From e3bf159572bc2dd72d97d33cf70ed0fe49746b4d Mon Sep 17 00:00:00 2001
From: Dr Maxim Orlovsky <orlovsky@lnp-bp.org>
Date: Sun, 19 Nov 2023 13:20:43 +0100
Subject: [PATCH] descriptor: add default_keychain method

---
 descriptors/src/descriptor.rs |  7 +++++++
 descriptors/src/segwit.rs     |  3 +++
 descriptors/src/taproot.rs    |  3 +++
 std/src/derive.rs             | 11 +++++++++++
 std/src/path.rs               |  8 ++++++++
 5 files changed, 32 insertions(+)

diff --git a/descriptors/src/descriptor.rs b/descriptors/src/descriptor.rs
index 251d4fe..2c874e1 100644
--- a/descriptors/src/descriptor.rs
+++ b/descriptors/src/descriptor.rs
@@ -140,6 +140,13 @@ pub enum StdDescr<S: DeriveSet = XpubDerivable> {
 }
 
 impl<S: DeriveSet> Derive<DerivedScript> for StdDescr<S> {
+    fn default_keychain(&self) -> Keychain {
+        match self {
+            StdDescr::Wpkh(d) => d.default_keychain(),
+            StdDescr::TrKey(d) => d.default_keychain(),
+        }
+    }
+
     fn keychains(&self) -> &BTreeSet<Keychain> {
         match self {
             StdDescr::Wpkh(d) => d.keychains(),
diff --git a/descriptors/src/segwit.rs b/descriptors/src/segwit.rs
index 67f13bf..e56cebc 100644
--- a/descriptors/src/segwit.rs
+++ b/descriptors/src/segwit.rs
@@ -41,6 +41,9 @@ impl<K: DeriveCompr> Wpkh<K> {
 }
 
 impl<K: DeriveCompr> Derive<DerivedScript> for Wpkh<K> {
+    #[inline]
+    fn default_keychain(&self) -> Keychain { self.0.default_keychain() }
+
     #[inline]
     fn keychains(&self) -> &BTreeSet<Keychain> { self.0.keychains() }
 
diff --git a/descriptors/src/taproot.rs b/descriptors/src/taproot.rs
index 73018e2..b6afaf8 100644
--- a/descriptors/src/taproot.rs
+++ b/descriptors/src/taproot.rs
@@ -41,6 +41,9 @@ impl<K: DeriveXOnly> TrKey<K> {
 }
 
 impl<K: DeriveXOnly> Derive<DerivedScript> for TrKey<K> {
+    #[inline]
+    fn default_keychain(&self) -> Keychain { self.0.default_keychain() }
+
     #[inline]
     fn keychains(&self) -> &BTreeSet<Keychain> { self.0.keychains() }
 
diff --git a/std/src/derive.rs b/std/src/derive.rs
index 9ea6978..ee98634 100644
--- a/std/src/derive.rs
+++ b/std/src/derive.rs
@@ -281,6 +281,8 @@ impl FromStr for DerivedAddr {
 }
 
 pub trait Derive<D> {
+    fn default_keychain(&self) -> Keychain;
+
     fn keychains(&self) -> &BTreeSet<Keychain>;
 
     fn derive(&self, keychain: impl Into<Keychain>, index: impl Into<NormalIndex>) -> D;
@@ -358,6 +360,9 @@ impl DeriveKey<XOnlyPk> for XpubDerivable {
 }
 
 impl Derive<LegacyPk> for XpubDerivable {
+    #[inline]
+    fn default_keychain(&self) -> Keychain { self.keychains.first() }
+
     #[inline]
     fn keychains(&self) -> &BTreeSet<Keychain> { self.keychains.as_ref() }
 
@@ -367,6 +372,9 @@ impl Derive<LegacyPk> for XpubDerivable {
 }
 
 impl Derive<CompressedPk> for XpubDerivable {
+    #[inline]
+    fn default_keychain(&self) -> Keychain { self.keychains.first() }
+
     #[inline]
     fn keychains(&self) -> &BTreeSet<Keychain> { self.keychains.as_ref() }
 
@@ -376,6 +384,9 @@ impl Derive<CompressedPk> for XpubDerivable {
 }
 
 impl Derive<XOnlyPk> for XpubDerivable {
+    #[inline]
+    fn default_keychain(&self) -> Keychain { self.keychains.first() }
+
     #[inline]
     fn keychains(&self) -> &BTreeSet<Keychain> { self.keychains.as_ref() }
 
diff --git a/std/src/path.rs b/std/src/path.rs
index 3bd07bf..983b30c 100644
--- a/std/src/path.rs
+++ b/std/src/path.rs
@@ -58,6 +58,14 @@ impl<I: IdxBase> DerivationSeg<I> {
 
     #[inline]
     pub fn at(&self, index: u8) -> Option<I> { self.0.iter().nth(index as usize).copied() }
+
+    #[inline]
+    pub fn first(&self) -> I {
+        *self
+            .0
+            .first()
+            .expect("confined type guarantees that there is at least one item in the collection")
+    }
 }
 
 impl DerivationSeg<NormalIndex> {