package models

import (
	. "battle_srv/common"
	"battle_srv/common/utils"
	. "battle_srv/protos"
	. "dnmshared"
	. "dnmshared/sharedprotos"
	"encoding/xml"
	"fmt"
	"github.com/golang/protobuf/proto"
	"github.com/gorilla/websocket"
	"github.com/solarlune/resolv"
	"go.uber.org/zap"
	"io/ioutil"
	"math/rand"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"sync/atomic"
	"time"
)

const (
	UPSYNC_MSG_ACT_HB_PING             = int32(1)
	UPSYNC_MSG_ACT_PLAYER_CMD          = int32(2)
	UPSYNC_MSG_ACT_PLAYER_COLLIDER_ACK = int32(3)

	DOWNSYNC_MSG_ACT_HB_REQ         = int32(1)
	DOWNSYNC_MSG_ACT_INPUT_BATCH    = int32(2)
	DOWNSYNC_MSG_ACT_BATTLE_STOPPED = int32(3)
	DOWNSYNC_MSG_ACT_FORCED_RESYNC  = int32(4)

	DOWNSYNC_MSG_ACT_BATTLE_READY_TO_START = int32(-1)
	DOWNSYNC_MSG_ACT_BATTLE_START          = int32(0)

	DOWNSYNC_MSG_ACT_PLAYER_ADDED_AND_ACKED = int32(-98)
)

const (
	MAGIC_JOIN_INDEX_DEFAULT = 0
	MAGIC_JOIN_INDEX_INVALID = -1
)

const (
	COLLISION_CATEGORY_CONTROLLED_PLAYER = (1 << 1)
	COLLISION_CATEGORY_BARRIER           = (1 << 2)

	COLLISION_MASK_FOR_CONTROLLED_PLAYER = (COLLISION_CATEGORY_BARRIER)
	COLLISION_MASK_FOR_BARRIER           = (COLLISION_CATEGORY_CONTROLLED_PLAYER)

	COLLISION_PLAYER_INDEX_PREFIX  = (1 << 17)
	COLLISION_BARRIER_INDEX_PREFIX = (1 << 16)
	COLLISION_BULLET_INDEX_PREFIX  = (1 << 15)
)

const (
	MAGIC_LAST_SENT_INPUT_FRAME_ID_NORMAL_ADDED = -1
	MAGIC_LAST_SENT_INPUT_FRAME_ID_READDED      = -2
)

const (
	ATK_CHARACTER_STATE_IDLE1        = 0
	ATK_CHARACTER_STATE_WALKING      = 1
	ATK_CHARACTER_STATE_ATK1         = 2
	ATK_CHARACTER_STATE_ATKED1       = 3
	ATK_CHARACTER_STATE_INAIR_IDLE1  = 4
	ATK_CHARACTER_STATE_INAIR_ATK1   = 5
	ATK_CHARACTER_STATE_INAIR_ATKED1 = 6
)

const (
	DEFAULT_PLAYER_RADIUS = float64(12)
)

// These directions are chosen such that when speed is changed to "(speedX+delta, speedY+delta)" for any of them, the direction is unchanged.
var DIRECTION_DECODER = [][]int32{
	{0, 0},
	{0, +2},
	{0, -2},
	{+2, 0},
	{-2, 0},
	{+1, +1},
	{-1, -1},
	{+1, -1},
	{-1, +1},
}

type RoomBattleState struct {
	IDLE                           int32
	WAITING                        int32
	PREPARE                        int32
	IN_BATTLE                      int32
	STOPPING_BATTLE_FOR_SETTLEMENT int32
	IN_SETTLEMENT                  int32
	IN_DISMISSAL                   int32
}

type BattleStartCbType func()
type SignalToCloseConnCbType func(customRetCode int, customRetMsg string)

// A single instance containing only "named constant integers" to be shared by all threads.
var RoomBattleStateIns RoomBattleState

func InitRoomBattleStateIns() {
	RoomBattleStateIns = RoomBattleState{
		IDLE:                           0,
		WAITING:                        -1,
		PREPARE:                        10000000,
		IN_BATTLE:                      10000001,
		STOPPING_BATTLE_FOR_SETTLEMENT: 10000002,
		IN_SETTLEMENT:                  10000003,
		IN_DISMISSAL:                   10000004,
	}
}

func calRoomScore(inRoomPlayerCount int32, roomPlayerCnt int, currentRoomBattleState int32) float32 {
	x := float32(inRoomPlayerCount) / float32(roomPlayerCnt)
	d := (x - 0.5)
	d2 := d * d
	return -7.8125*d2 + 5.0 - float32(currentRoomBattleState)
}

type Room struct {
	Id                    int32
	Capacity              int
	collisionSpaceOffsetX float64
	collisionSpaceOffsetY float64
	Players               map[int32]*Player
	PlayersArr            []*Player // ordered by joinIndex
	Space                 *resolv.Space
	CollisionSysMap       map[int32]*resolv.Object
	/**
		 * The following `PlayerDownsyncSessionDict` is NOT individually put
		 * under `type Player struct` for a reason.
		 *
		 * Upon each connection establishment, a new instance `player Player` is created for the given `playerId`.

