Merge rust-bitcoin/rust-bitcoin#926: Remove redundant code computing tap hashes

f3ebfd6 Remove repeated tap branch hash computing logic (Dr Maxim Orlovsky)
1b28375 Abstract tap branch hash computing into a dedicated method (Dr Maxim Orlovsky)

Pull request description:

ACKs for top commit:
  apoelstra:
    ACK f3ebfd6

Tree-SHA512: d66d544df324a7d25c8cc9dc48ddedf086ac87eb2ed09a8d7a568cc1488ae44e0807d53ccb7a6e61dbeef0d3d62a1cacf0d69ba7b8de9178ac5c13bae944d08b

Author: ChallengeDev210

src/util/taproot.rs

@@ -131,6 +131,21 @@ impl TapLeafHash {
     }
 }
 
+impl TapBranchHash {
+    /// Computes branch hash given two hashes of the nodes underneath it.
+    pub fn from_node_hashes(a: sha256::Hash, b: sha256::Hash) -> TapBranchHash {
+        let mut eng = TapBranchHash::engine();
+        if a < b {
+            eng.input(&a);
+            eng.input(&b);
+        } else {
+            eng.input(&b);
+            eng.input(&a);
+        };
+        TapBranchHash::from_engine(eng)
+    }
+}
+
 /// Maximum depth of a taproot tree script spend path.
 // https://github.com/bitcoin/bitcoin/blob/e826b22da252e0599c61d21c98ff89f366b3120f/src/script/interpreter.h#L229
 pub const TAPROOT_CONTROL_MAX_NODE_COUNT: usize = 128;
@@ -565,16 +580,9 @@ impl NodeInfo {
             b_leaf.merkle_branch.push(a.hash)?; // add hashing partner
             all_leaves.push(b_leaf);
         }
-        let mut eng = TapBranchHash::engine();
-        if a.hash < b.hash {
-            eng.input(&a.hash);
-            eng.input(&b.hash);
-        } else {
-            eng.input(&b.hash);
-            eng.input(&a.hash);
-        };
+        let hash = TapBranchHash::from_node_hashes(a.hash, b.hash);
         Ok(Self {
-            hash: sha256::Hash::from_engine(eng),
+            hash: sha256::Hash::from_inner(hash.into_inner()),
             leaves: all_leaves,
         })
     }
@@ -779,16 +787,11 @@ impl ControlBlock {
         let mut curr_hash = TapBranchHash::from_inner(leaf_hash.into_inner());
         // Verify the proof
         for elem in self.merkle_branch.as_inner() {
-            let mut eng = TapBranchHash::engine();
-            if curr_hash.as_inner() < elem.as_inner() {
-                eng.input(&curr_hash);
-                eng.input(elem);
-            } else {
-                eng.input(elem);
-                eng.input(&curr_hash);
-            }
             // Recalculate the curr hash as parent hash
-            curr_hash = TapBranchHash::from_engine(eng);
+            curr_hash = TapBranchHash::from_node_hashes(
+                sha256::Hash::from_inner(curr_hash.into_inner()),
+                *elem
+            );
         }
         // compute the taptweak
         let tweak = TapTweakHash::from_key_and_tweak(self.internal_key, Some(curr_hash));