// 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")
	}
}