validation: Extract basic block file logic into FlatFileSeq class.

src/flatfile.cpp

@@ -0,0 +1,23 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <stdexcept>
+
+#include <flatfile.h>
+#include <tinyformat.h>
+
+FlatFileSeq::FlatFileSeq(fs::path dir, const char* prefix, size_t chunk_size) :
+    m_dir(std::move(dir)),
+    m_prefix(prefix),
+    m_chunk_size(chunk_size)
+{
+    if (chunk_size == 0) {
+        throw std::invalid_argument("chunk_size must be positive");
+    }
+}
+
+fs::path FlatFileSeq::FileName(const CDiskBlockPos& pos) const
+{
+    return m_dir / strprintf("%s%05u.dat", m_prefix, pos.nFile);
+}