// 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))
		}
	}
}