Allow minimum-difficulty blocks on testnet and regtest

A block may be mined with nBits set to the minimum difficulty if its
nTime is set more than six block intervals (15 minutes) after its parent
block.

This is a consensus rule change on testnet that will result in a chain
split (as desired).

src/gtest/test_pow.cpp

@@ -68,3 +68,44 @@ TEST(PoW, DifficultyAveraging) {
                                         params),
               GetNextWorkRequired(&blocks[lastBlk], nullptr, params));
 }
+
+TEST(PoW, MinDifficultyRules) {
+    SelectParams(CBaseChainParams::TESTNET);
+    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);
+    }
+
+    // Create a new block at the target spacing
+    CBlockHeader next;
+    next.nTime = blocks[lastBlk].nTime + params.nPowTargetSpacing;
+
+    // Result should be unchanged, modulo integer division precision loss
+    arith_uint256 bnRes;
+    bnRes.SetCompact(0x1e7fffff);
+    bnRes /= params.AveragingWindowTimespan();
+    bnRes *= params.AveragingWindowTimespan();
+    EXPECT_EQ(GetNextWorkRequired(&blocks[lastBlk], &next, params), bnRes.GetCompact());
+
+    // Delay last block up to the edge of the min-difficulty limit
+    next.nTime += params.nPowTargetSpacing * 5;
+
+    // Result should be unchanged, modulo integer division precision loss
+    EXPECT_EQ(GetNextWorkRequired(&blocks[lastBlk], &next, params), bnRes.GetCompact());
+
+    // Delay last block over the min-difficulty limit
+    next.nTime += 1;
+
+    // Result should be the minimum difficulty
+    EXPECT_EQ(GetNextWorkRequired(&blocks[lastBlk], &next, params),
+              UintToArith256(params.powLimit).GetCompact());
+}

src/gtest/test_rpc.cpp

@@ -9,33 +9,6 @@
 #include "streams.h"
 #include "utilstrencodings.h"
 
-TEST(rpc, GetDifficultyTestnetRules) {
-    SelectParams(CBaseChainParams::TESTNET);
-
-    CBlockIndex prev;
-    prev.nTime = 1472700000;
-    prev.nBits = 0x201fffff;
-
-    CBlockIndex curr;
-    curr.pprev = &prev;
-    curr.nTime = 1472700300;
-    curr.nBits = 0x207fffff;
-
-    // Time interval is within 5 minutes, so the min-difficulty block should be
-    // interpreted as a valid network difficulty.
-    EXPECT_EQ(1, GetDifficulty(&curr));
-    EXPECT_EQ(1, GetNetworkDifficulty(&curr));
-
-    curr.nTime += 1;
-
-    // Time interval is over 5 minutes, so the min-difficulty block should be
-    // ignored for network difficulty determination.
-    EXPECT_EQ(1, GetDifficulty(&curr));
-    // We have to check this directly, because of some combination of rounding
-    // and truncation issues that result in Google Test displaying 4 != 4
-    EXPECT_EQ((double)0x7fffff/(double)0x1fffff, GetNetworkDifficulty(&curr));
-}
-
 extern UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool txDetails = false);
 
 TEST(rpc, check_blockToJSON_returns_minified_solution) {