feat:support for concurrent file-based queries

binding/csharp/IP2Region.Net/Internal/Abstractions/AbstractCacheStrategy.cs

@@ -0,0 +1,64 @@
+// Copyright 2023 The Ip2Region Authors. All rights reserved.
+// Use of this source code is governed by a Apache2.0-style
+// license that can be found in the LICENSE file.
+// @Author Alan <lzh.shap@gmail.com>
+// @Date   2023/07/25
+
+using System.Buffers;
+
+namespace IP2Region.Net.Internal.Abstractions;
+
+internal abstract class AbstractCacheStrategy
+{
+    protected const int HeaderInfoLength = 256;
+    protected const int VectorIndexRows = 256;
+    protected const int VectorIndexCols = 256;
+    protected const int VectorIndexSize = 8;
+
+    protected readonly FileStream XdbFileStream;
+    private const int BufferSize = 4096;
+
+    internal int IoCount { get; private set; }
+
+    protected AbstractCacheStrategy(string xdbPath)
+    {
+        XdbFileStream = new FileStream(xdbPath, FileMode.Open, FileAccess.Read, FileShare.Read, BufferSize,
+            useAsync: true);
+    }
+
+    protected int GetVectorIndexStartPos(uint ip)
+    {
+        var il0 = ip >> 24 & 0xFF;
+        var il1 = ip >> 16 & 0xFF;
+        var idx = il0 * VectorIndexCols * VectorIndexSize + il1 * VectorIndexSize;
+        return (int)idx;
+    }
+
+    internal abstract ReadOnlyMemory<byte> GetVectorIndex(uint ip);
+
+    internal virtual ReadOnlyMemory<byte> GetData(int offset, int length)
+    {
+        byte[] buffer = ArrayPool<byte>.Shared.Rent(length);
+        int totalBytesRead = 0;
+        try
+        {
+            XdbFileStream.Seek(offset, SeekOrigin.Begin);
+
+            int bytesRead;
+            do
+            {
+                int bytesToRead = Math.Min(BufferSize, length - totalBytesRead);
+                bytesRead = XdbFileStream.Read(buffer, totalBytesRead, bytesToRead);
+                totalBytesRead += bytesRead;
+            
+                IoCount++;
+            } while (bytesRead > 0 && totalBytesRead < length);
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(buffer);
+        }
+
+        return new ReadOnlyMemory<byte>(buffer, 0, totalBytesRead);
+    }
+}