Added some comments

Some clarifications after a code review by Mike Hearn.

src/main.cpp

@@ -698,6 +698,8 @@ bool CTxMemPool::accept(CTransaction &tx, bool fCheckInputs,
             return false;
 
         // do all inputs exist?
+        // Note that this does not check for the presence of actual outputs (see the next check for that),
+        // only helps filling in pfMissingInputs (to determine missing vs spent).
         BOOST_FOREACH(const CTxIn txin, tx.vin) {
             if (!view.HaveCoins(txin.prevout.hash)) {
                 if (pfMissingInputs)
@@ -706,6 +708,7 @@ bool CTxMemPool::accept(CTransaction &tx, bool fCheckInputs,
             }
         }
 
+        // are the actual inputs available?
         if (!tx.HaveInputs(view))
             return error("CTxMemPool::accept() : inputs already spent");
 
@@ -1956,9 +1959,13 @@ bool CBlock::CheckBlock(bool fCheckPOW, bool fCheckMerkleRoot) const
         if (!tx.CheckTransaction())
             return DoS(tx.nDoS, error("CheckBlock() : CheckTransaction failed"));
 
+    // Build the merkle tree already. We need it anyway later, and it makes the
+    // block cache the transaction hashes, which means they don't need to be
+    // recalculated many times during this block's validation.
+    BuildMerkleTree();
+
     // Check for duplicate txids. This is caught by ConnectInputs(),
     // but catching it earlier avoids a potential DoS attack:
-    BuildMerkleTree();
     set<uint256> uniqueTx;
     for (unsigned int i=0; i<vtx.size(); i++) {
         uniqueTx.insert(GetTxHash(i));

src/main.h

@@ -1820,7 +1820,11 @@ public:
 
     // Modify the currently active block index
     virtual bool SetBestBlock(CBlockIndex *pindex);
+
+    // Do a bulk modification (multiple SetCoins + one SetBestBlock)
     virtual bool BatchWrite(const std::map<uint256, CCoins> &mapCoins, CBlockIndex *pindex);
+
+    // Calculate statistics about the unspent transaction output set
     virtual bool GetStats(CCoinsStats &stats);
 };
 
@@ -1851,14 +1855,25 @@ protected:
 
 public:
     CCoinsViewCache(CCoinsView &baseIn, bool fDummy = false);
+
+    // Standard CCoinsView methods
     bool GetCoins(uint256 txid, CCoins &coins);
     bool SetCoins(uint256 txid, const CCoins &coins);
     bool HaveCoins(uint256 txid);
-    CCoins &GetCoins(uint256 txid);
     CBlockIndex *GetBestBlock();
     bool SetBestBlock(CBlockIndex *pindex);
     bool BatchWrite(const std::map<uint256, CCoins> &mapCoins, CBlockIndex *pindex);
+
+    // Return a modifiable reference to a CCoins. Check HaveCoins first.
+    // Many methods explicitly require a CCoinsViewCache because of this method, to reduce
+    // copying.
+    CCoins &GetCoins(uint256 txid);
+
+    // Push the modifications applied to this cache to its base.
+    // Failure to call this method before destruction will cause the changes to be forgotten.
     bool Flush();
+
+    // Calculate the size of the cache (in number of transactions)
     unsigned int GetCacheSize();
 
 private: