1 files changed, 155 insertions, 0 deletions

diff --git a/src/compactblock.cpp b/src/compactblock.cpp
new file mode 100644
index 0000000..3999ab7
--- /dev/null
+++ b/src/compactblock.cpp
@@ -0,0 +1,155 @@
+// Copyright (c) 2026 Bitflash developers
+// Distributed under the MIT/X11 software license.
+//
+// See compactblock.h.
+
+#include "headers.h"
+#include "compactblock.h"
+#include <sodium.h>
+#include <map>
+
+uint256 CCmpctBlock::HeaderHash() const
+{
+    // Same technique CBlock::GetHash() uses: the header fields are laid out
+    // contiguously and in the same order here as in CBlock, so hashing the
+    // raw member range gives the identical block-identity hash.
+    return Hash(BEGIN(nVersion), END(nNonce));
+}
+
+uint64 ComputeShortTxId(const uint256& hashPrevBlock, uint64 shortIdNonce, const uint256& txid)
+{
+    // Derive a 16-byte SipHash key from (hashPrevBlock, shortIdNonce), so
+    // short IDs are specific to this block: they can't be precomputed
+    // before the block (and its freshly-chosen nonce) exist, and the same
+    // transaction gets a different short ID in every block it appears in
+    // (relevant across reorgs/re-announcements).
+    unsigned char keyMaterial[32 + sizeof(uint64)];
+    memcpy(keyMaterial, BEGIN(hashPrevBlock), 32);
+    memcpy(keyMaterial + 32, &shortIdNonce, sizeof(uint64));
+    uint256 keyHash = Hash((const char*)keyMaterial, (const char*)keyMaterial + sizeof(keyMaterial));
+
+    unsigned char out[crypto_shorthash_BYTES]; // 8 bytes
+    crypto_shorthash(out, (const unsigned char*)BEGIN(txid), 32, (const unsigned char*)BEGIN(keyHash));
+
+    uint64 id = 0;
+    memcpy(&id, out, sizeof(uint64));
+    return id & 0xFFFFFFFFFFFFULL; // low 48 bits -- matches the wire field width
+}
+
+bool BuildCompactBlock(const CBlock& block, CCmpctBlock& out)
+{
+    if (block.vtx.empty())
+        return false; // every real block has at least the coinbase
+
+    out.nVersion      = block.nVersion;
+    out.hashPrevBlock  = block.hashPrevBlock;
+    out.hashMerkleRoot = block.hashMerkleRoot;
+    out.nTime          = block.nTime;
+    out.nBits          = block.nBits;
+    out.nNonce         = block.nNonce;
+
+    unsigned char nonceBytes[sizeof(uint64)];
+    RAND_bytes(nonceBytes, sizeof(nonceBytes));
+    memcpy(&out.shortIdNonce, nonceBytes, sizeof(uint64));
+
+    out.vPrefilledIndex.clear();
+    out.vPrefilledTx.clear();
+    out.vShortTxId.clear();
+
+    // Only the coinbase is prefilled (index 0) -- see file comment in
+    // compactblock.h for why that's enough for this chain's scale.
+    out.vPrefilledIndex.push_back(0);
+    out.vPrefilledTx.push_back(block.vtx[0]);
+
+    for (size_t i = 1; i < block.vtx.size(); i++)
+        out.vShortTxId.push_back(ComputeShortTxId(out.hashPrevBlock, out.shortIdNonce, block.vtx[i].GetHash()));
+
+    return true;
+}
+
+bool TryReconstructBlock(const CCmpctBlock& cmpct, CBlock& outBlock, std::vector<unsigned int>& vMissingIndex)
+{
+    vMissingIndex.clear();
+
+    size_t nTotal = cmpct.vPrefilledIndex.size() + cmpct.vShortTxId.size();
+    if (cmpct.vPrefilledIndex.size() != cmpct.vPrefilledTx.size() || nTotal == 0)
+        return false; // malformed
+
+    outBlock.SetNull();
+    outBlock.nVersion      = cmpct.nVersion;
+    outBlock.hashPrevBlock  = cmpct.hashPrevBlock;
+    outBlock.hashMerkleRoot = cmpct.hashMerkleRoot;
+    outBlock.nTime          = cmpct.nTime;
+    outBlock.nBits          = cmpct.nBits;
+    outBlock.nNonce         = cmpct.nNonce;
+    outBlock.vtx.assign(nTotal, CTransaction());
+
+    std::vector<bool> vFilled(nTotal, false);
+    for (size_t i = 0; i < cmpct.vPrefilledIndex.size(); i++)
+    {
+        unsigned int idx = cmpct.vPrefilledIndex[i];
+        if (idx >= nTotal)
+            return false; // malformed
+        outBlock.vtx[idx] = cmpct.vPrefilledTx[i];
+        vFilled[idx] = true;
+    }
+
+    // Index every mempool tx by its short ID under this block's keying, so
+    // resolving each block position is a map lookup rather than a linear
+    // scan of the mempool per short ID.
+    std::map<uint64, const CTransaction*> mapShortIdToTx;
+    for (std::map<uint256, CTransaction>::const_iterator mi = mapTransactions.begin();
+         mi != mapTransactions.end(); ++mi)
+    {
+        uint64 sid = ComputeShortTxId(cmpct.hashPrevBlock, cmpct.shortIdNonce, mi->first);
+        // A short-ID collision between two distinct mempool transactions is
+        // possible (48 bits, birthday bound ~2^24 mempool entries) though
+        // very unlikely at this chain's scale; on collision, prefer keeping
+        // neither mapped rather than risk resolving to the wrong tx --
+        // that position just falls through to getblocktxn like a genuine
+        // miss, which is always correct, only occasionally not optimal.
+        if (mapShortIdToTx.count(sid))
+            mapShortIdToTx[sid] = NULL;
+        else
+            mapShortIdToTx[sid] = &mi->second;
+    }
+
+    size_t shortIdPos = 0;
+    for (size_t idx = 0; idx < nTotal; idx++)
+    {
+        if (vFilled[idx])
+            continue;
+        if (shortIdPos >= cmpct.vShortTxId.size())
+            return false; // malformed: more unfilled positions than short IDs
+        uint64 sid = cmpct.vShortTxId[shortIdPos++];
+
+        std::map<uint64, const CTransaction*>::const_iterator found = mapShortIdToTx.find(sid);
+        if (found != mapShortIdToTx.end() && found->second != NULL)
+        {
+            outBlock.vtx[idx] = *found->second;
+            vFilled[idx] = true;
+        }
+        else
+        {
+            vMissingIndex.push_back(idx);
+        }
+    }
+
+    return vMissingIndex.empty();
+}
+
+bool FillMissingAndFinish(CBlock& outBlock, const std::vector<unsigned int>& vMissingIndex,
+                           const std::vector<CTransaction>& vTxn)
+{
+    if (vMissingIndex.size() != vTxn.size())
+        return false; // peer didn't answer what was asked
+
+    for (size_t i = 0; i < vMissingIndex.size(); i++)
+    {
+        unsigned int idx = vMissingIndex[i];
+        if (idx >= outBlock.vtx.size())
+            return false; // malformed
+        outBlock.vtx[idx] = vTxn[i];
+    }
+    return true;
+}