feat: initial release

Assisted-by: GLM 5.3 Flash

grad/hvp_shape_pin_test.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package grad
+
+import "testing"
+
+// TestHessianVectorProductZeroDirectionShape pins the repair: a zero
+// direction returned a flat length-n vector while
+// every other answer carries the point's own shape.
+func TestHessianVectorProductZeroDirectionShape(t *testing.T) {
+	x0 := []float64{0.5, -1.25, 2, -2}
+	xt, err := FromFloat64s(x0, false, 2, 2)
+	if err != nil {
+		t.Fatalf("FromFloat64s: %v", err)
+	}
+	f := func(z *Tensor) (*Tensor, error) { return z.Sum() }
+	v, err := FromFloat64s(make([]float64, 4), false, 2, 2)
+	if err != nil {
+		t.Fatalf("FromFloat64s: %v", err)
+	}
+	hv, err := HessianVectorProduct(f, xt, v, HessianOptions{})
+	if err != nil {
+		t.Fatalf("HessianVectorProduct: %v", err)
+	}
+	want := []int{2, 2}
+	got := hv.Shape()
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("H·0 shape = %v, want %v", got, want)
+		}
+	}
+	for i := range 4 {
+		if hv.FloatAt(i) != 0 {
+			t.Fatalf("H·0 = %v, want the zero matrix", hv.FloatAt(i))
+		}
+	}
+}