package cache

import (
	"context"
	"fmt"
	"log/slog"

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

// Subject prefix for cache invalidation events. The full subject is
//
//	cache.invalidate.<entity>.<id>
//
// where <entity> is "org", "node", "token", etc. Writers publish to the
// specific subject after a successful Postgres commit; every node's
// subscriber wildcards the prefix and drops matching cached entries.
const InvalidateSubjectPrefix = "cache.invalidate"

// Invalidator publishes cache-invalidation events across the cluster.
//
// Each writer holds one Invalidator shared across all caches; after a
// successful mutation it calls Emit("org", orgID) and every node sees
// the message within a NATS RTT (typically <1ms in-cluster).
type Invalidator struct {
	nc *nats.Conn
}

// NewInvalidator wraps a connected nats.Conn.
func NewInvalidator(nc *nats.Conn) *Invalidator {
	return &Invalidator{nc: nc}
}

// Emit publishes cache.invalidate.<entity>.<id>. The payload is empty —
// subscribers only need the subject to know which key to drop.
//
// Errors are returned rather than swallowed so the caller can decide
// whether to retry or log. For most write paths a best-effort log-only
// handling is fine because the cache entry will TTL out regardless.
func (i *Invalidator) Emit(entity, id string) error {
	if i == nil || i.nc == nil {
		return nil // no-op in tests / standalone mode
	}
	subject := fmt.Sprintf("%s.%s.%s", InvalidateSubjectPrefix, entity, id)
	return i.nc.Publish(subject, nil)
}

// Subscriber wires a NATS subscription to a cache's Delete method.
//
// The returned subscription must be drained / unsubscribed at shutdown.
type Subscriber interface {
	Unsubscribe() error
}

// WatchEntity starts a subscription that calls onInvalidate(id) for every
// message arriving on cache.invalidate.<entity>.*.
//
// Intended to be called once per cache per node during startup.
func WatchEntity(ctx context.Context, nc *nats.Conn, entity string, onInvalidate func(id string)) (Subscriber, error) {
	subject := fmt.Sprintf("%s.%s.*", InvalidateSubjectPrefix, entity)
	sub, err := nc.Subscribe(subject, func(m *nats.Msg) {
		// Subject is "cache.invalidate.<entity>.<id>" — the ID is the
		// trailing token.
		tokens := splitSubject(m.Subject)
		if len(tokens) < 4 {
			slog.Warn("invalidate: malformed subject", "subject", m.Subject)
			return
		}
		onInvalidate(tokens[len(tokens)-1])
	})
	if err != nil {
		return nil, fmt.Errorf("subscribe %s: %w", subject, err)
	}

	// Tie unsubscription to ctx cancellation so the caller doesn't have
	// to plumb sub.Unsubscribe() manually if they're already managing
	// lifecycles via context.
	go func() {
		<-ctx.Done()
		_ = sub.Unsubscribe()
	}()

	return sub, nil
}

func splitSubject(s string) []string {
	out := make([]string, 0, 4)
	start := 0
	for i := 0; i < len(s); i++ {
		if s[i] == '.' {
			out = append(out, s[start:i])
			start = i + 1
		}
	}
	out = append(out, s[start:])
	return out
}