		 * To be specific, if
		 *   - that `playerId == 42` accidentally reconnects in just several milliseconds after a passive disconnection, e.g. due to bad wireless signal strength, and
		 *   - that `type Player struct` contains a `DownsyncSession` field
		 *
		 * , then we might have to
		 *   - clean up `previousPlayerInstance.DownsyncSession`
		 *   - initialize `currentPlayerInstance.DownsyncSession`
		 *
		 * to avoid chaotic flaws.
	     *
	     * Moreover, during the invocation of `PlayerSignalToCloseDict`, the `Player` instance is supposed to be deallocated (though not synchronously).
	*/
	PlayerDownsyncSessionDict              map[int32]*websocket.Conn
	PlayerDownsyncChanDict                 map[int32](chan InputsBufferSnapshot)
	PlayerSignalToCloseDict                map[int32]SignalToCloseConnCbType
	Score                                  float32
	State                                  int32
	Index                                  int
	RenderFrameId                          int32
	CurDynamicsRenderFrameId               int32 // [WARNING] The dynamics of backend is ALWAYS MOVING FORWARD BY ALL-CONFIRMED INPUTFRAMES (either by upsync or forced), i.e. no rollback; Moreover when "true == BackendDynamicsEnabled" we always have "Room.CurDynamicsRenderFrameId >= Room.RenderFrameId" because each "all-confirmed inputFrame" is applied on "all applicable renderFrames" in one-go hence often sees a future "renderFrame" earlier
	EffectivePlayerCount                   int32
	DismissalWaitGroup                     sync.WaitGroup
	Barriers                               map[int32]*Barrier
	InputsBuffer                           *RingBuffer // Indices are STRICTLY consecutive
	InputsBufferLock                       sync.Mutex  // Guards [InputsBuffer, LastAllConfirmedInputFrameId, LastAllConfirmedInputList, LastAllConfirmedInputFrameIdWithChange]
	RenderFrameBuffer                      *RingBuffer // Indices are STRICTLY consecutive
	LastAllConfirmedInputFrameId           int32
	LastAllConfirmedInputFrameIdWithChange int32
	LastAllConfirmedInputList              []uint64
	JoinIndexBooleanArr                    []bool

	BackendDynamicsEnabled       bool
	LastRenderFrameIdTriggeredAt int64
	PlayerDefaultSpeed           int32

	BulletBattleLocalIdCounter      int32
	dilutedRollbackEstimatedDtNanos int64

	BattleColliderInfo // Compositing to send centralized magic numbers
}

func (pR *Room) updateScore() {
	pR.Score = calRoomScore(pR.EffectivePlayerCount, pR.Capacity, pR.State)
}

func (pR *Room) AddPlayerIfPossible(pPlayerFromDbInit *Player, session *websocket.Conn, signalToCloseConnOfThisPlayer SignalToCloseConnCbType) bool {
	playerId := pPlayerFromDbInit.Id
	// TODO: Any thread-safety concern for accessing "pR" here?
	if RoomBattleStateIns.IDLE != pR.State && RoomBattleStateIns.WAITING != pR.State {
		Logger.Warn("AddPlayerIfPossible error, roomState:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State), zap.Any("roomEffectivePlayerCount", pR.EffectivePlayerCount))
		return false
	}
	if _, existent := pR.Players[playerId]; existent {
		Logger.Warn("AddPlayerIfPossible error, existing in the room.PlayersDict:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State), zap.Any("roomEffectivePlayerCount", pR.EffectivePlayerCount))
		return false
	}

	defer pR.onPlayerAdded(playerId)
	pPlayerFromDbInit.AckingFrameId = -1
	pPlayerFromDbInit.AckingInputFrameId = -1
	pPlayerFromDbInit.LastSentInputFrameId = MAGIC_LAST_SENT_INPUT_FRAME_ID_NORMAL_ADDED
	pPlayerFromDbInit.BattleState = PlayerBattleStateIns.ADDED_PENDING_BATTLE_COLLIDER_ACK
	pPlayerFromDbInit.Speed = pR.PlayerDefaultSpeed          // Hardcoded
	pPlayerFromDbInit.ColliderRadius = DEFAULT_PLAYER_RADIUS // Hardcoded
	pPlayerFromDbInit.InAir = true                           // Hardcoded

	pR.Players[playerId] = pPlayerFromDbInit
	pR.PlayerDownsyncSessionDict[playerId] = session
	pR.PlayerSignalToCloseDict[playerId] = signalToCloseConnOfThisPlayer
	return true
}

