package p2p

import (
	"container/heap"
	"sort"
	"strconv"
	"time"

	"github.com/gogo/protobuf/proto"
	tmsync "github.com/tendermint/tendermint/internal/libs/sync"
	"github.com/tendermint/tendermint/libs/log"
)

// pqEnvelope defines a wrapper around an Envelope with priority to be inserted
// into a priority queue used for Envelope scheduling.
type pqEnvelope struct {
	envelope  Envelope
	priority  uint
	size      uint
	timestamp time.Time

	index int
}

// priorityQueue defines a type alias for a priority queue implementation.
type priorityQueue []*pqEnvelope

func (pq priorityQueue) get(i int) *pqEnvelope { return pq[i] }
func (pq priorityQueue) Len() int              { return len(pq) }

func (pq priorityQueue) Less(i, j int) bool {
	// if both elements have the same priority, prioritize based on most recent
	if pq[i].priority == pq[j].priority {
		return pq[i].timestamp.After(pq[j].timestamp)
	}

	// otherwise, pick the pqEnvelope with the higher priority
	return pq[i].priority > pq[j].priority
}

func (pq priorityQueue) Swap(i, j int) {
	pq[i], pq[j] = pq[j], pq[i]
	pq[i].index = i
	pq[j].index = j
}

func (pq *priorityQueue) Push(x interface{}) {
	n := len(*pq)
	pqEnv := x.(*pqEnvelope)
	pqEnv.index = n
	*pq = append(*pq, pqEnv)
}

func (pq *priorityQueue) Pop() interface{} {
	old := *pq
	n := len(old)
	pqEnv := old[n-1]
	old[n-1] = nil
	pqEnv.index = -1
	*pq = old[:n-1]
	return pqEnv
}

// Assert the priority queue scheduler implements the queue interface at
// compile-time.
var _ queue = (*pqScheduler)(nil)

type pqScheduler struct {
	logger       log.Logger
	metrics      *Metrics
	size         uint
	sizes        map[uint]uint // cumulative priority sizes
	pq           *priorityQueue
	chDescs      []ChannelDescriptor
	capacity     uint
	chPriorities map[ChannelID]uint

	enqueueCh chan Envelope
	dequeueCh chan Envelope
	closer    *tmsync.Closer
	done      *tmsync.Closer
}

func newPQScheduler(
	logger log.Logger,
	m *Metrics,
	chDescs []ChannelDescriptor,
	enqueueBuf, dequeueBuf, capacity uint,
) *pqScheduler {

	// copy each ChannelDescriptor and sort them by ascending channel priority
	chDescsCopy := make([]ChannelDescriptor, len(chDescs))
	copy(chDescsCopy, chDescs)
	sort.Slice(chDescsCopy, func(i, j int) bool { return chDescsCopy[i].Priority < chDescsCopy[j].Priority })

	var (
		chPriorities = make(map[ChannelID]uint)
		sizes        = make(map[uint]uint)
	)

	for _, chDesc := range chDescsCopy {
		chID := ChannelID(chDesc.ID)
		chPriorities[chID] = uint(chDesc.Priority)
		sizes[uint(chDesc.Priority)] = 0
	}

	pq := make(priorityQueue, 0)
	heap.Init(&pq)

	return &pqScheduler{
		logger:       logger.With("router", "scheduler"),
		metrics:      m,
		chDescs:      chDescsCopy,
		capacity:     capacity,
		chPriorities: chPriorities,
		pq:           &pq,
		sizes:        sizes,
		enqueueCh:    make(chan Envelope, enqueueBuf),
		dequeueCh:    make(chan Envelope, dequeueBuf),
		closer:       tmsync.NewCloser(),
		done:         tmsync.NewCloser(),
	}
}

func (s *pqScheduler) enqueue() chan<- Envelope {
	return s.enqueueCh
}

func (s *pqScheduler) dequeue() <-chan Envelope {
	return s.dequeueCh
}

func (s *pqScheduler) close() {
	s.closer.Close()
	<-s.done.Done()
}

func (s *pqScheduler) closed() <-chan struct{} {
	return s.closer.Done()
}

// start starts non-blocking process that starts the priority queue scheduler.
func (s *pqScheduler) start() {
	go s.process()
}

