From fca0f83e2ab85960552b584818dd45ab72778133 Mon Sep 17 00:00:00 2001
From: Jason Phan <jason@ket.so>
Date: Wed, 30 Dec 2020 23:12:02 -0600
Subject: [PATCH] memory: Add FileOffset back

---
 vmm/memory/detail/guest.hpp | 51 +++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/vmm/memory/detail/guest.hpp b/vmm/memory/detail/guest.hpp
index ac3751e..01d3066 100644
--- a/vmm/memory/detail/guest.hpp
+++ b/vmm/memory/detail/guest.hpp
@@ -72,4 +72,55 @@ class MemoryRegionAddress : public Address<MemoryRegionAddress, uint64_t>
         size_type m_addr{};
 };
 
+// The starting point of a file which backs a GuestMemoryRegion.
+class FileOffset
+{
+    private:
+        std::shared_ptr<std::fstream> m_fstream;
+        long m_start{};
+    public:
+        FileOffset(const char* filename, std::ios_base::openmode mode,
+                   const long start={})
+            : m_fstream{std::make_shared<std::fstream>(filename, mode)},
+              m_start{start}
+        {
+            if (start) {
+                // TODO (?): Only call these if the appropriate openmode is set.
+                m_fstream->seekg(start);
+                m_fstream->seekp(start);
+            }
+        }
+
+        explicit FileOffset(const char* filename, const long start={})
+            : FileOffset(filename, std::ios_base::in | std::ios_base::out, start) {}
+
+        explicit FileOffset(const std::string& filename, const long start=0)
+            : FileOffset(filename.c_str(), start) {}
+
+        FileOffset(const std::string& filename,
+                   const std::ios_base::openmode mode,
+                   const long start={})
+            : FileOffset(filename.c_str(), mode, start) {}
+
+        explicit FileOffset(const std::filesystem::path& filename, const long start=0)
+            : FileOffset(filename.c_str(), start) {}
+
+        FileOffset(const std::filesystem::path& filename,
+                   const std::ios_base::openmode mode,
+                   const long start={})
+            : FileOffset(filename.c_str(), mode, start) {}
+
+        // TODO: thread-safety
+        [[nodiscard]] auto data() const -> std::basic_filebuf<std::fstream::char_type,
+                                                              std::fstream::traits_type>*
+        {
+            return m_fstream->rdbuf();
+        }
+
+        [[nodiscard]] constexpr auto start() const noexcept -> long
+        {
+            return m_start;
+        }
+};
+
 }  // vmm::memory::detail
-- 
2.45.2