func (pR *Room) ReAddPlayerIfPossible(pTmpPlayerInstance *Player, session *websocket.Conn, signalToCloseConnOfThisPlayer SignalToCloseConnCbType) bool {
	playerId := pTmpPlayerInstance.Id
	// TODO: Any thread-safety concern for accessing "pR" and "pEffectiveInRoomPlayerInstance" here?
	if RoomBattleStateIns.PREPARE != pR.State && RoomBattleStateIns.WAITING != pR.State && RoomBattleStateIns.IN_BATTLE != pR.State && RoomBattleStateIns.IN_SETTLEMENT != pR.State && RoomBattleStateIns.IN_DISMISSAL != pR.State {
		Logger.Warn("ReAddPlayerIfPossible error due to roomState:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State), zap.Any("roomEffectivePlayerCount", pR.EffectivePlayerCount))
		return false
	}
	if _, existent := pR.Players[playerId]; !existent {
		Logger.Warn("ReAddPlayerIfPossible error due to player nonexistent for room:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State), zap.Any("roomEffectivePlayerCount", pR.EffectivePlayerCount))
		return false
	}
	/*
	 * WARNING: The "pTmpPlayerInstance *Player" used here is a temporarily constructed
	 * instance from "<proj-root>/battle_srv/ws/serve.go", which is NOT the same as "pR.Players[pTmpPlayerInstance.Id]".
	 * -- YFLu
	 */
	defer pR.onPlayerReAdded(playerId)
	pEffectiveInRoomPlayerInstance := pR.Players[playerId]
	pEffectiveInRoomPlayerInstance.AckingFrameId = -1
	pEffectiveInRoomPlayerInstance.AckingInputFrameId = -1
	pEffectiveInRoomPlayerInstance.LastSentInputFrameId = MAGIC_LAST_SENT_INPUT_FRAME_ID_READDED
	pEffectiveInRoomPlayerInstance.BattleState = PlayerBattleStateIns.READDED_PENDING_BATTLE_COLLIDER_ACK
	pEffectiveInRoomPlayerInstance.Speed = pR.PlayerDefaultSpeed          // Hardcoded
	pEffectiveInRoomPlayerInstance.ColliderRadius = DEFAULT_PLAYER_RADIUS // Hardcoded
	pEffectiveInRoomPlayerInstance.InAir = true                           // Hardcoded

	pR.PlayerDownsyncSessionDict[playerId] = session
	pR.PlayerSignalToCloseDict[playerId] = signalToCloseConnOfThisPlayer

	Logger.Warn("ReAddPlayerIfPossible finished.", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("joinIndex", pEffectiveInRoomPlayerInstance.JoinIndex), zap.Any("playerBattleState", pEffectiveInRoomPlayerInstance.BattleState), zap.Any("roomState", pR.State), zap.Any("roomEffectivePlayerCount", pR.EffectivePlayerCount), zap.Any("AckingFrameId", pEffectiveInRoomPlayerInstance.AckingFrameId), zap.Any("AckingInputFrameId", pEffectiveInRoomPlayerInstance.AckingInputFrameId), zap.Any("LastSentInputFrameId", pEffectiveInRoomPlayerInstance.LastSentInputFrameId))
	return true
}

func (pR *Room) ChooseStage() error {
	/*
	 * We use the verb "refresh" here to imply that upon invocation of this function, all colliders will be recovered if they were destroyed in the previous battle.
	 *
	 * -- YFLu, 2019-09-04
	 */
	pwd, err := os.Getwd()
	if nil != err {
		panic(err)
	}

	rand.Seed(time.Now().Unix())
	stageNameList := []string{"dungeon" /*"dungeon", "simple", "richsoil" */}
	chosenStageIndex := rand.Int() % len(stageNameList) // Hardcoded temporarily. -- YFLu

	pR.StageName = stageNameList[chosenStageIndex]

	relativePathForAllStages := "../frontend/assets/resources/map"
	relativePathForChosenStage := fmt.Sprintf("%s/%s", relativePathForAllStages, pR.StageName)

	pTmxMapIns := &TmxMap{}

	absDirPathContainingDirectlyTmxFile := filepath.Join(pwd, relativePathForChosenStage)
	absTmxFilePath := fmt.Sprintf("%s/map.tmx", absDirPathContainingDirectlyTmxFile)
	if !filepath.IsAbs(absTmxFilePath) {
		panic("Tmx filepath must be absolute!")
	}

	byteArr, err := ioutil.ReadFile(absTmxFilePath)
	if nil != err {
		panic(err)
	}
	err = xml.Unmarshal(byteArr, pTmxMapIns)
	if nil != err {
		panic(err)
	}