// process starts a block process where we listen for Envelopes to enqueue. If
// there is sufficient capacity, it will be enqueued into the priority queue,
// otherwise, we attempt to dequeue enough elements from the priority queue to
// make room for the incoming Envelope by dropping lower priority elements. If
// there isn't sufficient capacity at lower priorities for the incoming Envelope,
// it is dropped.
//
// After we attempt to enqueue the incoming Envelope, if the priority queue is
// non-empty, we pop the top Envelope and send it on the dequeueCh.
func (s *pqScheduler) process() {
	defer s.done.Close()

	for {
		select {
		case e := <-s.enqueueCh:
			chIDStr := strconv.Itoa(int(e.channelID))
			pqEnv := &pqEnvelope{
				envelope:  e,
				size:      uint(proto.Size(e.Message)),
				priority:  s.chPriorities[e.channelID],
				timestamp: time.Now().UTC(),
			}

			s.metrics.PeerPendingSendBytes.With("peer_id", string(pqEnv.envelope.To)).Add(float64(pqEnv.size))

			// enqueue

			// Check if we have sufficient capacity to simply enqueue the incoming
			// Envelope.
			if s.size+pqEnv.size <= s.capacity {
				// enqueue the incoming Envelope
				s.push(pqEnv)
			} else {
				// There is not sufficient capacity to simply enqueue the incoming
				// Envelope. So we have to attempt to make room for it by dropping lower
				// priority Envelopes or drop the incoming Envelope otherwise.

				// The cumulative size of all enqueue envelopes at the incoming envelope's
				// priority or lower.
				total := s.sizes[pqEnv.priority]

				if total >= pqEnv.size {
					// There is room for the incoming Envelope, so we drop as many lower
					// priority Envelopes as we need to.
					var (
						canEnqueue bool
						tmpSize    = s.size
						i          = s.pq.Len() - 1
					)

					// Drop lower priority Envelopes until sufficient capacity exists for
					// the incoming Envelope
					for i >= 0 && !canEnqueue {
						pqEnvTmp := s.pq.get(i)

						if pqEnvTmp.priority < pqEnv.priority {
							if tmpSize+pqEnv.size <= s.capacity {
								canEnqueue = true
							} else {
								pqEnvTmpChIDStr := strconv.Itoa(int(pqEnvTmp.envelope.channelID))
								s.metrics.PeerQueueDroppedMsgs.With("ch_id", pqEnvTmpChIDStr).Add(1)
								s.logger.Debug(
									"dropped envelope",
									"ch_id", pqEnvTmpChIDStr,
									"priority", pqEnvTmp.priority,
									"msg_size", pqEnvTmp.size,
									"capacity", s.capacity,
								)

								// dequeue/drop from the priority queue
								heap.Remove(s.pq, pqEnvTmp.index)

								// update the size tracker
								tmpSize -= pqEnvTmp.size

								// start from the end again
								i = s.pq.Len() - 1
							}
						} else {
							i--
						}
					}

					// enqueue the incoming Envelope
					s.push(pqEnv)
				} else {
					// There is not sufficient capacity to drop lower priority Envelopes,
					// so we drop the incoming Envelope.
					s.metrics.PeerQueueDroppedMsgs.With("ch_id", chIDStr).Add(1)
					s.logger.Debug(
						"dropped envelope",
						"ch_id", chIDStr,
						"priority", pqEnv.priority,
						"msg_size", pqEnv.size,
						"capacity", s.capacity,
					)
				}
			}

			// dequeue

			for s.pq.Len() > 0 {
				pqEnv = heap.Pop(s.pq).(*pqEnvelope)
				s.size -= pqEnv.size

				// deduct the Envelope size from all the relevant cumulative sizes
				for i := 0; i < len(s.chDescs) && pqEnv.priority <= uint(s.chDescs[i].Priority); i++ {
					s.sizes[uint(s.chDescs[i].Priority)] -= pqEnv.size
				}

				s.metrics.PeerSendBytesTotal.With(
					"chID", chIDStr,
					"peer_id", string(pqEnv.envelope.To)).Add(float64(pqEnv.size))
				select {
				case s.dequeueCh <- pqEnv.envelope:
				case <-s.closer.Done():
					return
				}
			}

		case <-s.closer.Done():
			return
		}
	}
}

func (s *pqScheduler) push(pqEnv *pqEnvelope) {
	chIDStr := strconv.Itoa(int(pqEnv.envelope.channelID))

	// enqueue the incoming Envelope
	heap.Push(s.pq, pqEnv)
	s.size += pqEnv.size
	s.metrics.PeerQueueMsgSize.With("ch_id", chIDStr).Add(float64(pqEnv.size))

	// Update the cumulative sizes by adding the Envelope's size to every
	// priority less than or equal to it.
	for i := 0; i < len(s.chDescs) && pqEnv.priority <= uint(s.chDescs[i].Priority); i++ {
		s.sizes[uint(s.chDescs[i].Priority)] += pqEnv.size
	}
}