Testnet difficulty calculation changes, to take effect Feb 15 2012

Allow mining of min-difficulty blocks if 20 minutes have gone by without mining a regular-difficulty block.
Normal rules apply every 2016 blocks, though, so there may be a very-slow-to-confirm block at the difficulty-adjustment blocks.

src/main.cpp

@@ -768,6 +768,11 @@ static const int64 nInterval = nTargetTimespan / nTargetSpacing;
 //
 unsigned int ComputeMinWork(unsigned int nBase, int64 nTime)
 {
+    // Testnet has min-difficulty blocks
+    // after nTargetSpacing*2 time between blocks:
+    if (fTestNet && nTime > nTargetSpacing*2)
+        return bnProofOfWorkLimit.GetCompact();
+
     CBigNum bnResult;
     bnResult.SetCompact(nBase);
     while (nTime > 0 && bnResult < bnProofOfWorkLimit)
@@ -782,16 +787,36 @@ unsigned int ComputeMinWork(unsigned int nBase, int64 nTime)
     return bnResult.GetCompact();
 }
 
-unsigned int static GetNextWorkRequired(const CBlockIndex* pindexLast)
+unsigned int static GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlock *pblock)
 {
+    unsigned int nProofOfWorkLimit = bnProofOfWorkLimit.GetCompact();
 
     // Genesis block
     if (pindexLast == NULL)
-        return bnProofOfWorkLimit.GetCompact();
+        return nProofOfWorkLimit;
 
     // Only change once per interval
     if ((pindexLast->nHeight+1) % nInterval != 0)
+    {
+        // Special rules for testnet after 15 Feb 2012:
+        if (fTestNet && pblock->nTime > 1329264000)
+        {
+            // If the new block's timestamp is more than 2* 10 minutes
+            // then allow mining of a min-difficulty block.
+            if (pblock->nTime - pindexLast->nTime > nTargetSpacing*2)
+                return nProofOfWorkLimit;
+            else
+            {
+                // Return the last non-special-min-difficulty-rules-block
+                const CBlockIndex* pindex = pindexLast;
+                while (pindex->pprev && pindex->nHeight % nInterval != 0 && pindex->nBits == nProofOfWorkLimit)
+                    pindex = pindex->pprev;
+                return pindex->nBits;
+            }
+        }
+
         return pindexLast->nBits;
+    }
 
     // Go back by what we want to be 14 days worth of blocks
     const CBlockIndex* pindexFirst = pindexLast;
@@ -1540,7 +1565,7 @@ bool CBlock::AcceptBlock()
     int nHeight = pindexPrev->nHeight+1;
 
     // Check proof of work
-    if (nBits != GetNextWorkRequired(pindexPrev))
+    if (nBits != GetNextWorkRequired(pindexPrev, this))
         return DoS(100, error("AcceptBlock() : incorrect proof of work"));
 
     // Check timestamp against prev
@@ -3120,7 +3145,7 @@ CBlock* CreateNewBlock(CReserveKey& reservekey)
     pblock->hashPrevBlock  = pindexPrev->GetBlockHash();
     pblock->hashMerkleRoot = pblock->BuildMerkleTree();
     pblock->nTime          = max(pindexPrev->GetMedianTimePast()+1, GetAdjustedTime());
-    pblock->nBits          = GetNextWorkRequired(pindexPrev);
+    pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock.get());
     pblock->nNonce         = 0;
 
     return pblock.release();