optimization: cache PresaltedSipHasher in CBlockHeaderAndShortTxIDs

Replaces separate `shorttxidk0`/`shorttxidk1` members with a cached `PresaltedSipHasher`, so `GetShortID()` reuses the precomputed `SipHash` state instead of rebuilding it on every call.

`CBlockHeaderAndShortTxIDs` was never intended to be used before `FillShortTxIDSelector()` runs; doing so already relied on indeterminate salt values.
The new `Assert(m_hasher)` just makes this invariant explicit and fails fast if the object is used in an uninitialized state.

src/blockencodings.cpp

@@ -17,11 +17,14 @@
 
 #include <unordered_map>
 
-CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce) :
+CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, uint64_t nonce)
-        nonce(nonce),
+    : nonce(nonce),
-        shorttxids(block.vtx.size() - 1), prefilledtxn(1), header(block) {
+      shorttxids(block.vtx.size() - 1),
+      prefilledtxn(1),
+      header(block)
+{
     FillShortTxIDSelector();
-    //TODO: Use our mempool prior to block acceptance to predictively fill more than just the coinbase
+    // TODO: Use our mempool prior to block acceptance to predictively fill more than just the coinbase
     prefilledtxn[0] = {0, block.vtx[0]};
     for (size_t i = 1; i < block.vtx.size(); i++) {
         const CTransaction& tx = *block.vtx[i];
@@ -29,21 +32,21 @@ CBlockHeaderAndShortTxIDs::CBlockHeaderAndShortTxIDs(const CBlock& block, const
     }
 }
 
-void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const {
+void CBlockHeaderAndShortTxIDs::FillShortTxIDSelector() const
+{
     DataStream stream{};
     stream << header << nonce;
     CSHA256 hasher;
     hasher.Write((unsigned char*)&(*stream.begin()), stream.end() - stream.begin());
     uint256 shorttxidhash;
     hasher.Finalize(shorttxidhash.begin());
-    shorttxidk0 = shorttxidhash.GetUint64(0);
+    m_hasher.emplace(shorttxidhash.GetUint64(0), shorttxidhash.GetUint64(1));
-    shorttxidk1 = shorttxidhash.GetUint64(1);
 }
 
-uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const {
+uint64_t CBlockHeaderAndShortTxIDs::GetShortID(const Wtxid& wtxid) const
+{
     static_assert(SHORTTXIDS_LENGTH == 6, "shorttxids calculation assumes 6-byte shorttxids");
-    PresaltedSipHasher hasher(shorttxidk0, shorttxidk1); // TODO extract
+    return (*Assert(m_hasher))(wtxid.ToUint256()) & 0xffffffffffffL;
-    return hasher(wtxid.ToUint256()) & 0xffffffffffffL;
 }
 
 /* Reconstructing a compact block is in the hot-path for block relay,

src/blockencodings.h

@@ -5,6 +5,7 @@
 #ifndef BITCOIN_BLOCKENCODINGS_H
 #define BITCOIN_BLOCKENCODINGS_H
 
+#include <crypto/siphash.h>
 #include <primitives/block.h>
 
 #include <functional>
@@ -87,8 +88,7 @@ typedef enum ReadStatus_t
 } ReadStatus;
 
 class CBlockHeaderAndShortTxIDs {
-private:
+    mutable std::optional<PresaltedSipHasher> m_hasher;
-    mutable uint64_t shorttxidk0, shorttxidk1;
     uint64_t nonce;
 
     void FillShortTxIDSelector() const;
@@ -112,7 +112,7 @@ public:
     /**
      * @param[in]  nonce  This should be randomly generated, and is used for the siphash secret key
      */
-    CBlockHeaderAndShortTxIDs(const CBlock& block, const uint64_t nonce);
+    CBlockHeaderAndShortTxIDs(const CBlock& block, uint64_t nonce);
 
     uint64_t GetShortID(const Wtxid& wtxid) const;