Add ScriptTree helper to find leaf by hash (#148)

This allows us to store a `ScriptTree` with only the hash of the leaf
we want to spend, and fetch the actual script from the tree.

This avoids duplicating the script in our encoded data in `eclair` or
`lightning-kmp`.

Author: t-bast

.gitignore

@@ -3,6 +3,7 @@ build/
 
 # Idea
 .idea
+.kotlin
 *.iml
 
 # Gradle

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

@@ -15,18 +15,17 @@
  */
 package fr.acinq.bitcoin
 
-import fr.acinq.bitcoin.io.ByteArrayInput
 import fr.acinq.bitcoin.io.ByteArrayOutput
 import fr.acinq.bitcoin.io.Input
 import fr.acinq.bitcoin.io.Output
 import kotlin.jvm.JvmStatic
 
 /** Simple binary tree structure containing taproot spending scripts. */
 public sealed class ScriptTree {
-    public abstract fun write(output: Output, level: Int): Unit
+    public abstract fun write(output: Output, level: Int)
 
     /**
-     * @return the tree serialised with the format defined in BIP 371
+     * @return the tree serialized with the format defined in BIP 371
      */
     public fun write(): ByteArray {
         val output = ByteArrayOutput()
@@ -46,15 +45,15 @@ public sealed class ScriptTree {
         public constructor(script: List<ScriptElt>, leafVersion: Int) : this(Script.write(script).byteVector(), leafVersion)
         public constructor(script: String, leafVersion: Int) : this(ByteVector.fromHex(script), leafVersion)
 
-        override fun write(output: Output, level: Int): Unit {
+        override fun write(output: Output, level: Int) {
             output.write(level)
             output.write(leafVersion)
             BtcSerializer.writeScript(script, output)
         }
     }
 
     public data class Branch(val left: ScriptTree, val right: ScriptTree) : ScriptTree() {
-        override fun write(output: Output, level: Int): Unit {
+        override fun write(output: Output, level: Int) {
             left.write(output, level + 1)
             right.write(output, level + 1)
         }
@@ -68,7 +67,6 @@ public sealed class ScriptTree {
             BtcSerializer.writeScript(this.script, buffer)
             Crypto.taggedHash(buffer.toByteArray(), "TapLeaf")
         }
-
         is Branch -> {
             val h1 = this.left.hash()
             val h2 = this.right.hash()
@@ -83,6 +81,12 @@ public sealed class ScriptTree {
         is Branch -> this.left.findScript(script) ?: this.right.findScript(script)
     }
 
+    /** Return the first leaf with a matching leaf hash, if any. */
+    public fun findScript(leafHash: ByteVector32): Leaf? = when (this) {
+        is Leaf -> if (this.hash() == leafHash) this else null
+        is Branch -> this.left.findScript(leafHash) ?: this.right.findScript(leafHash)
+    }
+
     /**
      * Compute a merkle proof for the given script leaf.
      * This merkle proof is encoded for creating control blocks in taproot script path witnesses.
@@ -128,7 +132,7 @@ public sealed class ScriptTree {
         public fun read(input: Input): ScriptTree {
             val leaves = readLeaves(input)
             merge(leaves)
-            require(leaves.size == 1) { "invalid serialised script tree" }
+            require(leaves.size == 1) { "invalid serialized script tree" }
             return leaves[0].second
         }
     }

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

@@ -415,17 +415,28 @@ class TaprootTestsCommon {
 
     @Test
     fun `serialize script tree -- reference test`() {
-        val tree =
-            ScriptTree.read(ByteArrayInput(Hex.decode("02c02220736e572900fe1252589a2143c8f3c79f71a0412d2353af755e9701c782694a02ac02c02220631c5f3b5832b8fbdebfb19704ceeb323c21f40f7a24f43d68ef0cc26b125969ac01c0222044faa49a0338de488c8dfffecdfb6f329f380bd566ef20c8df6d813eab1c4273ac")))
+        val encoded = "02c02220736e572900fe1252589a2143c8f3c79f71a0412d2353af755e9701c782694a02ac02c02220631c5f3b5832b8fbdebfb19704ceeb323c21f40f7a24f43d68ef0cc26b125969ac01c0222044faa49a0338de488c8dfffecdfb6f329f380bd566ef20c8df6d813eab1c4273ac"
+        val tree = ScriptTree.read(ByteArrayInput(Hex.decode(encoded)))
+        val leaves = listOf(
+            ScriptTree.Leaf("20736e572900fe1252589a2143c8f3c79f71a0412d2353af755e9701c782694a02ac", 0xc0),
+            ScriptTree.Leaf("20631c5f3b5832b8fbdebfb19704ceeb323c21f40f7a24f43d68ef0cc26b125969ac", 0xc0),
+            ScriptTree.Leaf("2044faa49a0338de488c8dfffecdfb6f329f380bd566ef20c8df6d813eab1c4273ac", 0xc0),
+        )
         assertEquals(
             ScriptTree.Branch(
                 ScriptTree.Branch(
-                    ScriptTree.Leaf("20736e572900fe1252589a2143c8f3c79f71a0412d2353af755e9701c782694a02ac", 0xc0),
-                    ScriptTree.Leaf("20631c5f3b5832b8fbdebfb19704ceeb323c21f40f7a24f43d68ef0cc26b125969ac", 0xc0),
+                    leaves[0],
+                    leaves[1],
                 ),
-                ScriptTree.Leaf("2044faa49a0338de488c8dfffecdfb6f329f380bd566ef20c8df6d813eab1c4273ac", 0xc0)
+                leaves[2]
             ), tree
         )
+        // We're able to find leaves in that script tree.
+        leaves.forEach { l ->
+            assertEquals(l, tree.findScript(l.script))
+            assertEquals(l, tree.findScript(l.hash()))
+        }
+        assertNull(tree.findScript(ByteVector.fromHex("deadbeef")))
     }
 
     @Test