package router

import (
	"context"
	sync "sync"

	"github.com/xtls/xray-core/features/extension"
	"github.com/xtls/xray-core/features/outbound"
)

type BalancingStrategy interface {
	PickOutbound([]string) string
}

type BalancingPrincipleTarget interface {
	GetPrincipleTarget([]string) []string
}

type RoundRobinStrategy struct {
	mu    sync.Mutex
	index int
}

func (s *RoundRobinStrategy) PickOutbound(tags []string) string {
	n := len(tags)
	if n == 0 {
		panic("0 tags")
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	tag := tags[s.index%n]
	s.index = (s.index + 1) % n
	return tag
}

type Balancer struct {
	selectors   []string
	strategy    BalancingStrategy
	ohm         outbound.Manager
	fallbackTag string

	override override
}

// PickOutbound picks the tag of a outbound
func (b *Balancer) PickOutbound() (string, error) {
	candidates, err := b.SelectOutbounds()
	if err != nil {
		if b.fallbackTag != "" {
			newError("fallback to [", b.fallbackTag, "], due to error: ", err).AtInfo().WriteToLog()
			return b.fallbackTag, nil
		}
		return "", err
	}
	var tag string
	if o := b.override.Get(); o != "" {
		tag = o
	} else {
		tag = b.strategy.PickOutbound(candidates)
	}
	if tag == "" {
		if b.fallbackTag != "" {
			newError("fallback to [", b.fallbackTag, "], due to empty tag returned").AtInfo().WriteToLog()
			return b.fallbackTag, nil
		}
		// will use default handler
		return "", newError("balancing strategy returns empty tag")
	}
	return tag, nil
}

func (b *Balancer) InjectContext(ctx context.Context) {
	if contextReceiver, ok := b.strategy.(extension.ContextReceiver); ok {
		contextReceiver.InjectContext(ctx)
	}
}

// SelectOutbounds select outbounds with selectors of the Balancer
func (b *Balancer) SelectOutbounds() ([]string, error) {
	hs, ok := b.ohm.(outbound.HandlerSelector)
	if !ok {
		return nil, newError("outbound.Manager is not a HandlerSelector")
	}
	tags := hs.Select(b.selectors)
	return tags, nil
}

// GetPrincipleTarget implements routing.BalancerPrincipleTarget
func (r *Router) GetPrincipleTarget(tag string) ([]string, error) {
	if b, ok := r.balancers[tag]; ok {
		if s, ok := b.strategy.(BalancingPrincipleTarget); ok {
			candidates, err := b.SelectOutbounds()
			if err != nil {
				return nil, newError("unable to select outbounds").Base(err)
			}
			return s.GetPrincipleTarget(candidates), nil
		}
		return nil, newError("unsupported GetPrincipleTarget")
	}
	return nil, newError("cannot find tag")
}

// SetOverrideTarget implements routing.BalancerOverrider
func (r *Router) SetOverrideTarget(tag, target string) error {
	if b, ok := r.balancers[tag]; ok {
		b.override.Put(target)
		return nil
	}
	return newError("cannot find tag")
}

// GetOverrideTarget implements routing.BalancerOverrider
func (r *Router) GetOverrideTarget(tag string) (string, error) {
	if b, ok := r.balancers[tag]; ok {
		return b.override.Get(), nil
	}
	return "", newError("cannot find tag")
}