Work on memmapped fs

src/TensorFlowNET.Core/IO/MemmappedFileSystem.cs

@@ -0,0 +1,70 @@
+/*****************************************************************************
+   Copyright 2021 The TensorFlow.NET Authors. All Rights Reserved.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+******************************************************************************/
+
+using System;
+using System.IO;
+using System.IO.MemoryMappedFiles;
+using System.Linq;
+using Tensorflow;
+
+namespace Tensorflow.IO
+{
+    public class MemmappedFileSystem
+    {
+        public const string MEMMAPPED_PACKAGE_DEFAULT_NAME = "memmapped_package://.";
+
+        private MemoryMappedFile _mmapFile;
+        private MemmappedFileSystemDirectory _directory;
+
+        public MemmappedFileSystem(string path)
+        {
+            using (var stream = File.OpenRead(path))
+            {
+                // Read the offset for the directory
+                var offsetData = new byte[sizeof(ulong)];
+                stream.Seek(-sizeof(ulong), SeekOrigin.End);
+                stream.Read(offsetData, 0, sizeof(ulong));
+                var offset = BitConverter.ToUInt64(offsetData, 0);
+
+                var dirLength = stream.Length - (long) offset - sizeof(ulong);
+                if (dirLength < 0)
+                {
+                    throw new InvalidDataException("Malformed mmapped filesystem!");
+                }
+
+                var dirData = new byte[dirLength];
+
+                stream.Seek((long) offset, SeekOrigin.Begin);
+                stream.Read(dirData, 0, (int) dirLength);
+
+                _directory = MemmappedFileSystemDirectory.Parser.ParseFrom(dirData);
+            }
+
+            _mmapFile = MemoryMappedFile.CreateFromFile(path, FileMode.Open);
+        }
+
+        public Stream OpenMemmapped(string filename)
+        {
+            var entry = _directory.Element.FirstOrDefault(x => x.Name == filename);
+            if (entry == null)
+            {
+                throw new FileNotFoundException($"Missing memmaped file entry: {filename}");
+            }
+
+            return _mmapFile.CreateViewStream((long) entry.Offset, (long) entry.Length);
+        }
+    }
+}