	// Obtain the content of `gidBoundariesMap`.
	gidBoundariesMap := make(map[int]StrToPolygon2DListMap, 0)
	for _, tileset := range pTmxMapIns.Tilesets {
		relativeTsxFilePath := fmt.Sprintf("%s/%s", filepath.Join(pwd, relativePathForChosenStage), tileset.Source) // Note that "TmxTileset.Source" can be a string of "relative path".
		absTsxFilePath, err := filepath.Abs(relativeTsxFilePath)
		if nil != err {
			panic(err)
		}
		if !filepath.IsAbs(absTsxFilePath) {
			panic("Filepath must be absolute!")
		}

		byteArrOfTsxFile, err := ioutil.ReadFile(absTsxFilePath)
		if nil != err {
			panic(err)
		}

		DeserializeTsxToColliderDict(pTmxMapIns, byteArrOfTsxFile, int(tileset.FirstGid), gidBoundariesMap)
	}

	stageDiscreteW, stageDiscreteH, stageTileW, stageTileH, strToVec2DListMap, strToPolygon2DListMap, err := ParseTmxLayersAndGroups(pTmxMapIns, gidBoundariesMap)
	if nil != err {
		panic(err)
	}

	pR.StageDiscreteW = stageDiscreteW
	pR.StageDiscreteH = stageDiscreteH
	pR.StageTileW = stageTileW
	pR.StageTileH = stageTileH
	pR.StrToVec2DListMap = strToVec2DListMap
	pR.StrToPolygon2DListMap = strToPolygon2DListMap

	barrierPolygon2DList := *(strToPolygon2DListMap["Barrier"])

	var barrierLocalIdInBattle int32 = 0
	for _, polygon2DUnaligned := range barrierPolygon2DList.Eles {
		polygon2D := AlignPolygon2DToBoundingBox(polygon2DUnaligned)
		/*
		   // For debug-printing only.
		   Logger.Info("ChooseStage printing polygon2D for barrierPolygon2DList", zap.Any("barrierLocalIdInBattle", barrierLocalIdInBattle), zap.Any("polygon2D.Anchor", polygon2D.Anchor), zap.Any("polygon2D.Points", polygon2D.Points))
		*/
		pR.Barriers[barrierLocalIdInBattle] = &Barrier{
			Boundary: polygon2D,
		}

		barrierLocalIdInBattle++
	}

	return nil
}

func (pR *Room) ConvertToInputFrameId(renderFrameId int32, inputDelayFrames int32) int32 {
	if renderFrameId < inputDelayFrames {
		return 0
	}
	return ((renderFrameId - inputDelayFrames) >> pR.InputScaleFrames)
}

func (pR *Room) ConvertToGeneratingRenderFrameId(inputFrameId int32) int32 {
	return (inputFrameId << pR.InputScaleFrames)
}

func (pR *Room) ConvertToFirstUsedRenderFrameId(inputFrameId int32, inputDelayFrames int32) int32 {
	return ((inputFrameId << pR.InputScaleFrames) + inputDelayFrames)
}

func (pR *Room) ConvertToLastUsedRenderFrameId(inputFrameId int32, inputDelayFrames int32) int32 {
	return ((inputFrameId << pR.InputScaleFrames) + inputDelayFrames + (1 << pR.InputScaleFrames) - 1)
}

func (pR *Room) RenderFrameBufferString() string {
	return fmt.Sprintf("{renderFrameId: %d, stRenderFrameId: %d, edRenderFrameId: %d, curDynamicsRenderFrameId: %d}", pR.RenderFrameId, pR.RenderFrameBuffer.StFrameId, pR.RenderFrameBuffer.EdFrameId, pR.CurDynamicsRenderFrameId)
}

func (pR *Room) InputsBufferString(allDetails bool) string {
	if allDetails {
		// Appending of the array of strings can be very SLOW due to on-demand heap allocation! Use this printing with caution.
		s := make([]string, 0)
		s = append(s, fmt.Sprintf("{renderFrameId: %v, stInputFrameId: %v, edInputFrameId: %v, lastAllConfirmedInputFrameIdWithChange: %v, lastAllConfirmedInputFrameId: %v}", pR.RenderFrameId, pR.InputsBuffer.StFrameId, pR.InputsBuffer.EdFrameId, pR.LastAllConfirmedInputFrameIdWithChange, pR.LastAllConfirmedInputFrameId))
		for playerId, player := range pR.PlayersArr {
			s = append(s, fmt.Sprintf("{playerId: %v, ackingFrameId: %v, ackingInputFrameId: %v, lastSentInputFrameId: %v}", playerId, player.AckingFrameId, player.AckingInputFrameId, player.LastSentInputFrameId))
		}
		for i := pR.InputsBuffer.StFrameId; i < pR.InputsBuffer.EdFrameId; i++ {
			tmp := pR.InputsBuffer.GetByFrameId(i)
			if nil == tmp {
				break
			}
			f := tmp.(*InputFrameDownsync)
			//s = append(s, fmt.Sprintf("{inputFrameId: %v, inputList: %v, &inputList: %p, confirmedList: %v}", f.InputFrameId, f.InputList, &(f.InputList), f.ConfirmedList))
			s = append(s, fmt.Sprintf("{inputFrameId: %v, inputList: %v, confirmedList: %v}", f.InputFrameId, f.InputList, f.ConfirmedList))
		}

