Let the Cholesky decomposition fail gracefully

app-proto/src/engine.rs

@@ -490,13 +490,22 @@ pub fn realize_gram(
         if state.loss < tol { break; }
         
         // compute the Newton step
+        /* TO DO */
         /*
-          we need to either handle or eliminate the case where the minimum
-          eigenvalue of the Hessian is zero, so the regularized Hessian is
-          singular. right now, this causes the Cholesky decomposition to return
-          `None`, leading to a panic when we unrap
+          we should change our regularization to ensure that the Hessian is
+          is positive-definite, rather than just positive-semidefinite. ideally,
+          that would guarantee the success of the Cholesky decomposition---
+          although we'd still need the error-handling routine in case of
+          numerical hiccups
         */
-        let base_step_stacked = hess.clone().cholesky().unwrap().solve(&neg_grad_stacked);
+        let hess_cholesky = match hess.clone().cholesky() {
+            Some(cholesky) => cholesky,
+            None => return RealizationResult {
+                result: Err("Cholesky decomposition failed".to_string()),
+                history
+            }
+        };
+        let base_step_stacked = hess_cholesky.solve(&neg_grad_stacked);
         let base_step = base_step_stacked.reshape_generic(Dyn(element_dim), Dyn(assembly_dim));
         history.base_step.push(base_step.clone());