add tsection type interfaces

This interface combines grids which do not have clean interfaces.

The type of interface has not been tested or verified here.

multigrid/examples/tsection.json

@@ -0,0 +1,48 @@
+{
+    "grids": {
+        "default": {
+            "operators": {
+                "xi": "upwind9",
+                "eta": "upwind9"
+            }
+        },
+        "grid0": {
+            "x": "linspace:-5:5:101",
+            "y": "linspace:0:5:50",
+            "boundary_conditions": {
+                "south": "multi:grid1(0,61):grid2(0,41)",
+                "north": "vortex",
+                "east": "vortex",
+                "west": "vortex"
+            }
+        },
+        "grid1": {
+            "x": "linspace:-5:1:61",
+            "y": "linspace:-5:0:50",
+            "boundary_conditions": {
+                "south": "vortex",
+                "north": "multi:grid0(0,61)",
+                "east": "grid2",
+                "west": "vortex"
+            }
+        },
+        "grid2": {
+            "x": "linspace:1:5:41",
+            "y": "linspace:-5:0:50",
+            "boundary_conditions": {
+                "south": "vortex",
+                "north": "multi:grid0(60,101)",
+                "east": "vortex",
+                "west": "grid1"
+            }
+        }
+    },
+    "integration_time": 2.0,
+    "vortex": {
+        "x0": 0.0,
+        "y0": 0.0,
+        "mach": 0.5,
+        "rstar": 0.5,
+        "eps": 1.0
+    }
+}

multigrid/src/parsing.rs

@@ -143,6 +143,27 @@ pub fn json_to_grids(
                     names.iter().position(|other| other == grid).unwrap(),
                     int_op,
                 )
+            } else if let Some(multigrid) = dir.strip_prefix("multi:") {
+                let grids = multigrid.split(":");
+                sbp::euler::BoundaryCharacteristic::MultiGrid(
+                    grids
+                        .map(|g| {
+                            let rparen = g.find('(').unwrap();
+                            let gridname = &g[..rparen];
+
+                            let gridnumber =
+                                names.iter().position(|other| other == gridname).unwrap();
+
+                            let paren = &g[rparen + 1..];
+                            let paren = &paren[..paren.len() - 1];
+                            let mut pareni = paren.split(',');
+                            let start = pareni.next().unwrap().parse::<usize>().unwrap();
+                            let end = pareni.next().unwrap().parse::<usize>().unwrap();
+
+                            (gridnumber, start, end)
+                        })
+                        .collect::<Vec<_>>(),
+                )
             } else {
                 sbp::euler::BoundaryCharacteristic::Grid(
                     names.iter().position(|other| other == dir).unwrap(),