Add test of difficulty averaging

src/Makefile.gtest.include

@@ -13,6 +13,7 @@ zcash_gtest_SOURCES = \
 	gtest/test_keystore.cpp \
 	gtest/test_noteencryption.cpp \
 	gtest/test_merkletree.cpp \
+	gtest/test_pow.cpp \
 	gtest/test_rpc.cpp \
 	gtest/test_circuit.cpp \
 	gtest/test_txid.cpp \

src/gtest/test_pow.cpp

@@ -0,0 +1,62 @@
+#include <gtest/gtest.h>
+
+#include "chain.h"
+#include "chainparams.h"
+#include "pow.h"
+#include "random.h"
+
+TEST(PoW, DifficultyAveraging) {
+    SelectParams(CBaseChainParams::MAIN);
+    const Consensus::Params& params = Params().GetConsensus();
+    size_t lastBlk = 2*params.nPowAveragingWindow;
+    size_t firstBlk = lastBlk - params.nPowAveragingWindow;
+
+    // Start with blocks evenly-spaced and equal difficulty
+    std::vector<CBlockIndex> blocks(lastBlk+1);
+    for (int i = 0; i <= lastBlk; i++) {
+        blocks[i].pprev = i ? &blocks[i - 1] : nullptr;
+        blocks[i].nHeight = i;
+        blocks[i].nTime = 1269211443 + i * params.nPowTargetSpacing;
+        blocks[i].nBits = 0x1e7fffff; /* target 0x007fffff000... */
+        blocks[i].nChainWork = i ? blocks[i - 1].nChainWork + GetBlockProof(blocks[i - 1]) : arith_uint256(0);
+    }
+
+    // Result should be the same as if last difficulty was used
+    EXPECT_EQ(CalculateNextWorkRequired(&blocks[lastBlk],
+                                        blocks[firstBlk].GetMedianTimePast(),
+                                        params),
+              GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+    // Result should be unchanged
+    // TODO: This should be 0x1e7fffff, and just before GetNextWorkRequired()
+    //       returns, it is. Somehow it ends up off by one....
+    EXPECT_EQ(0x1e7ffffe, GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+
+    // Randomise the final block time
+    blocks[lastBlk].nTime += GetRand(params.nPowTargetSpacing)
+                           - GetRand(params.nPowTargetSpacing/2);
+
+    // Result should be the same as if last difficulty was used
+    EXPECT_EQ(CalculateNextWorkRequired(&blocks[lastBlk],
+                                        blocks[firstBlk].GetMedianTimePast(),
+                                        params),
+              GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+    // Result should not be unchanged
+    EXPECT_NE(0x1e7fffff, GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+
+    // Change the final block difficulty
+    blocks[lastBlk].nBits = 0x1e0fffff;
+
+    // Result should not be the same as if last difficulty was used
+    EXPECT_NE(CalculateNextWorkRequired(&blocks[lastBlk],
+                                        blocks[firstBlk].GetMedianTimePast(),
+                                        params),
+              GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+
+    // Result should be the same as if the average difficulty was used
+    arith_uint256 average = UintToArith256(uint256S("0000796968696969696969696969696969696969696969696969696969696969"));
+    EXPECT_EQ(CalculateNextWorkRequired(average,
+                                        blocks[lastBlk].GetMedianTimePast(),
+                                        blocks[firstBlk].GetMedianTimePast(),
+                                        params),
+              GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
+}