIP84Descriptors(chainHash: ByteVector32, fingerprint: Long, accountPub: DeterministicWallet.ExtendedPublicKey): Pair<String, String> {
+    public fun createBIP84Descriptor(chainHash: ByteVector32, fingerprint: Long, accountPub: DeterministicWallet.ExtendedPublicKey): String {
         val (keyPath, prefix) = getKeyPath(chainHash)
-        val accountDesc = "wpkh([${fingerprint.toString(16)}/$keyPath]${DeterministicWallet.encode(accountPub, prefix)}/0/*)"
-        val changeDesc = "wpkh([${fingerprint.toString(16)}/$keyPath]${DeterministicWallet.encode(accountPub, prefix)}/1/*)"
-        return Pair(
-            "$accountDesc#${checksum(accountDesc)}",
-            "$changeDesc#${checksum(changeDesc)}"
-        )
+        val accountDesc = "wpkh([${fingerprint.toString(16)}/$keyPath]${DeterministicWallet.encode(accountPub, prefix)}/<0;1>/*)"
+        return "$accountDesc#${checksum(accountDesc)}"
     }
 }

src/commonTest/kotlin/fr/acinq/bitcoin/DescriptorTestsCommon.kt

@@ -19,16 +19,14 @@ class DescriptorTestsCommon {
         data.forEach { dnc ->
             val (desc, checksum) = dnc.split('#').toTypedArray()
             assertEquals(checksum, Descriptor.checksum(desc))
-       }
+        }
     }
 
     @Test
     fun `compute BIP84 descriptors`() {
         val seed = ByteVector.fromHex("817a9c8e6ba36f083d7e68b5ee89ce74fde9ef294a724a5efc5cef2b88db057f")
         val master = DeterministicWallet.generate(seed)
-        val (accountDesc, changeDesc) = Descriptor.BIP84Descriptors(Block.RegtestGenesisBlock.hash, master)
-        assertEquals("wpkh([189ef5fe/84'/1'/0'/0]tpubDFTu6FhLqfTBLMd7BvGkyH1h4XBw7XoKWfnNNWw5Sp8V6aC55EhgPTVNAYvBwBXQ8EGnMqaZi3dpdSzhMbD4Z7ivZiaVKNMUkXVjDU1CDuE/0/*)#uysr3s9y", accountDesc)
-        assertEquals("wpkh([189ef5fe/84'/1'/0'/0]tpubDFTu6FhLqfTBLMd7BvGkyH1h4XBw7XoKWfnNNWw5Sp8V6aC55EhgPTVNAYvBwBXQ8EGnMqaZi3dpdSzhMbD4Z7ivZiaVKNMUkXVjDU1CDuE/1/*)#ds4zv94u", changeDesc)
+        val descriptor = Descriptor.createBIP84Descriptor(Block.RegtestGenesisBlock.hash, master)
+        assertEquals("wpkh([189ef5fe/84'/1'/0'/0]tpubDFTu6FhLqfTBLMd7BvGkyH1h4XBw7XoKWfnNNWw5Sp8V6aC55EhgPTVNAYvBwBXQ8EGnMqaZi3dpdSzhMbD4Z7ivZiaVKNMUkXVjDU1CDuE/<0;1>/*)#rw8h72wu", descriptor)
     }
-
 }