Port getchaintxstats from BTC master (#1328)

This will allow explorers to present interesting analytics about
transaction volume in various time frames.

src/rpc/blockchain.cpp

@@ -1655,6 +1655,87 @@ UniValue getmempoolinfo(const UniValue& params, bool fHelp)
     return mempoolInfoToJSON();
 }
 
+inline CBlockIndex* LookupBlockIndex(const uint256& hash)
+{
+    AssertLockHeld(cs_main);
+    BlockMap::const_iterator it = mapBlockIndex.find(hash);
+    return it == mapBlockIndex.end() ? nullptr : it->second;
+}
+
+UniValue getchaintxstats(const UniValue& params, bool fHelp)
+{
+    if (fHelp || params.size() > 2)
+        throw runtime_error(
+                "getchaintxstats\n"
+                "\nCompute statistics about the total number and rate of transactions in the chain.\n"
+                "\nArguments:\n"
+                "1. nblocks   (numeric, optional) Number of blocks in averaging window.\n"
+                "2. blockhash (string, optional) The hash of the block which ends the window.\n"
+                "\nResult:\n"
+            "{\n"
+            "  \"time\": xxxxx,                         (numeric) The timestamp for the final block in the window in UNIX format.\n"
+            "  \"txcount\": xxxxx,                      (numeric) The total number of transactions in the chain up to that point.\n"
+            "  \"window_final_block_hash\": \"...\",      (string) The hash of the final block in the window.\n"
+            "  \"window_block_count\": xxxxx,           (numeric) Size of the window in number of blocks.\n"
+            "  \"window_tx_count\": xxxxx,              (numeric) The number of transactions in the window. Only returned if \"window_block_count\" is > 0.\n"
+            "  \"window_interval\": xxxxx,              (numeric) The elapsed time in the window in seconds. Only returned if \"window_block_count\" is > 0.\n"
+            "  \"txrate\": x.xx,                        (numeric) The average rate of transactions per second in the window. Only returned if \"window_interval\" is > 0.\n"
+            "}\n"
+            "\nExamples:\n"
+            + HelpExampleCli("getchaintxstats", "")
+            + HelpExampleRpc("getchaintxstats", "2016")
+        );
+
+    const CBlockIndex* pindex;
+    int blockcount = 30 * 24 * 60 * 60 / Params().GetConsensus().nPowTargetSpacing; // By default: 1 month
+
+    if (params[1].isNull()) {
+        LOCK(cs_main);
+        pindex = chainActive.Tip();
+    } else {
+        uint256 hash(ParseHashV(params[1], "blockhash"));
+        LOCK(cs_main);
+        pindex = LookupBlockIndex(hash);
+        if (!pindex) {
+            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
+        }
+        if (!chainActive.Contains(pindex)) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Block is not in main chain");
+        }
+    }
+
+    assert(pindex != nullptr);
+
+    if (params[0].isNull()) {
+        blockcount = std::max(0, std::min(blockcount, pindex->GetHeight() - 1));
+    } else {
+        blockcount = params[0].get_int();
+
+        if (blockcount < 0 || (blockcount > 0 && blockcount >= pindex->GetHeight())) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid block count: should be between 0 and the block's height - 1");
+        }
+    }
+
+    const CBlockIndex* pindexPast = pindex->GetAncestor(pindex->GetHeight() - blockcount);
+    int nTimeDiff = pindex->GetMedianTimePast() - pindexPast->GetMedianTimePast();
+    int nTxDiff = pindex->nChainTx - pindexPast->nChainTx;
+
+    UniValue ret(UniValue::VOBJ);
+    ret.pushKV("time", (int64_t)pindex->nTime);
+    ret.pushKV("txcount", (int64_t)pindex->nChainTx);
+    ret.pushKV("window_final_block_hash", pindex->GetBlockHash().GetHex());
+    ret.pushKV("window_block_count", blockcount);
+    if (blockcount > 0) {
+        ret.pushKV("window_tx_count", nTxDiff);
+        ret.pushKV("window_interval", nTimeDiff);
+        if (nTimeDiff > 0) {
+            ret.pushKV("txrate", ((double)nTxDiff) / nTimeDiff);
+        }
+    }
+
+    return ret;
+}
+
 UniValue invalidateblock(const UniValue& params, bool fHelp)
 {
     if (fHelp || params.size() != 1)
@@ -1742,6 +1823,7 @@ static const CRPCCommand commands[] =
     { "blockchain",         "getblockhash",           &getblockhash,           true  },
     { "blockchain",         "getblockheader",         &getblockheader,         true  },
     { "blockchain",         "getchaintips",           &getchaintips,           true  },
+    { "blockchain",         "getchaintxstats",        &getchaintxstats,        true  },
     { "blockchain",         "getdifficulty",          &getdifficulty,          true  },
     { "blockchain",         "getmempoolinfo",         &getmempoolinfo,         true  },
     { "blockchain",         "getrawmempool",          &getrawmempool,          true  },

src/rpc/client.cpp

@@ -89,6 +89,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "listunspent", 2 },
     { "getblock", 1 },
     { "getblockheader", 1 },
+    { "getchaintxstats", 0  },
     { "getlastsegidstakes", 0 },
     { "gettransaction", 1 },
     { "getrawtransaction", 1 },