Limit memory usage of mapZerocoinSupply

The current code retains functionality to validate past zerocoin transactions.
The ported code uses too much memory to store trivial zerocoin related data.

Supply items in the map are now only stored:
- When zerocoin spends were possible
- When the supply denomination <> 0

Author: FornaxA

src/chain.h

@@ -219,7 +219,7 @@ class CBlockIndex
     unsigned int nStakeModifierChecksum; // checksum of index; in-memeory only
 
     //! zerocoin specific fields
-    std::map<libzerocoin::CoinDenomination, int64_t> mapZerocoinSupply;
+    std::map<libzerocoin::CoinDenomination, uint16_t> mapZerocoinSupply;
     std::vector<libzerocoin::CoinDenomination> vMintDenominationsInBlock;
 
     //! ATP specific fields
@@ -267,10 +267,7 @@ class CBlockIndex
         nStakeModifierV2 = uint256();
         nStakeModifierChecksum = 0;
 
-        // Start supply of each denomination with 0s
-        for (auto& denom : libzerocoin::zerocoinDenomList) {
-            mapZerocoinSupply.insert(std::make_pair(denom, 0));
-        }
+        mapZerocoinSupply.clear();
         vMintDenominationsInBlock.clear();
         nAccumulatorCheckpoint = uint256();
 
@@ -419,7 +416,12 @@ class CBlockIndex
      */
     int64_t GetZcMints(libzerocoin::CoinDenomination denom) const
     {
-        return mapZerocoinSupply.at(denom);
+        auto it = mapZerocoinSupply.find(denom);
+        if (it != mapZerocoinSupply.end()) {
+            return it->second;
+        } else {
+            return 0;
+        }
     }
 
     /**
@@ -529,7 +531,7 @@ class CDiskBlockIndex : public CBlockIndex
         READWRITE(nNonce);
 
         READWRITE(VARINT(nFlags));
-        if(this->nVersion > 7) {
+        if(this->nVersion > 7 && this->nVersion <= 11) {
             READWRITE(nAccumulatorCheckpoint);
             READWRITE(mapZerocoinSupply);
             READWRITE(vMintDenominationsInBlock);

src/xion/xionchain.cpp

@@ -373,6 +373,9 @@ std::list<libzerocoin::CoinDenomination> ZerocoinSpendListFromBlock(const CBlock
 
 bool UpdateXIONSupply(const CBlock& block, CBlockIndex* pindex, bool fJustCheck)
 {
+    // Only update xION supply when zerocoin mint amount can change
+    if (pindex->nVersion <= 3 || pindex->nVersion > 11) return true;
+
     std::list<CZerocoinMint> listMints;
     bool fFilterInvalid = false;//pindex->nHeight >= Params().Zerocoin_Block_RecalculateAccumulators();
     BlockToZerocoinMintList(block, listMints, fFilterInvalid);
@@ -381,7 +384,8 @@ bool UpdateXIONSupply(const CBlock& block, CBlockIndex* pindex, bool fJustCheck)
     // Initialize zerocoin supply to the supply from previous block
     if (pindex->pprev && pindex->pprev->GetBlockHeader().nVersion > 3) {
         for (auto& denom : libzerocoin::zerocoinDenomList) {
-            pindex->mapZerocoinSupply.at(denom) = pindex->pprev->GetZcMints(denom);
+            uint16_t nMints = pindex->pprev->GetZcMints(denom);
+            if (nMints != 0) pindex->mapZerocoinSupply[denom] = nMints;
         }
     }
 
@@ -393,7 +397,7 @@ bool UpdateXIONSupply(const CBlock& block, CBlockIndex* pindex, bool fJustCheck)
         for (auto& m : listMints) {
             libzerocoin::CoinDenomination denom = m.GetDenomination();
             pindex->vMintDenominationsInBlock.push_back(m.GetDenomination());
-            pindex->mapZerocoinSupply.at(denom)++;
+            pindex->mapZerocoinSupply[denom] = pindex->GetZcMints(denom) + 1;
 
 /*
             //Remove any of our own mints from the mintpool
@@ -420,17 +424,20 @@ bool UpdateXIONSupply(const CBlock& block, CBlockIndex* pindex, bool fJustCheck)
         }
 
         for (auto& denom : listSpends) {
-            pindex->mapZerocoinSupply.at(denom)--;
-            nAmountZerocoinSpent += libzerocoin::ZerocoinDenominationToAmount(denom);
+            uint16_t nMints = pindex->GetZcMints(denom);
 
             // zerocoin failsafe
-            if (pindex->GetZcMints(denom) < 0)
+            if (nMints == 0)
                 return error("Block contains zerocoins that spend more than are in the available supply to spend");
+
+            pindex->mapZerocoinSupply[denom] = nMints - 1;
+            nAmountZerocoinSpent += libzerocoin::ZerocoinDenominationToAmount(denom);
+
         }
     }
 
     for (auto& denom : libzerocoin::zerocoinDenomList)
-        LogPrint(BCLog::ZEROCOIN, "%s coins for denomination %d pubcoin %s\n", __func__, denom, pindex->mapZerocoinSupply.at(denom));
+        LogPrint(BCLog::ZEROCOIN, "%s coins for denomination %d pubcoin %s\n", __func__, denom, pindex->GetZcMints(denom));
 
     return true;
 }