validation: Extract basic block file logic into FlatFileSeq class.

src/flatfile.h

@@ -0,0 +1,36 @@
+// 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.
+
+#ifndef BITCOIN_FLATFILE_H
+#define BITCOIN_FLATFILE_H
+
+#include <chain.h>
+#include <fs.h>
+
+/**
+ * FlatFileSeq represents a sequence of numbered files storing raw data. This class facilitates
+ * access to and efficient management of these files.
+ */
+class FlatFileSeq
+{
+private:
+    const fs::path m_dir;
+    const char* const m_prefix;
+    const size_t m_chunk_size;
+
+public:
+    /**
+     * Constructor
+     *
+     * @param dir The base directory that all files live in.
+     * @param prefix A short prefix given to all file names.
+     * @param chunk_size Disk space is pre-allocated in multiples of this amount.
+     */
+    FlatFileSeq(fs::path dir, const char* prefix, size_t chunk_size);
+
+    /** Get the name of the file at the given position. */
+    fs::path FileName(const CDiskBlockPos& pos) const;
+};
+
+#endif // BITCOIN_FLATFILE_H