		return strings.Join(s, "; ")
	} else {
		return fmt.Sprintf("{renderFrameId: %d, stInputFrameId: %d, edInputFrameId: %d, lastAllConfirmedInputFrameIdWithChange: %d, lastAllConfirmedInputFrameId: %d}", pR.RenderFrameId, pR.InputsBuffer.StFrameId, pR.InputsBuffer.EdFrameId, pR.LastAllConfirmedInputFrameIdWithChange, pR.LastAllConfirmedInputFrameId)
	}
}

func (pR *Room) StartBattle() {
	if RoomBattleStateIns.WAITING != pR.State {
		Logger.Debug("[StartBattle] Battle not started due to not being WAITING!", zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State))
		return
	}

	pR.RenderFrameId = 0

	// Initialize the "collisionSys" as well as "RenderFrameBuffer"
	pR.CurDynamicsRenderFrameId = 0
	kickoffFrame := &RoomDownsyncFrame{
		Id:             pR.RenderFrameId,
		Players:        toPbPlayers(pR.Players, false),
		CountdownNanos: pR.BattleDurationNanos,
	}
	pR.RenderFrameBuffer.Put(kickoffFrame)

	// Refresh "Colliders"
	spaceW := pR.StageDiscreteW * pR.StageTileW
	spaceH := pR.StageDiscreteH * pR.StageTileH

	pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY = float64(spaceW)*0.5, float64(spaceH)*0.5
	pR.refreshColliders(spaceW, spaceH)

	/**
		 * Will be triggered from a goroutine which executes the critical `Room.AddPlayerIfPossible`, thus the `battleMainLoop` should be detached.
		 * All of the consecutive stages, e.g. settlement, dismissal, should share the same goroutine with `battleMainLoop`.
	     *
	     * As "defer" is only applicable to function scope, the use of "pR.InputsBufferLock" within "battleMainLoop" is embedded into each subroutine call.
	*/
	battleMainLoop := func() {
		defer func() {
			if r := recover(); r != nil {
				Logger.Error("battleMainLoop, recovery spot#1, recovered from: ", zap.Any("roomId", pR.Id), zap.Any("panic", r))
			}
			pR.StopBattleForSettlement()
			Logger.Info(fmt.Sprintf("The `battleMainLoop` for roomId=%v is stopped@renderFrameId=%v, with battleDurationFrames=%v:\n%v", pR.Id, pR.RenderFrameId, pR.BattleDurationFrames, pR.InputsBufferString(false))) // This takes sometime to print
			pR.onBattleStoppedForSettlement()
		}()

		pR.LastRenderFrameIdTriggeredAt = utils.UnixtimeNano()
		battleStartedAt := utils.UnixtimeNano()
		Logger.Info("The `battleMainLoop` is started for:", zap.Any("roomId", pR.Id))
		for {
			stCalculation := utils.UnixtimeNano()
			elapsedNanosSinceLastFrameIdTriggered := stCalculation - pR.LastRenderFrameIdTriggeredAt
			if elapsedNanosSinceLastFrameIdTriggered < pR.RollbackEstimatedDtNanos {
				Logger.Debug(fmt.Sprintf("renderFrameId=%v@roomId=%v: Is backend running too fast? elapsedNanosSinceLastFrameIdTriggered=%v", pR.RenderFrameId, pR.Id, elapsedNanosSinceLastFrameIdTriggered))
			}

			if pR.RenderFrameId > pR.BattleDurationFrames {
				return
			}

			if swapped := atomic.CompareAndSwapInt32(&pR.State, RoomBattleStateIns.IN_BATTLE, RoomBattleStateIns.IN_BATTLE); !swapped {
				return
			}

