// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package optim

import (
	"strings"
	"testing"
	"time"

	"sourcedock.dev/petrbalvin/tensor/internal/core"
)

// Callback-contract pins: a gradient callback that returns a shorter
// array than the problem has dimensions used to index past it, and a
// differential-evolution population below four has no way to draw
// three distinct others.

// TestLBFGSGradientLength pins the callback contract.
func TestLBFGSGradientLength(t *testing.T) {
	f := func(p *core.Array) (float64, error) {
		x, y := p.FloatAt(0), p.FloatAt(1)
		return x*x + y*y, nil
	}
	short, err := core.FromFloats([]float64{1}, 1)
	if err != nil {
		t.Fatal(err)
	}
	grad := func(*core.Array) (*core.Array, error) { return short, nil }
	start, err := core.FromFloats([]float64{1, 1}, 2)
	if err != nil {
		t.Fatal(err)
	}
	if _, _, err := MinimiseLBFGS(f, grad, start, LBFGSOptions{}); err == nil {
		t.Fatal("expected an error for a gradient of the wrong length")
	} else if !strings.Contains(err.Error(), "gradient callback") {
		t.Fatalf("error = %v, want a callback-length refusal", err)
	}
}

// TestDifferentialEvolutionPopulationFloor pins the option contract: a
// population below four has no way to draw three distinct others, and
// the search used to spin in the picking loop for ever.
func TestDifferentialEvolutionPopulationFloor(t *testing.T) {
	lo, err := core.FromFloats([]float64{-5}, 1)
	if err != nil {
		t.Fatal(err)
	}
	hi, err := core.FromFloats([]float64{5}, 1)
	if err != nil {
		t.Fatal(err)
	}
	f := func(p *core.Array) (float64, error) { v := p.FloatAt(0); return v * v, nil }
	done := make(chan error, 1)
	go func() {
		_, _, err := MinimiseDifferentialEvolution(f, lo, hi, DifferentialEvolutionOptions{Population: 3, Generations: 5})
		done <- err
	}()
	select {
	case err := <-done:
		if err == nil {
			t.Fatal("expected an error for a population of 3")
		} else if !strings.Contains(err.Error(), "population") {
			t.Fatalf("error = %v, want a population refusal", err)
		}
	case <-time.After(10 * time.Second):
		t.Fatal("MinimiseDifferentialEvolution did not return: the picking loop spins")
	}
	// The floor itself still works.
	if _, _, err := MinimiseDifferentialEvolution(f, lo, hi, DifferentialEvolutionOptions{Population: 4, Generations: 20}); err != nil {
		t.Fatalf("population 4: %v", err)
	}
}