package htlcswitch

import (
	"container/list"
	"errors"
	"sync"
	"sync/atomic"
	"time"

	"github.com/lightningnetwork/lnd/lnwire"
)

// ErrMailBoxShuttingDown is returned when the mailbox is interrupted by a
// shutdown request.
var ErrMailBoxShuttingDown = errors.New("mailbox is shutting down")

// MailBox is an interface which represents a concurrent-safe, in-order
// delivery queue for messages from the network and also from the main switch.
// This struct servers as a buffer between incoming messages, and messages to
// the handled by the link. Each of the mutating methods within this interface
// should be implemented in a non-blocking manner.
type MailBox interface {
	// AddMessage appends a new message to the end of the message queue.
	AddMessage(msg lnwire.Message) error

	// AddPacket appends a new message to the end of the packet queue.
	AddPacket(pkt *htlcPacket) error

	// HasPacket queries the packets for a circuit key, this is used to drop
	// packets bound for the switch that already have a queued response.
	HasPacket(CircuitKey) bool

	// AckPacket removes a packet from the mailboxes in-memory replay
	// buffer. This will prevent a packet from being delivered after a link
	// restarts if the switch has remained online.
	AckPacket(CircuitKey) error

	// MessageOutBox returns a channel that any new messages ready for
	// delivery will be sent on.
	MessageOutBox() chan lnwire.Message

	// PacketOutBox returns a channel that any new packets ready for
	// delivery will be sent on.
	PacketOutBox() chan *htlcPacket

	// Clears any pending wire messages from the inbox.
	ResetMessages() error

	// Reset the packet head to point at the first element in the list.
	ResetPackets() error

	// Start starts the mailbox and any goroutines it needs to operate
	// properly.
	Start() error

	// Stop signals the mailbox and its goroutines for a graceful shutdown.
	Stop() error
}

// memoryMailBox is an implementation of the MailBox struct backed by purely
// in-memory queues.
type memoryMailBox struct {
	started uint32
	stopped uint32

	wireMessages *list.List
	wireHead     *list.Element
	wireMtx      sync.Mutex
	wireCond     *sync.Cond

	messageOutbox chan lnwire.Message
	msgReset      chan chan struct{}

	htlcPkts *list.List
	pktIndex map[CircuitKey]*list.Element
	pktHead  *list.Element
	pktMtx   sync.Mutex
	pktCond  *sync.Cond

	pktOutbox chan *htlcPacket
	pktReset  chan chan struct{}

	wg   sync.WaitGroup
	quit chan struct{}
}

// newMemoryMailBox creates a new instance of the memoryMailBox.
func newMemoryMailBox() *memoryMailBox {
	box := &memoryMailBox{
		wireMessages:  list.New(),
		htlcPkts:      list.New(),
		messageOutbox: make(chan lnwire.Message),
		pktOutbox:     make(chan *htlcPacket),
		msgReset:      make(chan chan struct{}, 1),
		pktReset:      make(chan chan struct{}, 1),
		pktIndex:      make(map[CircuitKey]*list.Element),
		quit:          make(chan struct{}),
	}
	box.wireCond = sync.NewCond(&box.wireMtx)
	box.pktCond = sync.NewCond(&box.pktMtx)

	return box
}

// A compile time assertion to ensure that memoryMailBox meets the MailBox
// interface.
var _ MailBox = (*memoryMailBox)(nil)

// courierType is an enum that reflects the distinct types of messages a
// MailBox can handle. Each type will be placed in an isolated mail box and
// will have a dedicated goroutine for delivering the messages.
type courierType uint8

const (
	// wireCourier is a type of courier that handles wire messages.
	wireCourier courierType = iota

	// pktCourier is a type of courier that handles htlc packets.
	pktCourier
)

// Start starts the mailbox and any goroutines it needs to operate properly.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) Start() error {
	if !atomic.CompareAndSwapUint32(&m.started, 0, 1) {
		return nil
	}

	m.wg.Add(2)
	go m.mailCourier(wireCourier)
	go m.mailCourier(pktCourier)

	return nil
}

// ResetMessages blocks until all buffered wire messages are cleared.
func (m *memoryMailBox) ResetMessages() error {
	msgDone := make(chan struct{})
	select {
	case m.msgReset <- msgDone:
		return m.signalUntilReset(wireCourier, msgDone)
	case <-m.quit:
		return ErrMailBoxShuttingDown
	}
}

// ResetPackets blocks until the head of packets buffer is reset, causing the
// packets to be redelivered in order.
func (m *memoryMailBox) ResetPackets() error {
	pktDone := make(chan struct{})
	select {
	case m.pktReset <- pktDone:
		return m.signalUntilReset(pktCourier, pktDone)
	case <-m.quit:
		return ErrMailBoxShuttingDown
	}
}

// signalUntilReset strobes the condition variable for the specified inbox type
// until receiving a response that the mailbox has processed a reset.
func (m *memoryMailBox) signalUntilReset(cType courierType,
	done chan struct{}) error {

	for {
		switch cType {
		case wireCourier:
			m.wireCond.Signal()
		case pktCourier:
			m.pktCond.Signal()
		}

		select {
		case <-time.After(time.Millisecond):
			continue
		case <-done:
			return nil
		case <-m.quit:
			return ErrMailBoxShuttingDown
		}
	}
}

