/*
 *     Copyright 2020 The Dragonfly 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 evaluator

import (
	"math/big"

	"github.com/montanaflynn/stats"

	logger "d7y.io/dragonfly/v2/internal/dflog"
	"d7y.io/dragonfly/v2/scheduler/resource/persistentcache"
	"d7y.io/dragonfly/v2/scheduler/resource/standard"
)

const (
	// DefaultAlgorithm is a rule-based scheduling algorithm.
	DefaultAlgorithm = "default"

	// MLAlgorithm is a machine learning scheduling algorithm.
	MLAlgorithm = "ml"

	// PluginAlgorithm is a scheduling algorithm based on plugin extension.
	PluginAlgorithm = "plugin"
)

const (
	// Maximum score.
	maxScore float64 = 1

	// Minimum score.
	minScore = 0
)

const (
	// Maximum number of elements.
	maxElementLen = 5

	// If the number of samples is greater than or equal to 30,
	// it is close to the normal distribution.
	normalDistributionLen = 30

	// When costs len is greater than or equal to 2,
	// the last cost can be compared and calculated.
	minAvailableCostLen = 2
)

// Evaluator is an interface that evaluates the parents.
type Evaluator interface {
	// EvaluateParents sort parents by evaluating multiple feature scores.
	EvaluateParents(parents []*standard.Peer, child *standard.Peer, taskPieceCount uint32) []*standard.Peer

	// IsBadParent determine if peer is a bad parent, it can not be selected as a parent.
	IsBadParent(peer *standard.Peer) bool

	// EvaluatePersistentCacheParents sort persistent cache parents by evaluating multiple feature scores.
	EvaluatePersistentCacheParents(parents []*persistentcache.Peer, child *persistentcache.Peer, taskPieceCount uint32) []*persistentcache.Peer

	// IsBadPersistentCacheParent determine if persistent cache peer is a bad parent, it can not be selected as a parent.
	IsBadPersistentCacheParent(peer *persistentcache.Peer) bool
}

// evaluator is an implementation of Evaluator.
type evaluator struct{}

// New returns a new Evaluator.
func New(algorithm string, pluginDir string) Evaluator {
	switch algorithm {
	case PluginAlgorithm:
		if plugin, err := LoadPlugin(pluginDir); err == nil {
			return plugin
		}
	// TODO Implement MLAlgorithm.
	case MLAlgorithm, DefaultAlgorithm:
		return newEvaluatorBase()
	}

	return newEvaluatorBase()
}

// IsBadParent determine if peer is a bad parent, it can not be selected as a parent.
func (e *evaluator) IsBadParent(peer *standard.Peer) bool {
	if peer.FSM.Is(standard.PeerStateFailed) || peer.FSM.Is(standard.PeerStateLeave) || peer.FSM.Is(standard.PeerStatePending) ||
		peer.FSM.Is(standard.PeerStateReceivedTiny) || peer.FSM.Is(standard.PeerStateReceivedSmall) ||
		peer.FSM.Is(standard.PeerStateReceivedNormal) || peer.FSM.Is(standard.PeerStateReceivedEmpty) {
		peer.Log.Debugf("peer is bad node because peer status is %s", peer.FSM.Current())
		return true
	}

	// Determine whether to bad node based on piece download costs.
	costs := stats.LoadRawData(peer.PieceCosts())
	len := len(costs)
	// Peer has not finished downloading enough piece.
	if len < minAvailableCostLen {
		logger.Debugf("peer %s has not finished downloading enough piece, it can't be bad node", peer.ID)
		return false
	}

	lastCost := costs[len-1]
	mean, _ := stats.Mean(costs[:len-1]) // nolint: errcheck

	// Download costs does not meet the normal distribution,
	// if the last cost is twenty times more than mean, it is bad node.
	if len < normalDistributionLen {
		isBadParent := big.NewFloat(lastCost).Cmp(big.NewFloat(mean*20)) > 0
		logger.Debugf("peer %s mean is %.2f and it is bad node: %t", peer.ID, mean, isBadParent)
		return isBadParent
	}

	// Download costs satisfies the normal distribution,
	// last cost falling outside of three-sigma effect need to be adjusted parent,
	// refer to https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule.
	stdev, _ := stats.StandardDeviation(costs[:len-1]) // nolint: errcheck
	isBadParent := big.NewFloat(lastCost).Cmp(big.NewFloat(mean+3*stdev)) > 0
	logger.Debugf("peer %s meet the normal distribution, costs mean is %.2f and standard deviation is %.2f, peer is bad node: %t",
		peer.ID, mean, stdev, isBadParent)
	return isBadParent
}

// IsBadPersistentCacheParent determine if persistent cache peer is a bad parent, it can not be selected as a parent.
func (e *evaluator) IsBadPersistentCacheParent(peer *persistentcache.Peer) bool {
	if peer.FSM.Is(persistentcache.PeerStatePending) || peer.FSM.Is(persistentcache.PeerStateUploading) || peer.FSM.Is(persistentcache.PeerStateReceivedEmpty) ||
		peer.FSM.Is(persistentcache.PeerStateReceivedNormal) || peer.FSM.Is(persistentcache.PeerStateFailed) {
		peer.Log.Debugf("persistent cache peer is bad node because peer status is %s", peer.FSM.Current())
		return true
	}

	return false
}