From 29ec54fad6466c81e2f38bd24ae18cc5886997d6 Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@z.cash>
Date: Sat, 12 Jun 2021 19:23:31 +0100
Subject: [PATCH] ZIP 244 hashAuthDataRoot computation

---
 src/primitives/block.cpp | 44 ++++++++++++++++++++++++++++++++++++++++
 src/primitives/block.h   |  5 +++++
 2 files changed, 49 insertions(+)

diff --git a/src/primitives/block.cpp b/src/primitives/block.cpp
index 3e685b6c3..a40f46457 100644
--- a/src/primitives/block.cpp
+++ b/src/primitives/block.cpp
@@ -10,6 +10,9 @@
 #include "utilstrencodings.h"
 #include "crypto/common.h"
 
+const unsigned char ZCASH_AUTH_DATA_HASH_PERSONALIZATION[BLAKE2bPersonalBytes] =
+    {'Z','c','a','s','h','A','u','t','h','D','a','t','H','a','s','h'};
+
 uint256 CBlockHeader::GetHash() const
 {
     return SerializeHash(*this);
@@ -109,6 +112,47 @@ uint256 CBlock::CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMer
     return hash;
 }
 
+static uint64_t next_pow2(uint64_t x)
+{
+    x -= 1;
+    x |= (x >> 1);
+    x |= (x >> 2);
+    x |= (x >> 4);
+    x |= (x >> 8);
+    x |= (x >> 16);
+    x |= (x >> 32);
+    return x + 1;
+}
+
+uint256 CBlock::BuildAuthDataMerkleTree() const
+{
+    std::vector<uint256> tree;
+    auto perfectSize = next_pow2(vtx.size());
+    tree.reserve(perfectSize * 2); // Safe upper bound for the number of total nodes.
+
+    // Add the leaves to the tree. v1-v4 transactions will append empty leaves.
+    for (auto &tx : vtx) {
+        tree.push_back(tx.GetAuthDigest());
+    }
+    // Append empty leaves until we get a perfect tree.
+    tree.insert(tree.end(), perfectSize - vtx.size(), uint256());
+
+    int j = 0;
+    for (int layerWidth = tree.size(); layerWidth > 1; layerWidth = (layerWidth + 1) / 2) {
+        for (int i = 0; i < layerWidth; i += 2) {
+            CBLAKE2bWriter ss(SER_GETHASH, 0, ZCASH_AUTH_DATA_HASH_PERSONALIZATION);
+            ss << tree[j + i];
+            ss << tree[j + i + 1];
+            tree.push_back(ss.GetHash());
+        }
+
+        // Move to the next layer.
+        j += layerWidth;
+    }
+
+    return (tree.empty() ? uint256() : tree.back());
+}
+
 std::string CBlock::ToString() const
 {
     std::stringstream s;
diff --git a/src/primitives/block.h b/src/primitives/block.h
index c4464c68d..dfba56833 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -134,6 +134,11 @@ public:
 
     std::vector<uint256> GetMerkleBranch(int nIndex) const;
     static uint256 CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMerkleBranch, int nIndex);
+
+    // Build the authorizing data Merkle tree for this block and return its
+    // root.
+    uint256 BuildAuthDataMerkleTree() const;
+
     std::string ToString() const;
 };