			/*
			   [WARNING]
			   Golang "time.Sleep" is known to be taking longer than specified time to wake up at millisecond granularity, as discussed in https://github.com/golang/go/issues/44343
			   However, we assume that while "time.Sleep(16.67 ms)" might wake up after ~30ms, it still only covers at most 1 inputFrame generation.
			*/
			totalElapsedNanos := utils.UnixtimeNano() - battleStartedAt
			nextRenderFrameId := int32((totalElapsedNanos + pR.dilutedRollbackEstimatedDtNanos - 1) / pR.dilutedRollbackEstimatedDtNanos) // fast ceiling
			toSleepNanos := int64(0)
			if nextRenderFrameId > pR.RenderFrameId {
				if 0 == pR.RenderFrameId {
					// It's important to send kickoff frame iff  "0 == pR.RenderFrameId && nextRenderFrameId > pR.RenderFrameId", otherwise it might send duplicate kickoff frames
					for _, player := range pR.PlayersArr {
						playerId := player.Id
						thatPlayerBattleState := atomic.LoadInt32(&(player.BattleState)) // Might be changed in "OnPlayerDisconnected/OnPlayerLost" from other threads
						// [WARNING] DON'T try to send any message to an inactive player!
						switch thatPlayerBattleState {
						case PlayerBattleStateIns.DISCONNECTED:
						case PlayerBattleStateIns.LOST:
						case PlayerBattleStateIns.EXPELLED_DURING_GAME:
						case PlayerBattleStateIns.EXPELLED_IN_DISMISSAL:
							continue
						}
						kickoffFrame := pR.RenderFrameBuffer.GetByFrameId(0).(*RoomDownsyncFrame)
						pR.sendSafely(kickoffFrame, nil, DOWNSYNC_MSG_ACT_BATTLE_START, playerId, true)
					}
					Logger.Info(fmt.Sprintf("In `battleMainLoop` for roomId=%v sent out kickoffFrame", pR.Id))
				}
				prevRenderFrameId := pR.RenderFrameId
				pR.RenderFrameId = nextRenderFrameId

				dynamicsDuration := int64(0)

				// Prefab and buffer backend inputFrameDownsync
				if pR.BackendDynamicsEnabled {
					pR.doBattleMainLoopPerTickBackendDynamicsWithProperLocking(prevRenderFrameId, &dynamicsDuration)
				}

				pR.LastRenderFrameIdTriggeredAt = utils.UnixtimeNano()

				elapsedInCalculation := (utils.UnixtimeNano() - stCalculation)
				toSleepNanos = pR.dilutedRollbackEstimatedDtNanos - elapsedInCalculation // don't sleep if "nextRenderFrame == pR.RenderFrameId"
				if elapsedInCalculation > pR.RollbackEstimatedDtNanos {
					Logger.Warn(fmt.Sprintf("SLOW FRAME! Elapsed time statistics: roomId=%v, room.RenderFrameId=%v, elapsedInCalculation=%v ns, dynamicsDuration=%v ns, RollbackEstimatedDtNanos=%v, dilutedRollbackEstimatedDtNanos=%v", pR.Id, pR.RenderFrameId, elapsedInCalculation, dynamicsDuration, pR.RollbackEstimatedDtNanos, pR.dilutedRollbackEstimatedDtNanos))
				}
			}

			time.Sleep(time.Duration(toSleepNanos))
		}
	}

	downsyncLoop := func(playerId int32, player *Player, playerDownsyncChan chan InputsBufferSnapshot) {
		defer func() {
			if r := recover(); r != nil {
				Logger.Error("downsyncLoop, recovery spot#1, recovered from: ", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("panic", r))
			}
			Logger.Info(fmt.Sprintf("The `downsyncLoop` for (roomId=%v, playerId=%v) is stopped@renderFrameId=%v", pR.Id, playerId, pR.RenderFrameId))
		}()

		Logger.Debug(fmt.Sprintf("Started downsyncLoop for (roomId: %d, playerId:%d, playerDownsyncChan:%p)", pR.Id, playerId, playerDownsyncChan))

		for {
			select {
			case inputsBufferSnapshot := <-playerDownsyncChan:
				nowBattleState := atomic.LoadInt32(&pR.State)
				switch nowBattleState {
				case RoomBattleStateIns.IDLE:
				case RoomBattleStateIns.STOPPING_BATTLE_FOR_SETTLEMENT:
				case RoomBattleStateIns.IN_SETTLEMENT:
				case RoomBattleStateIns.IN_DISMISSAL:
					Logger.Warn(fmt.Sprintf("Battle is not waiting/preparing/active for playerDownsyncChan for (roomId: %d, playerId:%d)", pR.Id, playerId))
					return
				}

				pR.downsyncToSinglePlayer(playerId, player, inputsBufferSnapshot.RefRenderFrameId, inputsBufferSnapshot.UnconfirmedMask, inputsBufferSnapshot.ToSendInputFrameDownsyncs)
				Logger.Debug(fmt.Sprintf("Sent inputsBufferSnapshot(refRenderFrameId:%d, unconfirmedMask:%v) to for (roomId: %d, playerId:%d)#2", inputsBufferSnapshot.RefRenderFrameId, inputsBufferSnapshot.UnconfirmedMask, pR.Id, playerId))
			default:
			}
		}
	}

	for playerId, player := range pR.Players {
		/*
		   Always instantiates a new channel and let the old one die out due to not being retained by any root reference.

