package pubsub

import (
	"context"
	"fmt"
	"runtime"
	"time"

	"github.com/libp2p/go-libp2p-core/peer"
)

const (
	defaultValidateConcurrency = 1024
	defaultValidateThrottle    = 8192
)

// Validator is a function that validates a message.
type Validator func(context.Context, peer.ID, *Message) bool

// ValidatorOpt is an option for RegisterTopicValidator.
type ValidatorOpt func(addVal *addValReq) error

// validation represents the validator pipeline.
// The validator pipeline performs signature validation and runs a
// sequence of user-configured validators per-topic. It is possible to
// adjust various concurrency parameters, such as the number of
// workers and the max number of simultaneous validations. The user
// can also attach inline validators that will be executed
// synchronously; this may be useful to prevent superfluous
// context-switching for lightweight tasks.
type validation struct {
	p *PubSub

	tracer *pubsubTracer

	// topicVals tracks per topic validators
	topicVals map[string]*topicVal

	// validateQ is the front-end to the validation pipeline
	validateQ chan *validateReq

	// validateThrottle limits the number of active validation goroutines
	validateThrottle chan struct{}

	// this is the number of synchronous validation workers
	validateWorkers int
}

// validation requests
type validateReq struct {
	vals []*topicVal
	src  peer.ID
	msg  *Message
}

// representation of topic validators
type topicVal struct {
	topic            string
	validate         Validator
	validateTimeout  time.Duration
	validateThrottle chan struct{}
	validateInline   bool
}

// async request to add a topic validators
type addValReq struct {
	topic    string
	validate Validator
	timeout  time.Duration
	throttle int
	inline   bool
	resp     chan error
}

// async request to remove a topic validator
type rmValReq struct {
	topic string
	resp  chan error
}

// newValidation creates a new validation pipeline
func newValidation() *validation {
	return &validation{
		topicVals:        make(map[string]*topicVal),
		validateQ:        make(chan *validateReq, 32),
		validateThrottle: make(chan struct{}, defaultValidateThrottle),
		validateWorkers:  runtime.NumCPU(),
	}
}

// Start attaches the validation pipeline to a pubsub instance and starts background
// workers
func (v *validation) Start(p *PubSub) {
	v.p = p
	v.tracer = p.tracer
	for i := 0; i < v.validateWorkers; i++ {
		go v.validateWorker()
	}
}

// AddValidator adds a new validator
func (v *validation) AddValidator(req *addValReq) {
	topic := req.topic

	_, ok := v.topicVals[topic]
	if ok {
		req.resp <- fmt.Errorf("Duplicate validator for topic %s", topic)
		return
	}

	val := &topicVal{
		topic:            topic,
		validate:         req.validate,
		validateTimeout:  0,
		validateThrottle: make(chan struct{}, defaultValidateConcurrency),
		validateInline:   req.inline,
	}

	if req.timeout > 0 {
		val.validateTimeout = req.timeout
	}

	if req.throttle > 0 {
		val.validateThrottle = make(chan struct{}, req.throttle)
	}

	v.topicVals[topic] = val
	req.resp <- nil
}

// RemoveValidator removes an existing validator
func (v *validation) RemoveValidator(req *rmValReq) {
	topic := req.topic

	_, ok := v.topicVals[topic]
	if ok {
		delete(v.topicVals, topic)
		req.resp <- nil
	} else {
		req.resp <- fmt.Errorf("No validator for topic %s", topic)
	}
}

// Push pushes a message into the validation pipeline.
// It returns true if the message can be forwarded immediately without validation.
func (v *validation) Push(src peer.ID, msg *Message) bool {
	vals := v.getValidators(msg)

	if len(vals) > 0 || msg.Signature != nil {
		select {
		case v.validateQ <- &validateReq{vals, src, msg}:
		default:
			log.Warningf("message validation throttled; dropping message from %s", src)
			v.tracer.RejectMessage(msg, "validation throttled")
		}
		return false
	}

	return true
}

// getValidators returns all validators that apply to a given message
func (v *validation) getValidators(msg *Message) []*topicVal {
	var vals []*topicVal

	for _, topic := range msg.GetTopicIDs() {
		val, ok := v.topicVals[topic]
		if !ok {
			continue
		}

		vals = append(vals, val)
	}

	return vals
}

// validateWorker is an active goroutine performing inline validation
func (v *validation) validateWorker() {
	for {
		select {
		case req := <-v.validateQ:
			v.validate(req.vals, req.src, req.msg)
		case <-v.p.ctx.Done():
			return
		}
	}
}

// validate performs validation and only sends the message if all validators succeed
// signature validation is performed synchronously, while user validators are invoked
// asynchronously, throttled by the global validation throttle.
func (v *validation) validate(vals []*topicVal, src peer.ID, msg *Message) {
	if msg.Signature != nil {
		if !v.validateSignature(msg) {
			log.Warningf("message signature validation failed; dropping message from %s", src)
			v.tracer.RejectMessage(msg, "invalid signature")
			return
		}
	}

