Add unit tests for next difficulty calculations

Split GetNextWorkRequired() into two functions to allow the difficulty calculations to
be tested without requiring a full blockchain.
Add unit tests to cover basic difficulty calculation, plus each of the min/max actual
time, and maximum difficulty target conditions.

src/Makefile.test.include

@@ -55,6 +55,7 @@ BITCOIN_TESTS =\
   test/multisig_tests.cpp \
   test/netbase_tests.cpp \
   test/pmt_tests.cpp \
+  test/pow_tests.cpp \
   test/rpc_tests.cpp \
   test/sanity_tests.cpp \
   test/script_P2SH_tests.cpp \

src/pow.cpp

@@ -48,8 +48,13 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
         pindexFirst = pindexFirst->pprev;
     assert(pindexFirst);
 
+    return CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime());
+}
+
+unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime)
+{
     // Limit adjustment step
-    int64_t nActualTimespan = pindexLast->GetBlockTime() - pindexFirst->GetBlockTime();
+    int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
     LogPrintf("  nActualTimespan = %d  before bounds\n", nActualTimespan);
     if (nActualTimespan < Params().TargetTimespan()/4)
         nActualTimespan = Params().TargetTimespan()/4;

src/pow.h

@@ -14,6 +14,7 @@ class uint256;
 class arith_uint256;
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock);
+unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime);
 
 /** Check whether a block hash satisfies the proof-of-work requirement specified by nBits */
 bool CheckProofOfWork(uint256 hash, unsigned int nBits);

src/test/pow_tests.cpp

@@ -0,0 +1,66 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include "main.h"
+#include "pow.h"
+#include "util.h"
+
+#include <boost/test/unit_test.hpp>
+
+using namespace std;
+
+BOOST_AUTO_TEST_SUITE(pow_tests)
+
+/* Test calculation of next difficulty target with no constraints applying */
+BOOST_AUTO_TEST_CASE(get_next_work)
+{
+    SelectParams(CBaseChainParams::MAIN);
+
+    int64_t nLastRetargetTime = 1261130161; // Block #30240
+    CBlockIndex pindexLast;
+    pindexLast.nHeight = 32255;
+    pindexLast.nTime = 1262152739;  // Block #32255
+    pindexLast.nBits = 0x1d00ffff;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime), 0x1d00d86a);
+}
+
+/* Test the constraint on the upper bound for next work */
+BOOST_AUTO_TEST_CASE(get_next_work_pow_limit)
+{
+    SelectParams(CBaseChainParams::MAIN);
+
+    int64_t nLastRetargetTime = 1231006505; // Block #0
+    CBlockIndex pindexLast;
+    pindexLast.nHeight = 2015;
+    pindexLast.nTime = 1233061996;  // Block #2015
+    pindexLast.nBits = 0x1d00ffff;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime), 0x1d00ffff);
+}
+
+/* Test the constraint on the lower bound for actual time taken */
+BOOST_AUTO_TEST_CASE(get_next_work_lower_limit_actual)
+{
+    SelectParams(CBaseChainParams::MAIN);
+
+    int64_t nLastRetargetTime = 1279008237; // Block #66528
+    CBlockIndex pindexLast;
+    pindexLast.nHeight = 68543;
+    pindexLast.nTime = 1279297671;  // Block #68543
+    pindexLast.nBits = 0x1c05a3f4;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime), 0x1c0168fd);
+}
+
+/* Test the constraint on the upper bound for actual time taken */
+BOOST_AUTO_TEST_CASE(get_next_work_upper_limit_actual)
+{
+    SelectParams(CBaseChainParams::MAIN);
+    int64_t nLastRetargetTime = 1263163443; // NOTE: Not an actual block time
+    CBlockIndex pindexLast;
+    pindexLast.nHeight = 46367;
+    pindexLast.nTime = 1269211443;  // Block #46367
+    pindexLast.nBits = 0x1c387f6f;
+    BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime), 0x1d00e1fd);
+}
+
+BOOST_AUTO_TEST_SUITE_END()