Fix issue with writeVtk and add file writing test

zigsdof/src/stl.zig

@@ -125,8 +125,8 @@ pub fn writeVtk(allocator: std.mem.Allocator, io: std.Io, mesh: *Mesh, file_path
     const file = try cwd.openFile(io, file_path, .{ .mode = .read_write });
     defer file.close(io);
 
-    const buf: [266]u8 = undefined;
+    var buf: [256]u8 = undefined;
-    const writer = file.writer(io, buf);
+    var writer = file.writer(io, &buf);
     try writer.interface.writeAll(out_string);
     try writer.interface.flush();
     return;
@@ -364,3 +364,35 @@ test "vtk string is written correctly" {
 
     try std.testing.expect(std.mem.eql(u8, vtk_string, expected));
 }
+
+test "writeVtk writes the correct file contents" {
+    const allocator = std.testing.allocator;
+    const io = std.testing.io;
+
+    var mesh = try createDummyMesh(allocator);
+    defer mesh.deinit(allocator);
+
+    const file_path = "/tmp/zig_write_vtk_test.vtk";
+    // writeVtk opens the file in read_write mode, so it must exist first.
+    {
+        const cwd = std.Io.Dir.cwd();
+        const file = try cwd.createFile(io, file_path, .{});
+        defer file.close(io);
+    }
+
+    try writeVtk(allocator, io, &mesh, file_path);
+
+    // Read the file back and compare it to the expected VTK string.
+    const cwd = std.Io.Dir.cwd();
+    const file = try cwd.openFile(io, file_path, .{ .mode = .read_only });
+    defer file.close(io);
+
+    var reader = file.reader(io, &.{});
+    const contents = try reader.interface.allocRemaining(allocator, .unlimited);
+    defer allocator.free(contents);
+
+    const expected = try createVtkString(allocator, &mesh);
+    defer allocator.free(expected);
+
+    try std.testing.expect(std.mem.eql(u8, contents, expected));
+}