	// we can mark the message as seen now that we have verified the signature
	// and avoid invoking user validators more than once
	id := msgID(msg.Message)
	if !v.p.markSeen(id) {
		v.tracer.DuplicateMessage(msg)
		return
	}

	var inline, async []*topicVal
	for _, val := range vals {
		if val.validateInline {
			inline = append(inline, val)
		} else {
			async = append(async, val)
		}
	}

	// apply inline (synchronous) validators
	for _, val := range inline {
		if !val.validateMsg(v.p.ctx, src, msg) {
			log.Debugf("message validation failed; dropping message from %s", src)
			v.tracer.RejectMessage(msg, "validation failed")
			return
		}
	}

	// apply async validators
	if len(async) > 0 {
		select {
		case v.validateThrottle <- struct{}{}:
			go func() {
				v.doValidateTopic(async, src, msg)
				<-v.validateThrottle
			}()
		default:
			log.Warningf("message validation throttled; dropping message from %s", src)
			v.tracer.RejectMessage(msg, "validation throttled")
		}
		return
	}

	// no async validators, send the message
	v.p.sendMsg <- msg
}

func (v *validation) validateSignature(msg *Message) bool {
	err := verifyMessageSignature(msg.Message)
	if err != nil {
		log.Debugf("signature verification error: %s", err.Error())
		return false
	}

	return true
}

func (v *validation) doValidateTopic(vals []*topicVal, src peer.ID, msg *Message) {
	if !v.validateTopic(vals, src, msg) {
		log.Warningf("message validation failed; dropping message from %s", src)
		v.tracer.RejectMessage(msg, "validation failed")
		return
	}

	v.p.sendMsg <- msg
}

func (v *validation) validateTopic(vals []*topicVal, src peer.ID, msg *Message) bool {
	if len(vals) == 1 {
		return v.validateSingleTopic(vals[0], src, msg)
	}

	ctx, cancel := context.WithCancel(v.p.ctx)
	defer cancel()

	rch := make(chan bool, len(vals))
	rcount := 0
	throttle := false

loop:
	for _, val := range vals {
		rcount++

		select {
		case val.validateThrottle <- struct{}{}:
			go func(val *topicVal) {
				rch <- val.validateMsg(ctx, src, msg)
				<-val.validateThrottle
			}(val)

		default:
			log.Debugf("validation throttled for topic %s", val.topic)
			throttle = true
			break loop
		}
	}

	if throttle {
		v.tracer.RejectMessage(msg, "validation throttled")
		return false
	}

	for i := 0; i < rcount; i++ {
		valid := <-rch
		if !valid {
			return false
		}
	}

	return true
}

// fast path for single topic validation that avoids the extra goroutine
func (v *validation) validateSingleTopic(val *topicVal, src peer.ID, msg *Message) bool {
	select {
	case val.validateThrottle <- struct{}{}:
		res := val.validateMsg(v.p.ctx, src, msg)
		<-val.validateThrottle

		return res

	default:
		log.Debugf("validation throttled for topic %s", val.topic)
		v.tracer.RejectMessage(msg, "validation throttled")
		return false
	}
}

func (val *topicVal) validateMsg(ctx context.Context, src peer.ID, msg *Message) bool {
	if val.validateTimeout > 0 {
		var cancel func()
		ctx, cancel = context.WithTimeout(ctx, val.validateTimeout)
		defer cancel()
	}

	valid := val.validate(ctx, src, msg)
	if !valid {
		log.Debugf("validation failed for topic %s", val.topic)
	}

	return valid
}

/// Options

// WithValidateThrottle sets the upper bound on the number of active validation
// goroutines across all topics. The default is 8192.
func WithValidateThrottle(n int) Option {
	return func(ps *PubSub) error {
		ps.val.validateThrottle = make(chan struct{}, n)
		return nil
	}
}

// WithValidateWorkers sets the number of synchronous validation worker goroutines.
// Defaults to NumCPU.
//
// The synchronous validation workers perform signature validation, apply inline
// user validators, and schedule asynchronous user validators.
// You can adjust this parameter to devote less cpu time to synchronous validation.
func WithValidateWorkers(n int) Option {
	return func(ps *PubSub) error {
		if n > 0 {
			ps.val.validateWorkers = n
			return nil
		}
		return fmt.Errorf("number of validation workers must be > 0")
	}
}

// WithValidatorTimeout is an option that sets a timeout for an (asynchronous) topic validator.
// By default there is no timeout in asynchronous validators.
func WithValidatorTimeout(timeout time.Duration) ValidatorOpt {
	return func(addVal *addValReq) error {
		addVal.timeout = timeout
		return nil
	}
}

// WithValidatorConcurrency is an option that sets the topic validator throttle.
// This controls the number of active validation goroutines for the topic; the default is 1024.
func WithValidatorConcurrency(n int) ValidatorOpt {
	return func(addVal *addValReq) error {
		addVal.throttle = n
		return nil
	}
}

// WithValidatorInline is an option that sets the validation disposition to synchronous:
// it will be executed inline in validation front-end, without spawning a new goroutine.
// This is suitable for simple or cpu-bound validators that do not block.
func WithValidatorInline(inline bool) ValidatorOpt {
	return func(addVal *addValReq) error {
		addVal.inline = inline
		return nil
	}
}