package ultraviolet

import (
	"context"
	"fmt"
	"sync"
)

type ValueReactive struct {
	ctxGlobal context.Context
	ctxLocal  context.Context
	cancel    func()
	waitGroup sync.WaitGroup
	source    *Source
	out       <-chan any
	selector  Selector
	value     any
	change    bool
}

func NewValueReactive(ctxGlobal context.Context, source *Source, selector Selector) *ValueReactive {
	ctxLocal, cancel := context.WithCancel(context.Background())
	return &ValueReactive{
		ctxGlobal: ctxGlobal,
		ctxLocal:  ctxLocal,
		cancel:    cancel,
		source:    source,
		selector:  selector,
		value:     nil,
		change:    true,
	}
}

func (this *ValueReactive) Start() error {
	this.out = this.source.Subscribe(this)
	this.waitGroup.Add(1)
	go this.worker()
	return nil
}

func (this *ValueReactive) Stop() error {
	err := this.source.Unsubscribe(this)
	if err != nil {
		return fmt.Errorf("can't stop reactive value: %w", err)
	}

	this.cancel()
	this.waitGroup.Wait()
	return nil
}

func (this *ValueReactive) Value() any {
	return this.value
}

func (this *ValueReactive) Invalidate() bool {
	result := this.change
	this.change = false
	return result
}

func (this *ValueReactive) worker() {
	defer this.waitGroup.Done()

	for {
		select {
		case <-this.ctxLocal.Done():
			return
		case event, ok := <-this.out:
			if !ok {
				return
			}

			value := this.selector(event).Value()
			if this.value != value {
				this.value = value
				this.change = true
			}
		}
	}
}