package desired

import (
	"context"
	"errors"
	"io"
	"log/slog"
	"testing"

	"gitea.dooplex.hu/admin/felhom-agent/internal/hub"
	"gitea.dooplex.hu/admin/felhom-agent/internal/reconcile"
)

func quiet() *slog.Logger { return slog.New(slog.NewTextHandler(io.Discard, nil)) }

// fakeFetcher counts FetchDesiredState calls and returns a fixed response (or error).
type fakeFetcher struct {
	resp  *hub.DesiredStateResponse
	err   error
	calls int
}

func (f *fakeFetcher) FetchDesiredState(context.Context) (*hub.DesiredStateResponse, error) {
	f.calls++
	return f.resp, f.err
}

func env(gen int64, signed bool) *hub.ControlEnvelope {
	return &hub.ControlEnvelope{DesiredGeneration: gen, HasSignedOps: signed}
}

// The headline caching behaviour: desired-state is fetched ONCE when the generation advances, and
// NOT re-fetched while the generation is unchanged (the heartbeat stays light).
func TestSyncer_FetchesOnceOnGenerationAdvance(t *testing.T) {
	run := "running"
	f := &fakeFetcher{resp: &hub.DesiredStateResponse{
		Generation: 1,
		DesiredState: hub.WireDesiredState{Guests: []hub.WireDesiredGuest{
			{VMID: 100, Run: run},
			{VMID: 200, Decommission: true},
		}},
	}}
	p := reconcile.NewCachingProvider()
	s := NewSyncer(f, p, quiet())
	ctx := context.Background()

	// Generation 0 (fresh host, no intent) → NO fetch.
	s.OnEnvelope(ctx, env(0, false))
	if f.calls != 0 {
		t.Fatalf("fetched %d times at generation 0, want 0", f.calls)
	}

	// Generation advances to 1 → fetch exactly once, cache updated.
	s.OnEnvelope(ctx, env(1, false))
	if f.calls != 1 {
		t.Fatalf("fetched %d times on advance, want 1", f.calls)
	}
	if p.Generation() != 1 {
		t.Errorf("cached generation = %d, want 1", p.Generation())
	}
	st, _ := p.Desired(ctx)
	if st.Guests[100].Run != reconcile.RunRunning {
		t.Errorf("guest 100 run = %q, want running", st.Guests[100].Run)
	}
	if !st.Guests[200].Decommission {
		t.Errorf("guest 200 decommission = false, want true")
	}

	// Same generation on the next heartbeats → NO re-fetch (cached).
	s.OnEnvelope(ctx, env(1, false))
	s.OnEnvelope(ctx, env(1, false))
	if f.calls != 1 {
		t.Errorf("re-fetched on an unchanged generation (calls=%d, want 1)", f.calls)
	}

	// A further advance → one more fetch.
	f.resp = &hub.DesiredStateResponse{Generation: 2, DesiredState: hub.WireDesiredState{}}
	s.OnEnvelope(ctx, env(2, false))
	if f.calls != 2 || p.Generation() != 2 {
		t.Errorf("second advance: calls=%d gen=%d, want 2/2", f.calls, p.Generation())
	}
}

// A fetch failure keeps the last-known cache (the engine keeps reconciling toward it) and does not
// advance the cached generation (so the next heartbeat retries).
func TestSyncer_FetchFailureKeepsCache(t *testing.T) {
	p := reconcile.NewCachingProvider()
	p.Update(1, reconcile.DesiredState{Guests: map[int]reconcile.DesiredGuest{100: {VMID: 100, Run: reconcile.RunRunning}}})
	f := &fakeFetcher{err: errors.New("hub down")}
	s := NewSyncer(f, p, quiet())

	s.OnEnvelope(context.Background(), env(5, false)) // generation jumped, but fetch fails
	if p.Generation() != 1 {
		t.Errorf("generation advanced to %d despite fetch failure, want kept at 1", p.Generation())
	}
	st, _ := p.Desired(context.Background())
	if st.Guests[100].Run != reconcile.RunRunning {
		t.Errorf("cache lost on fetch failure: %+v", st.Guests)
	}
}

// The fetched generation (not the envelope's) is what gets cached — robust to a generation that
// advanced again between the heartbeat and the fetch.
func TestSyncer_CachesFetchedGeneration(t *testing.T) {
	f := &fakeFetcher{resp: &hub.DesiredStateResponse{Generation: 7, DesiredState: hub.WireDesiredState{}}}
	p := reconcile.NewCachingProvider()
	s := NewSyncer(f, p, quiet())
	s.OnEnvelope(context.Background(), env(5, false)) // envelope said 5, fetch returned 7
	if p.Generation() != 7 {
		t.Errorf("cached generation = %d, want 7 (the fetched generation)", p.Generation())
	}
	// A later envelope at generation 6 must NOT trigger a re-fetch (we already have 7).
	s.OnEnvelope(context.Background(), env(6, false))
	if f.calls != 1 {
		t.Errorf("re-fetched at generation 6 when cache is 7 (calls=%d)", f.calls)
	}
}