		   Each "playerDownsyncChan" stays alive through out the lifecycle of room instead of each "playerDownsyncSession", i.e. not closed or dereferenced upon disconnection.
		*/
		pR.PlayerDownsyncChanDict[playerId] = make(chan InputsBufferSnapshot, pR.InputsBuffer.N)
		go downsyncLoop(playerId, player, pR.PlayerDownsyncChanDict[playerId])
	}

	pR.onBattlePrepare(func() {
		pR.onBattleStarted() // NOTE: Deliberately not using `defer`.
		go battleMainLoop()
	})
}

func (pR *Room) toDiscreteInputsBufferIndex(inputFrameId int32, joinIndex int32) int32 {
	return (inputFrameId << 2) + joinIndex // allowing joinIndex upto 15
}

func (pR *Room) OnBattleCmdReceived(pReq *WsReq) {
	/*
	   [WARNING] This function "OnBattleCmdReceived" could be called by different ws sessions and thus from different threads!

	   That said, "markConfirmationIfApplicable" will still work as expected. Here's an example of weird call orders.
	   ---------------------------------------------------
	   now lastAllConfirmedInputFrameId: 42; each "()" below indicates a "Lock/Unlock cycle of InputsBufferLock", and "x" indicates no new all-confirmed snapshot is created
	   A: ([44,50],x)                                            ([49,54],snapshot=[51,53])
	   B:           ([54,58],x)
	   C:                               ([42,53],snapshot=[43,50])
	   D:                     ([51,55],x)
	   ---------------------------------------------------
	*/
	// TODO: Put a rate limiter on this function!
	if swapped := atomic.CompareAndSwapInt32(&pR.State, RoomBattleStateIns.IN_BATTLE, RoomBattleStateIns.IN_BATTLE); !swapped {
		return
	}

	playerId := pReq.PlayerId
	var player *Player = nil
	var existent bool = false
	inputFrameUpsyncBatch := pReq.InputFrameUpsyncBatch
	ackingFrameId := pReq.AckingFrameId
	ackingInputFrameId := pReq.AckingInputFrameId

	if player, existent = pR.Players[playerId]; !existent {
		Logger.Warn(fmt.Sprintf("upcmd player doesn't exist: roomId=%v, playerId=%v", pR.Id, playerId))
		return
	}

	atomic.StoreInt32(&(player.AckingFrameId), ackingFrameId)
	atomic.StoreInt32(&(player.AckingInputFrameId), ackingInputFrameId)

	//Logger.Debug(fmt.Sprintf("OnBattleCmdReceived-InputsBufferLock about to lock: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	pR.InputsBufferLock.Lock()
	//Logger.Debug(fmt.Sprintf("OnBattleCmdReceived-InputsBufferLock locked: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	defer func() {
		pR.InputsBufferLock.Unlock()
		//Logger.Debug(fmt.Sprintf("OnBattleCmdReceived-InputsBufferLock unlocked: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	}()

	inputsBufferSnapshot := pR.markConfirmationIfApplicable(inputFrameUpsyncBatch, playerId, player)
	if nil != inputsBufferSnapshot {
		pR.downsyncToAllPlayers(inputsBufferSnapshot)
	}
}

func (pR *Room) onInputFrameDownsyncAllConfirmed(inputFrameDownsync *InputFrameDownsync, playerId int32) {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	inputFrameId := inputFrameDownsync.InputFrameId
	if -1 == pR.LastAllConfirmedInputFrameIdWithChange || false == pR.equalInputLists(inputFrameDownsync.InputList, pR.LastAllConfirmedInputList) {
		if -1 == playerId {
			Logger.Debug(fmt.Sprintf("Key inputFrame change: roomId=%v, newInputFrameId=%v, lastInputFrameId=%v, newInputList=%v, lastInputList=%v, InputsBuffer=%v", pR.Id, inputFrameId, pR.LastAllConfirmedInputFrameId, inputFrameDownsync.InputList, pR.LastAllConfirmedInputList, pR.InputsBufferString(false)))
		} else {
			Logger.Debug(fmt.Sprintf("Key inputFrame change: roomId=%v, playerId=%v, newInputFrameId=%v, lastInputFrameId=%v, newInputList=%v, lastInputList=%v, InputsBuffer=%v", pR.Id, playerId, inputFrameId, pR.LastAllConfirmedInputFrameId, inputFrameDownsync.InputList, pR.LastAllConfirmedInputList, pR.InputsBufferString(false)))
		}
		pR.LastAllConfirmedInputFrameIdWithChange = inputFrameId
	}
	pR.LastAllConfirmedInputFrameId = inputFrameId
	for i, v := range inputFrameDownsync.InputList {
		// To avoid potential misuse of pointers
		pR.LastAllConfirmedInputList[i] = v
	}
	if -1 == playerId {
		Logger.Debug(fmt.Sprintf("inputFrame lifecycle#2[forced-allconfirmed]: roomId=%v, InputsBuffer=%v", pR.Id, pR.InputsBufferString(false)))
	} else {
		Logger.Debug(fmt.Sprintf("inputFrame lifecycle#2[allconfirmed]: roomId=%v, playerId=%v, InputsBuffer=%v", pR.Id, playerId, pR.InputsBufferString(false)))
	}
}

