refactor: Move GetDifficulty out of `rpc/server.h`

It has no business in `rpcserver.h`. Define it in the interface header
of the implementation unit `rpcblockchain` where it is defined.

Also modernize the signature to:

    double GetDifficulty(const CBlockIndex* blockindex = nullptr);

(remove `extern`, replace `NULL` with `nullptr`)

1 files changed, 20 insertions, 0 deletions

diff --git a/src/rpc/blockchain.h b/src/rpc/blockchain.h
new file mode 100644
index 0000000000..69cba8f019
--- /dev/null
+++ b/src/rpc/blockchain.h
@@ -0,0 +1,20 @@
+// Copyright (c) 2017 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_RPC_BLOCKCHAIN_H
+#define BITCOIN_RPC_BLOCKCHAIN_H
+
+class CBlockIndex;
+
+/**
+ * Get the difficulty of the net wrt to the given block index, or the chain tip if
+ * not provided.
+ *
+ * @return A floating point number that is a multiple of the main net minimum
+ * difficulty (4295032833 hashes).
+ */
+double GetDifficulty(const CBlockIndex* blockindex = nullptr);
+
+#endif
+