package link

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"time"

	amqp "github.com/rabbitmq/amqp091-go"
)

// Emitting a module event from Go.
//
// **Every event rides one topic exchange** (novox/hq ADR 0042), which is not the direct exchange
// nodes and the control plane speak over. A module that announces something publishes here, and
// consumers bind their own durable queue to a pattern over it.
//
// This exists because the builder is a module written in Go while every other emitter is
// TypeScript on the sdk. The envelope is the sdk's, reproduced exactly: the body is the payload
// alone and everything about the event travels as headers. A second shape would be a second thing
// for consumers to handle, and they are written against the first.
const (
	// EventsExchange is where every event rides. Named here rather than imported from the broker
	// package for the same reason BuildQueueName is duplicated there — one direction of dependency.
	EventsExchange = "mesh.events"
)

// EmitEvent publishes one module event, in the envelope the sdk's consumers expect.
//
// Persistent, because an event that a broker restart loses is not an announcement. The publish is
// not confirmed here: the caller has already done the work the event describes, and a build that
// succeeded must not be reported as failed because saying so failed.
func EmitEvent(ctx context.Context, channel *amqp.Channel, eventType, source, node string, body any) error {
	payload, err := json.Marshal(body)
	if err != nil {
		return fmt.Errorf("cannot serialise a %s event: %w", eventType, err)
	}
	id, err := eventID()
	if err != nil {
		return err
	}
	return channel.PublishWithContext(ctx, EventsExchange, eventType, false, false, amqp.Publishing{
		ContentType:  "application/json",
		DeliveryMode: amqp.Persistent,
		MessageId:    id,
		Timestamp:    time.Now().UTC(),
		Body:         payload,
		Headers: amqp.Table{
			"x-event-id":   id,
			"x-source":     source,
			"x-node":       node,
			"x-time":       time.Now().UTC().Format(time.RFC3339),
			"content-type": "application/json",
		},
	})
}

// eventID is what a consumer deduplicates on: delivery is at-least-once, so a handler must be able
// to tell a redelivery from a second event, and only the emitter can say which it is.
func eventID() (string, error) {
	raw := make([]byte, 16)
	if _, err := rand.Read(raw); err != nil {
		return "", fmt.Errorf("cannot make an event id: %w", err)
	}
	return hex.EncodeToString(raw), nil
}

// KeyModuleBuilt is what the builder announces when it has built something. The catalogue places
// it in the module graph; nothing else need care.
const KeyModuleBuilt = "module.builder.built"

// KeyModuleUpgraded is the catalogue saying a module's current version has moved.
//
// **The control plane hooks the meaning, not the build.** The builder says what it built; the
// catalogue decides whether that was an upgrade — a rebuild producing the commit already current
// is not one — and only this says anything the control plane can act on. Consuming the build
// directly would make the control plane re-derive a decision another module already made, and the
// two would eventually disagree (novox/hq ADR 0072).
const KeyModuleUpgraded = "module.mesh-catalog.upgraded"

// KeyCatchingUp is the catalogue saying it has just started and may have missed things.
//
// **A durable queue only keeps what arrived after it existed.** The catalogue's own queue is
// durable, so nothing is lost once it is running — but the modules built before it first ran were
// announced to a queue that did not exist yet, and on a fresh mesh those are, necessarily, the
// shared base, the store the catalogue runs on, and the catalogue itself. The graph's foundation
// is the part it never hears about (novox/hq 04-ISSUES/050).
//
// So it asks, and the control plane answers with what it recorded. Asking rather than being told
// because only the catalogue knows it has a gap; the control plane cannot tell a fresh catalogue
// from one that is merely quiet.
const KeyCatchingUp = "module.mesh-catalog.catching-up"

// CatchUpQueue is where that lands. Durable, for the same reason the upgrade queue is: a catalogue
// that started while the control plane was restarting is exactly the one with a gap to fill.
const CatchUpQueue = "control.catchup"

// UpgradeQueue is where those land. Durable and named, not a temporary queue: an upgrade announced
// while the control plane is restarting is exactly the one that must not be missed.
const UpgradeQueue = "control.upgrades"

// Replayer answers a catalogue that says it has just started.
//
// It is handed every build the mesh recorded, oldest first, and re-announces each. The catalogue
// registers them as history: a replayed build changed nothing in the world, so announcing it as an
// upgrade would have the mesh act on news that is years old.
type Replayer interface {
	// Announceable is every build worth re-announcing, oldest first.
	//
	// It hands them back rather than publishing them: the wire belongs to this package, and a
	// replay that built its own announcements could drift from what the builder emits — which is
	// the one thing it must match exactly, because the catalogue has a single handler for both.
	Announceable(ctx context.Context) ([]Announcement, error)
}

// Announcement is a build, in the shape the builder announces one.
//
// The field names are the wire's, not Go's, because a catalogue reads these and a rename here is
// an event nobody handles.
type Announcement struct {
	Module     string          `json:"module"`
	Commit     string          `json:"commit"`
	Repository string          `json:"repository"`
	Path       string          `json:"path"`
	Ref        string          `json:"ref"`
	Manifest   json.RawMessage `json:"manifest,omitempty"`
	Against    []string        `json:"against,omitempty"`
	Made       []MadeArtifact  `json:"made,omitempty"`
	// Replay says this is history rather than news: it was built once, and this is the mesh
	// telling a catalogue that missed it. A consumer registers it and announces nothing — an
	// upgrade that happened months ago is not one anything should act on now.
	Replay bool `json:"replay,omitempty"`
}

// Upgraded is what the catalogue says when a module's current version moves.
type Upgraded struct {
	Module   string `json:"module"`
	Commit   string `json:"commit"`
	Previous string `json:"previous"`
}