package link

import (
	"context"
	"encoding/json"
	"os"
	"testing"
	"time"

	"github.com/nats-io/nats.go"
)

// **Both implementations, one fixture.** The point of the seam is not that the transport can be
// swapped — it is that the two can be held to the same envelope while both ship, so the day the
// bus moves is a configuration change rather than a discovery.
//
// Against a real server, because what the fixture pins is what reaches the wire:
//
//	docker run -d --rm --name t -p 14222:4222 nats:2.10-alpine -js
//	MESH_TEST_NATS=nats://127.0.0.1:14222 go test ./internal/link/ -run TestTheNatsBus
func TestTheNatsBusEmitsTheEnvelopeTheFixturePins(t *testing.T) {
	url := os.Getenv("MESH_TEST_NATS")
	if url == "" {
		t.Skip("MESH_TEST_NATS unset")
	}
	f := loadFixture(t, "events/module-event.json")

	conn, err := nats.Connect(url)
	if err != nil {
		t.Fatal(err)
	}
	defer conn.Close()
	js, err := conn.JetStream()
	if err != nil {
		t.Fatal(err)
	}
	if _, err := js.AddStream(&nats.StreamConfig{
		Name: "EVENTS", Subjects: []string{"mesh.mod.*.event.>"},
	}); err != nil && err != nats.ErrStreamNameAlreadyInUse {
		t.Fatal(err)
	}

	bus := OverNATS{JS: js}
	body, _ := json.Marshal(f.Given.Body)
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	if err := EmitEvent(ctx, bus, f.Given.Key, f.Given.Module, f.Given.Node, f.Given.Body); err != nil {
		t.Fatal(err)
	}

	// Read back from the stream, not from the thing that wrote it.
	raw, err := js.GetLastMsg("EVENTS", f.Wire.Subject)
	if err != nil {
		t.Fatalf("nothing landed on %s, which the fixture names: %v", f.Wire.Subject, err)
	}
	for _, h := range f.Wire.RequiredHeaders {
		if raw.Header.Get(h) == "" {
			t.Errorf("%s is not set on the wire, and the fixture requires it", h)
		}
	}
	if got := raw.Header.Get("x-source"); got != f.Given.Module {
		t.Errorf("x-source is %q; the subject says %q", got, f.Given.Module)
	}
	if string(raw.Data) != string(body) {
		t.Errorf("the payload is %s, expected the body alone: %s", raw.Data, body)
	}
	// The envelope must not also be nested inside the payload.
	var nested map[string]any
	if json.Unmarshal(raw.Data, &nested) == nil {
		if _, has := nested["key"]; has {
			t.Error("the payload carries the envelope's own fields, which the fixture refuses")
		}
	}
}

// The subject a declaration lands on is one node's, and nothing else's — the state shape.
func TestADeclarationIsAddressedToOneNode(t *testing.T) {
	if got := DeclareSubject("anchor"); got != "mesh.node.anchor.declare" {
		t.Fatalf("a declaration would go to %q", got)
	}
	if DeclareSubject("anchor") == DeclareSubject("laptop") {
		t.Fatal("two nodes share a declaration subject, so each would apply the other's")
	}
}

// A module cannot emit under another's name: the subject is derived from the source, and the
// server's permissions make that subject the authority.
func TestAnEventsSubjectIsDerivedFromItsSource(t *testing.T) {
	if got := EventSubject("shop", "order.placed"); got != "mesh.mod.shop.event.order.placed" {
		t.Fatalf("an event would land on %q", got)
	}
	if EventSubject("shop", "x") == EventSubject("billing", "x") {
		t.Fatal("two modules share an event subject, so neither owns its own name")
	}
}