Auto merge of #887 - str4d:mining-slow-start, r=ebfull

Implement mining slow start

Closes #762.

src/main.cpp

@@ -1327,12 +1327,27 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex)
 
 CAmount GetBlockSubsidy(int nHeight, const Consensus::Params& consensusParams)
 {
-    int halvings = nHeight / consensusParams.nSubsidyHalvingInterval;
+    CAmount nSubsidy = 50 * COIN;
+
+    // Mining slow start
+    // The subsidy is ramped up linearly, skipping the middle payout of
+    // MAX_SUBSIDY/2 to keep the monetary curve consistent with no slow start.
+    if (nHeight < consensusParams.nSubsidySlowStartInterval / 2) {
+        nSubsidy /= consensusParams.nSubsidySlowStartInterval;
+        nSubsidy *= nHeight;
+        return nSubsidy;
+    } else if (nHeight < consensusParams.nSubsidySlowStartInterval) {
+        nSubsidy /= consensusParams.nSubsidySlowStartInterval;
+        nSubsidy *= (nHeight+1);
+        return nSubsidy;
+    }
+
+    assert(nHeight > consensusParams.SubsidySlowStartShift());
+    int halvings = (nHeight - consensusParams.SubsidySlowStartShift()) / consensusParams.nSubsidyHalvingInterval;
     // Force block reward to zero when right shift is undefined.
     if (halvings >= 64)
         return 0;
 
-    CAmount nSubsidy = 50 * COIN;
     // Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years.
     nSubsidy >>= halvings;
     return nSubsidy;