Added faiss_write_index_fname

FaissMask.Test/IndexPersistencyTests.cs

@@ -0,0 +1,26 @@
+using System;
+using System.IO;
+using Xunit;
+
+namespace FaissMask.Test;
+
+public class IndexPersistencyTests
+{
+    private const string _indexPath = "data/faiss.index";
+    private const string _regeneratedIndexPath = "data/faiss_regenerated.index";
+
+    [Fact]
+    public void EnsureRegeneratedIndexIsTheSame()
+    {
+        using var index = IndexFlat.Read(_indexPath);
+        index.WriteIndex(_regeneratedIndexPath);
+        Assert.True(File.Exists(_regeneratedIndexPath));
+
+        byte[] originalIndexBytes = File.ReadAllBytes(_regeneratedIndexPath);
+        byte[] regenratedIndexBytes = File.ReadAllBytes(_regeneratedIndexPath);
+        Assert.Equal(originalIndexBytes, regenratedIndexBytes);
+
+        File.Delete(_regeneratedIndexPath);
+    }
+
+}

FaissMask/Index.cs

@@ -112,6 +112,11 @@ public float[] DecodeVector(byte[] bytes)
             return Handle.DecodeVector(bytes);
         }
 
+        public void WriteIndex(string filename)
+        {
+            Handle.WriteIndex(filename);
+        }
+
         public void Dispose()
         {
             Handle?.Free();

FaissMask/Internal/IndexSafeHandle.cs

@@ -145,5 +145,30 @@ public ulong SaCodeSize
                 return ptr.ToUInt64();
             }
         }
+
+        public void WriteIndex(string filename)
+        {
+            if (string.IsNullOrEmpty(filename))
+            {
+                throw new ArgumentNullException(nameof(filename));
+            }
+
+            var returnCode = NativeMethods.faiss_write_index_fname(this, filename);
+            if (returnCode != 0)
+            {
+                var lastError = NativeMethods.faiss_get_last_error();
+
+                if (string.IsNullOrEmpty(lastError))
+                {
+                    throw new IOException(
+                        $"An unknown error occurred trying to write the index '{filename}' (return code {returnCode})");
+                }
+                else
+                {
+                    throw new IOException(
+                        $"An error occurred trying to write the index '{filename}': {lastError} (return code {returnCode})");
+                }
+            }
+        }
     }
 }

FaissMask/Internal/NativeMethods.cs

@@ -48,6 +48,8 @@ internal static class NativeMethods
         [DllImport("faiss_c", SetLastError = true)]
         public static extern int faiss_read_index_fname(string fname, int io_flags, ref IntPtr p_out);
         [DllImport("faiss_c", SetLastError = true)]
+        public static extern int faiss_write_index_fname(IndexSafeHandle index, string fname);
+        [DllImport("faiss_c", SetLastError = true)]
         public static extern int faiss_read_VectorTransform_fname(string fname, ref IntPtr p_out);
         [DllImport("faiss_c", SetLastError = true)]
         public static extern string faiss_get_last_error();