// Package notify is the backend's in-process live-event seam. Domain services
// publish Intents after a successful commit; the gRPC push server (internal
// /pushgrpc) subscribes to the hub and streams them to the gateway, which fans
// them out to clients (docs/ARCHITECTURE.md §10). Event payloads are
// FlatBuffers-encoded by the typed constructors in events.go, so the domain
// services stay free of the wire schema and only depend on this package.
//
// Publishing is best-effort and non-blocking: a live event is a convenience, not
// a correctness requirement, so a slow or absent subscriber never blocks a game
// transition. The default Publisher is Nop, which keeps every domain service (and
// its tests) runnable without a live channel.
package notify

import (
	"sync"

	"github.com/google/uuid"
)

// Notification kinds — the catalog in docs/ARCHITECTURE.md §10.
const (
	KindYourTurn      = "your_turn"
	KindOpponentMoved = "opponent_moved"
	KindChatMessage   = "chat_message"
	KindNudge         = "nudge"
	KindMatchFound    = "match_found"
)

// Intent is one live event destined for a single user. Payload is the
// FlatBuffers-encoded body (a scrabblefb.* table) that the gateway forwards
// verbatim to the client; EventID is a correlation id carried through unchanged.
type Intent struct {
	UserID  uuid.UUID
	Kind    string
	Payload []byte
	EventID string
}

// Publisher accepts live-event intents. Implementations must be safe for
// concurrent use and must not block the caller.
type Publisher interface {
	Publish(intents ...Intent)
}

// Nop is the default Publisher: it discards every intent.
type Nop struct{}

// Publish discards the intents.
func (Nop) Publish(...Intent) {}

// Hub is the in-process fan-in/fan-out between the domain publishers and the
// push subscribers (the gRPC stream). It is safe for concurrent use.
type Hub struct {
	mu      sync.Mutex
	subs    map[int]chan Intent
	nextID  int
	bufSize int
}

// defaultBuffer is the per-subscriber queue depth used when NewHub is given a
// non-positive size.
const defaultBuffer = 256

// NewHub returns a Hub whose per-subscriber buffer holds bufSize intents before
// dropping (a slow subscriber never blocks a publisher).
func NewHub(bufSize int) *Hub {
	if bufSize <= 0 {
		bufSize = defaultBuffer
	}
	return &Hub{subs: make(map[int]chan Intent), bufSize: bufSize}
}

// Publish delivers each intent to every current subscriber, dropping it for any
// subscriber whose buffer is full (best-effort live delivery).
func (h *Hub) Publish(intents ...Intent) {
	h.mu.Lock()
	defer h.mu.Unlock()
	for _, in := range intents {
		for _, ch := range h.subs {
			select {
			case ch <- in:
			default:
			}
		}
	}
}

// Subscribe registers a new subscriber and returns its intent channel and an
// unsubscribe func that closes the channel. The caller reads the channel until
// it is closed or its own context ends, then calls unsubscribe.
func (h *Hub) Subscribe() (<-chan Intent, func()) {
	h.mu.Lock()
	defer h.mu.Unlock()
	id := h.nextID
	h.nextID++
	ch := make(chan Intent, h.bufSize)
	h.subs[id] = ch
	return ch, func() { h.unsubscribe(id) }
}

// unsubscribe removes and closes the subscriber's channel. It holds the same
// lock as Publish, so it never closes a channel mid-send.
func (h *Hub) unsubscribe(id int) {
	h.mu.Lock()
	defer h.mu.Unlock()
	if ch, ok := h.subs[id]; ok {
		delete(h.subs, id)
		close(ch)
	}
}