Make in-memory buffers seekable to fix fog of war

components/files/memorystream.hpp

@@ -9,11 +9,31 @@ namespace Files
     struct MemBuf : std::streambuf
     {
         MemBuf(char const* buffer, size_t size)
-        {
             // a streambuf isn't specific to istreams, so we need a non-const pointer :/
-            char* nonconstBuffer = (const_cast<char*>(buffer));
-            this->setg(nonconstBuffer, nonconstBuffer, nonconstBuffer + size);
+            : bufferStart(const_cast<char*>(buffer))
+            , bufferEnd(bufferStart + size)
+        {
+            this->setg(bufferStart, bufferStart, bufferEnd);
+        }
+
+        pos_type seekoff(off_type off, std::ios_base::seekdir dir, std::ios_base::openmode which) override
+        {
+            if (dir == std::ios_base::cur)
+                gbump(off);
+            else
+                setg(bufferStart, (dir == std::ios_base::beg ? bufferStart : bufferEnd) + off, bufferEnd);
+
+            return gptr() - bufferStart;
         }
+
+        pos_type seekpos(pos_type pos, std::ios_base::openmode which) override
+        {
+            return seekoff(pos, std::ios_base::beg, which);
+        }
+
+    protected:
+        char* bufferStart;
+        char* bufferEnd;
     };
 
     /// @brief A variant of std::istream that reads from a constant in-memory buffer.