// AckPacket removes the packet identified by it's incoming circuit key from the
// queue of packets to be delivered.
//
// NOTE: It is safe to call this method multiple times for the same circuit key.
func (m *memoryMailBox) AckPacket(inKey CircuitKey) error {
	m.pktCond.L.Lock()
	entry, ok := m.pktIndex[inKey]
	if !ok {
		m.pktCond.L.Unlock()
		return nil
	}

	m.htlcPkts.Remove(entry)
	delete(m.pktIndex, inKey)
	m.pktCond.L.Unlock()

	return nil
}

// HasPacket queries the packets for a circuit key, this is used to drop packets
// bound for the switch that already have a queued response.
func (m *memoryMailBox) HasPacket(inKey CircuitKey) bool {
	m.pktCond.L.Lock()
	_, ok := m.pktIndex[inKey]
	m.pktCond.L.Unlock()

	return ok
}

// Stop signals the mailbox and its goroutines for a graceful shutdown.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) Stop() error {
	if !atomic.CompareAndSwapUint32(&m.stopped, 0, 1) {
		return nil
	}

	close(m.quit)

	m.wireCond.Signal()
	m.pktCond.Signal()

	return nil
}

// mailCourier is a dedicated goroutine whose job is to reliably deliver
// messages of a particular type. There are two types of couriers: wire
// couriers, and mail couriers. Depending on the passed courierType, this
// goroutine will assume one of two roles.
func (m *memoryMailBox) mailCourier(cType courierType) {
	defer m.wg.Done()

	// TODO(roasbeef): refactor...

	for {
		// First, we'll check our condition. If our target mailbox is
		// empty, then we'll wait until a new item is added.
		switch cType {
		case wireCourier:
			m.wireCond.L.Lock()
			for m.wireMessages.Front() == nil {
				m.wireCond.Wait()

				select {
				case msgDone := <-m.msgReset:
					m.wireMessages.Init()

					close(msgDone)
				case <-m.quit:
					m.wireCond.L.Unlock()
					return
				default:
				}
			}

		case pktCourier:
			m.pktCond.L.Lock()
			for m.pktHead == nil {
				m.pktCond.Wait()

				select {
				// Resetting the packet queue means just moving
				// our pointer to the front. This ensures that
				// any un-ACK'd messages are re-delivered upon
				// reconnect.
				case pktDone := <-m.pktReset:
					m.pktHead = m.htlcPkts.Front()

					close(pktDone)
				case <-m.quit:
					m.pktCond.L.Unlock()
					return
				default:
				}
			}
		}

		var (
			nextPkt *htlcPacket
			nextMsg lnwire.Message
		)
		switch cType {
		// Grab the datum off the front of the queue, shifting the
		// slice's reference down one in order to remove the datum from
		// the queue.
		case wireCourier:
			entry := m.wireMessages.Front()
			nextMsg = m.wireMessages.Remove(entry).(lnwire.Message)

		// For packets, we actually never remove an item until it has
		// been ACK'd by the link. This ensures that if a read packet
		// doesn't make it into a commitment, then it'll be
		// re-delivered once the link comes back online.
		case pktCourier:
			nextPkt = m.pktHead.Value.(*htlcPacket)
			m.pktHead = m.pktHead.Next()
		}

		// Now that we're done with the condition, we can unlock it to
		// allow any callers to append to the end of our target queue.
		switch cType {
		case wireCourier:
			m.wireCond.L.Unlock()
		case pktCourier:
			m.pktCond.L.Unlock()
		}

		// With the next message obtained, we'll now select to attempt
		// to deliver the message. If we receive a kill signal, then
		// we'll bail out.
		switch cType {
		case wireCourier:
			select {
			case m.messageOutbox <- nextMsg:
			case msgDone := <-m.msgReset:
				m.wireCond.L.Lock()
				m.wireMessages.Init()
				m.wireCond.L.Unlock()

				close(msgDone)
			case <-m.quit:
				return
			}

		case pktCourier:
			select {
			case m.pktOutbox <- nextPkt:
			case pktDone := <-m.pktReset:
				m.pktCond.L.Lock()
				m.pktHead = m.htlcPkts.Front()
				m.pktCond.L.Unlock()

				close(pktDone)
			case <-m.quit:
				return
			}
		}

	}
}

// AddMessage appends a new message to the end of the message queue.
//
// NOTE: This method is safe for concrete use and part of the MailBox
// interface.
func (m *memoryMailBox) AddMessage(msg lnwire.Message) error {
	// First, we'll lock the condition, and add the message to the end of
	// the wire message inbox.
	m.wireCond.L.Lock()
	m.wireMessages.PushBack(msg)
	m.wireCond.L.Unlock()

	// With the message added, we signal to the mailCourier that there are
	// additional messages to deliver.
	m.wireCond.Signal()

	return nil
}

// AddPacket appends a new message to the end of the packet queue.
//
// NOTE: This method is safe for concrete use and part of the MailBox
// interface.
func (m *memoryMailBox) AddPacket(pkt *htlcPacket) error {
	// First, we'll lock the condition, and add the packet to the end of
	// the htlc packet inbox.
	m.pktCond.L.Lock()
	if _, ok := m.pktIndex[pkt.inKey()]; ok {
		m.pktCond.L.Unlock()
		return nil
	}

	entry := m.htlcPkts.PushBack(pkt)
	m.pktIndex[pkt.inKey()] = entry
	if m.pktHead == nil {
		m.pktHead = entry
	}
	m.pktCond.L.Unlock()

	// With the packet added, we signal to the mailCourier that there are
	// additional packets to consume.
	m.pktCond.Signal()

	return nil
}

// MessageOutBox returns a channel that any new messages ready for delivery
// will be sent on.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) MessageOutBox() chan lnwire.Message {
	return m.messageOutbox
}

// PacketOutBox returns a channel that any new packets ready for delivery will
// be sent on.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) PacketOutBox() chan *htlcPacket {
	return m.pktOutbox
}