package utils

import (
	"sync"

	"cloud.o-forge.io/core/oc-lib/tools"
)

// ChangeEvent is fired whenever a DB object is created, updated or deleted
// within this process. Deleted=true means the object was removed; Object is
// the last known snapshot before deletion.
type ChangeEvent struct {
	DataType tools.DataType
	ID       string
	Object   ShallowDBObject // nil only when the load after the write failed
	Deleted  bool
}

var (
	changeBusMu sync.RWMutex
	changeBus   = map[tools.DataType][]chan ChangeEvent{}
)

// SubscribeChanges returns a channel that receives ChangeEvents for dt
// whenever an object of that type is written or deleted in this process.
// Call the returned cancel function to unsubscribe; after that the channel
// will no longer receive events (it is not closed — use a context to stop
// reading).
func SubscribeChanges(dt tools.DataType) (<-chan ChangeEvent, func()) {
	ch := make(chan ChangeEvent, 32)
	changeBusMu.Lock()
	changeBus[dt] = append(changeBus[dt], ch)
	changeBusMu.Unlock()
	return ch, func() {
		changeBusMu.Lock()
		subs := changeBus[dt]
		for i, c := range subs {
			if c == ch {
				changeBus[dt] = append(subs[:i], subs[i+1:]...)
				break
			}
		}
		changeBusMu.Unlock()
	}
}

// NotifyChange broadcasts a ChangeEvent to all current subscribers for dt.
// Non-blocking: events are dropped for subscribers whose buffer is full.
func NotifyChange(dt tools.DataType, id string, obj ShallowDBObject, deleted bool) {
	changeBusMu.RLock()
	subs := changeBus[dt]
	changeBusMu.RUnlock()
	evt := ChangeEvent{DataType: dt, ID: id, Object: obj, Deleted: deleted}
	for _, ch := range subs {
		select {
		case ch <- evt:
		default:
		}
	}
}