Minor update of implementation for BIP68

This patch syncronised latest implementation
Adds comments, adds assert()

bip-0068.mediawiki

@@ -89,10 +89,10 @@ static const uint32_t SEQUENCE_LOCKTIME_MASK = 0x0000ffff;
 static const int SEQUENCE_LOCKTIME_GRANULARITY = 9;
 
 /**
- * Calculates the block height and time which the transaction must be later than
- * in order to be considered final in the context of BIP 68.  It also removes
- * from the vector of input heights any entries which did not correspond to sequence
- * locked inputs as they do not affect the calculation.
+ * Calculates the block height and previous block's median time past at
+ * which the transaction will be considered final in the context of BIP 68.
+ * Also removes from the vector of input heights any entries which did not
+ * correspond to sequence locked inputs as they do not affect the calculation.
  */
 static std::pair<int, int64_t> CalculateSequenceLocks(const CTransaction &tx, int flags, std::vector<int>* prevHeights, const CBlockIndex& block)
 {
@@ -134,17 +134,21 @@ static std::pair<int, int64_t> CalculateSequenceLocks(const CTransaction &tx, in
 
         if (txin.nSequence & CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG) {
             int64_t nCoinTime = block.GetAncestor(std::max(nCoinHeight-1, 0))->GetMedianTimePast();
+            // NOTE: Subtract 1 to maintain nLockTime semantics
+            // BIP 68 relative lock times have the semantics of calculating
+            // the first block or time at which the transaction would be
+            // valid. When calculating the effective block time or height
+            // for the entire transaction, we switch to using the
+            // semantics of nLockTime which is the last invalid block
+            // time or height.  Thus we subtract 1 from the calculated
+            // time or height.
 
             // Time-based relative lock-times are measured from the
             // smallest allowed timestamp of the block containing the
             // txout being spent, which is the median time past of the
             // block prior.
             nMinTime = std::max(nMinTime, nCoinTime + (int64_t)((txin.nSequence & CTxIn::SEQUENCE_LOCKTIME_MASK) << CTxIn::SEQUENCE_LOCKTIME_GRANULARITY) - 1);
         } else {
-            // We subtract 1 from relative lock-times because a lock-
-            // time of 0 has the semantics of "same block," so a lock-
-            // time of 1 should mean "next block," but nLockTime has
-            // the semantics of "last invalid block height."
             nMinHeight = std::max(nMinHeight, nCoinHeight + (int)(txin.nSequence & CTxIn::SEQUENCE_LOCKTIME_MASK) - 1);
         }
     }
@@ -154,7 +158,8 @@ static std::pair<int, int64_t> CalculateSequenceLocks(const CTransaction &tx, in
 
 static bool EvaluateSequenceLocks(const CBlockIndex& block, std::pair<int, int64_t> lockPair)
 {
-    int64_t nBlockTime = block.pprev ? block.pprev->GetMedianTimePast() : 0;
+    assert(block.pprev);
+    int64_t nBlockTime = block.pprev->GetMedianTimePast();
     if (lockPair.first >= block.nHeight || lockPair.second >= nBlockTime)
         return false;
 
@@ -189,7 +194,7 @@ bool CheckSequenceLocks(const CTransaction &tx, int flags)
         const CTxIn& txin = tx.vin[txinIndex];
         CCoins coins;
         if (!viewMemPool.GetCoins(txin.prevout.hash, coins)) {
-            return  error("%s: Missing input", __func__);
+            return error("%s: Missing input", __func__);
         }
         if (coins.nHeight == MEMPOOL_HEIGHT) {
             // Assume all mempool transaction confirm in the next block