// Copyright 2022 TiKV Project Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package schedulers

import (
	"math/rand"

	"github.com/pingcap/errors"
	"github.com/pingcap/log"
	"github.com/tikv/pd/pkg/core"
	"github.com/tikv/pd/pkg/core/constant"
	"github.com/tikv/pd/pkg/errs"
	sche "github.com/tikv/pd/pkg/schedule/core"
	"github.com/tikv/pd/pkg/schedule/filter"
	"github.com/tikv/pd/pkg/schedule/operator"
	"github.com/tikv/pd/pkg/schedule/plan"
	"github.com/tikv/pd/pkg/schedule/types"
)

const (
	// TransferWitnessLeaderBatchSize is the number of operators to to transfer
	// leaders by one scheduling
	transferWitnessLeaderBatchSize = 3
	// TODO: When we prepare to use Ranges, we will need to implement the ReloadConfig function for this scheduler.
	// TransferWitnessLeaderRecvMaxRegionSize is the max number of region can receive
	// TODO: make it a reasonable value
	transferWitnessLeaderRecvMaxRegionSize = 10000
)

type transferWitnessLeaderScheduler struct {
	*BaseScheduler
	regions chan *core.RegionInfo
}

// newTransferWitnessLeaderScheduler creates an admin scheduler that transfers witness leader of a region.
func newTransferWitnessLeaderScheduler(opController *operator.Controller, conf schedulerConfig) Scheduler {
	return &transferWitnessLeaderScheduler{
		BaseScheduler: NewBaseScheduler(opController, types.TransferWitnessLeaderScheduler, conf),
		regions:       make(chan *core.RegionInfo, transferWitnessLeaderRecvMaxRegionSize),
	}
}

// IsScheduleAllowed implements the Scheduler interface.
func (*transferWitnessLeaderScheduler) IsScheduleAllowed(sche.SchedulerCluster) bool {
	return true
}

// Schedule implements the Scheduler interface.
func (s *transferWitnessLeaderScheduler) Schedule(cluster sche.SchedulerCluster, _ bool) ([]*operator.Operator, []plan.Plan) {
	transferWitnessLeaderCounter.Inc()
	return s.scheduleTransferWitnessLeaderBatch(s.GetName(), cluster, transferWitnessLeaderBatchSize), nil
}

func (s *transferWitnessLeaderScheduler) scheduleTransferWitnessLeaderBatch(name string, cluster sche.SchedulerCluster, batchSize int) []*operator.Operator {
	var ops []*operator.Operator
batchLoop:
	for range batchSize {
		select {
		case region := <-s.regions:
			op, err := scheduleTransferWitnessLeader(s.R, name, cluster, region)
			if err != nil {
				log.Debug("fail to create transfer leader operator", errs.ZapError(err))
				continue
			}
			if op != nil {
				op.SetPriorityLevel(constant.Urgent)
				op.Counters = append(op.Counters, transferWitnessLeaderNewOperatorCounter)
				ops = append(ops, op)
			}
		default:
			break batchLoop
		}
	}
	return ops
}

func scheduleTransferWitnessLeader(r *rand.Rand, name string, cluster sche.SchedulerCluster, region *core.RegionInfo) (*operator.Operator, error) {
	var filters []filter.Filter
	unhealthyPeerStores := make(map[uint64]struct{})
	for _, peer := range region.GetDownPeers() {
		unhealthyPeerStores[peer.GetPeer().GetStoreId()] = struct{}{}
	}
	for _, peer := range region.GetPendingPeers() {
		unhealthyPeerStores[peer.GetStoreId()] = struct{}{}
	}
	filters = append(filters, filter.NewExcludedFilter(name, nil, unhealthyPeerStores),
		&filter.StoreStateFilter{ActionScope: name, TransferLeader: true, OperatorLevel: constant.Urgent})
	candidates := filter.NewCandidates(r, cluster.GetFollowerStores(region)).FilterTarget(cluster.GetSchedulerConfig(), nil, nil, filters...)
	// Compatible with old TiKV transfer leader logic.
	target := candidates.RandomPick()
	targets := candidates.PickAll()
	// `targets` MUST contains `target`, so only needs to check if `target` is nil here.
	if target == nil {
		transferWitnessLeaderNoTargetStoreCounter.Inc()
		return nil, errors.New("no target store to schedule")
	}
	targetIDs := make([]uint64, 0, len(targets))
	for _, t := range targets {
		targetIDs = append(targetIDs, t.GetID())
	}
	return operator.CreateTransferLeaderOperator(name, cluster, region, target.GetID(), targetIDs, operator.OpWitnessLeader)
}

// RecvRegionInfo receives a checked region from coordinator
func RecvRegionInfo(s Scheduler) chan<- *core.RegionInfo {
	return s.(*transferWitnessLeaderScheduler).regions
}