Prevent int overflow at large model sizes

gprMax/grid/fdtd_grid.py

@@ -776,7 +776,7 @@ class FDTDGrid:
         mem_use = 0
 
         for vol in self.fractalvolumes:
-            mem_use += vol.nx * vol.ny * vol.nz * vol.dtype.itemsize
+            mem_use += np.prod(vol.start) * vol.dtype.itemsize
             for surface in vol.fractalsurfaces:
                 surfacedims = surface.get_surface_dims()
                 mem_use += surfacedims[0] * surfacedims[1] * surface.dtype.itemsize

gprMax/model.py

@@ -98,6 +98,10 @@ class Model:
     def nz(self, value: int):
         self.G.nz = value
 
+    @property
+    def cells(self) -> np.int64:
+        return np.prod(self.G.size)
+
     @property
     def dx(self) -> float:
         return self.G.dl[0]

gprMax/user_objects/cmds_singleuse.py

@@ -133,7 +133,7 @@ class Domain(ModelUserObject):
         logger.info(
             f"Domain size: {self.domain_size[0]:g} x {self.domain_size[1]:g} x "
             + f"{self.domain_size[2]:g}m ({model.nx:d} x {model.ny:d} x {model.nz:d} = "
-            + f"{(model.nx * model.ny * model.nz):g} cells)"
+            + f"{(model.cells):g} cells)"
         )
 
         # Set mode and switch off appropriate PMLs for 2D models