Write and read mesh using ADIOS2

cpp/demo/checkpointing/main.cpp

@@ -40,6 +40,13 @@ int main(int argc, char* argv[])
     adios2::Engine engine = io.Open("mesh.bp", adios2::Mode::Append);
 
     io::native::write_mesh(io, engine, *mesh);
+    std::span<float> x = mesh->geometry().x();
+    for (std::size_t i = 0; i < x.size(); ++i)
+    {
+      x[i] *= 4;
+    }
+
+    io::native::write_mesh(io, engine, *mesh, 0.5);
 
     engine.Close();
   }
@@ -51,28 +58,71 @@ int main(int argc, char* argv[])
 
   try
   {
-    // Set up ADIOS2 IO and Engine
+    // Read mode : set up ADIOS2 IO and Engine
     adios2::ADIOS adios_read(MPI_COMM_WORLD);
     adios2::IO io_read = adios_read.DeclareIO("mesh-read");
     io_read.SetEngine("BP5");
     adios2::Engine engine_read = io_read.Open("mesh.bp", adios2::Mode::Read);
 
+    // ReadRandomAccess mode : set up ADIOS2 IO and Engine
+    adios2::IO io_rra = adios_read.DeclareIO("mesh-rra");
+    io_rra.SetEngine("BP5");
+    adios2::Engine engine_rra
+        = io_rra.Open("mesh.bp", adios2::Mode::ReadRandomAccess);
+
+    // Write mode : set up ADIOS2 IO and Engine
+    adios2::IO io_write = adios_read.DeclareIO("mesh-write");
+    io_write.SetEngine("BP5");
+    adios2::Engine engine_write
+        = io_write.Open("mesh2.bp", adios2::Mode::Write);
+
+    // Find the time stamps array
+    auto var_time = io_rra.InquireVariable<double>("time");
+    const std::vector<std::vector<adios2::Variable<double>::Info>> timestepsinfo
+        = var_time.AllStepsBlocksInfo();
+
+    std::size_t num_steps = timestepsinfo.size();
+    std::vector<double> times(num_steps);
+
+    for (std::size_t step = 0; step < num_steps; ++step)
+    {
+      var_time.SetStepSelection({step, 1});
+      engine_rra.Get(var_time, times[step]);
+    }
+    engine_rra.Close();
+
+    // Read mesh
     engine_read.BeginStep();
     auto mesh_read
         = io::native::read_mesh<float>(io_read, engine_read, MPI_COMM_WORLD);
     if (engine_read.BetweenStepPairs())
     {
       engine_read.EndStep();
     }
+    // Write mesh
+    io::native::write_mesh(io_write, engine_write, mesh_read);
 
-    engine_read.Close();
+    // Update mesh
+    double time = 0.5;
+    std::size_t querystep;
+    auto pos = std::ranges::find(times, time);
+    if (pos != times.end())
+    {
+      querystep = std::ranges::distance(times.begin(), pos);
+      std::cout << "Query step is : " << querystep << "\n";
+    }
+    else
+    {
+      throw std::runtime_error("Step corresponding to time : "
+                               + std::to_string(time) + " not found");
+    }
 
-    adios2::IO io_write = adios_read.DeclareIO("mesh-write");
-    io_write.SetEngine("BP5");
-    adios2::Engine engine_write
-        = io_write.Open("mesh2.bp", adios2::Mode::Write);
+    io::native::update_mesh<float>(io_read, engine_read, mesh_read, querystep);
 
-    io::native::write_mesh(io_write, engine_write, mesh_read);
+    // Write updated mesh
+    io::native::write_mesh(io_write, engine_write, mesh_read, time);
+
+    engine_read.Close();
     engine_write.Close();
   }
   catch (std::exception& e)