feat: initial release

Assisted-by: GLM 5.3 Flash

grad/hessian_nil_guard_test.go

@@ -0,0 +1,40 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package grad
+
+import (
+	"strings"
+	"testing"
+)
+
+// Regression tests: an objective whose graph
+// never reaches x left xt.Grad() nil, and the second-order helpers
+// dereferenced it.
+
+// TestHessianDisconnectedObjective pins the error: an objective that
+// ignores its argument has no gradient to differentiate.
+func TestHessianDisconnectedObjective(t *testing.T) {
+	x, err := FromFloat64s([]float64{1, 2}, true, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c, err := FromFloat64s([]float64{3}, true, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	objective := func(*Tensor) (*Tensor, error) { return c.Mul(c) }
+
+	if _, err := Hessian(objective, x, HessianOptions{}); err == nil {
+		t.Fatal("expected an error when the objective does not depend on x")
+	} else if !strings.Contains(err.Error(), "does not depend") {
+		t.Fatalf("error = %v, want a disconnected-graph refusal", err)
+	}
+	v, err := FromFloat64s([]float64{1, 0}, true, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := HessianVectorProduct(objective, x, v, HessianOptions{}); err == nil {
+		t.Fatal("expected an error when the objective does not depend on x")
+	}
+}