func (pR *Room) equalInputLists(lhs []uint64, rhs []uint64) bool {
	if len(lhs) != len(rhs) {
		return false
	}
	for i, _ := range lhs {
		if lhs[i] != rhs[i] {
			return false
		}
	}
	return true
}

func (pR *Room) StopBattleForSettlement() {
	if RoomBattleStateIns.IN_BATTLE != pR.State {
		return
	}
	pR.State = RoomBattleStateIns.STOPPING_BATTLE_FOR_SETTLEMENT
	Logger.Info("Stopping the `battleMainLoop` for:", zap.Any("roomId", pR.Id))
	pR.RenderFrameId++
	for playerId, _ := range pR.Players {
		assembledFrame := RoomDownsyncFrame{
			Id:             pR.RenderFrameId,
			Players:        toPbPlayers(pR.Players, false),
			CountdownNanos: -1, // TODO: Replace this magic constant!
		}
		pR.sendSafely(&assembledFrame, nil, DOWNSYNC_MSG_ACT_BATTLE_STOPPED, playerId, true)
	}
	// Note that `pR.onBattleStoppedForSettlement` will be called by `battleMainLoop`.
}

func (pR *Room) onBattleStarted() {
	if RoomBattleStateIns.PREPARE != pR.State {
		return
	}
	pR.State = RoomBattleStateIns.IN_BATTLE
	pR.updateScore()
}

func (pR *Room) onBattlePrepare(cb BattleStartCbType) {
	if RoomBattleStateIns.WAITING != pR.State {
		Logger.Warn("[onBattlePrepare] Battle not started after all players' battle state checked!", zap.Any("roomId", pR.Id), zap.Any("roomState", pR.State))
		return
	}
	pR.State = RoomBattleStateIns.PREPARE
	Logger.Info("Battle state transitted to RoomBattleStateIns.PREPARE for:", zap.Any("roomId", pR.Id))

	battleReadyToStartFrame := &RoomDownsyncFrame{
		Id:             DOWNSYNC_MSG_ACT_BATTLE_READY_TO_START,
		Players:        toPbPlayers(pR.Players, true),
		CountdownNanos: pR.BattleDurationNanos,
	}

	Logger.Info("Sending out frame for RoomBattleState.PREPARE:", zap.Any("battleReadyToStartFrame", battleReadyToStartFrame))
	for _, player := range pR.Players {
		pR.sendSafely(battleReadyToStartFrame, nil, DOWNSYNC_MSG_ACT_BATTLE_READY_TO_START, player.Id, true)
	}

	battlePreparationNanos := int64(6000000000)
	preparationLoop := func() {
		defer func() {
			Logger.Info("The `preparationLoop` is stopped for:", zap.Any("roomId", pR.Id))
			cb()
		}()
		preparationLoopStartedNanos := utils.UnixtimeNano()
		totalElapsedNanos := int64(0)
		for {
			if totalElapsedNanos > battlePreparationNanos {
				break
			}
			now := utils.UnixtimeNano()
			totalElapsedNanos = (now - preparationLoopStartedNanos)
			time.Sleep(time.Duration(battlePreparationNanos - totalElapsedNanos))
		}
	}
	go preparationLoop()
}

func (pR *Room) onBattleStoppedForSettlement() {
	if RoomBattleStateIns.STOPPING_BATTLE_FOR_SETTLEMENT != pR.State {
		return
	}
	defer func() {
		pR.onSettlementCompleted()
	}()
	pR.State = RoomBattleStateIns.IN_SETTLEMENT
	Logger.Info("The room is in settlement:", zap.Any("roomId", pR.Id))
	// TODO: Some settlement labor.
}

func (pR *Room) onSettlementCompleted() {
	pR.Dismiss()
}

func (pR *Room) Dismiss() {
	if RoomBattleStateIns.IN_SETTLEMENT != pR.State {
		return
	}
	pR.State = RoomBattleStateIns.IN_DISMISSAL
	if 0 < len(pR.Players) {
		Logger.Info("The room is in dismissal:", zap.Any("roomId", pR.Id))
		for playerId, _ := range pR.Players {
			Logger.Info("Adding 1 to pR.DismissalWaitGroup:", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId))
			pR.DismissalWaitGroup.Add(1)
			pR.expelPlayerForDismissal(playerId)
			pR.DismissalWaitGroup.Done()
			Logger.Info("Decremented 1 to pR.DismissalWaitGroup:", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId))
		}
		pR.DismissalWaitGroup.Wait()
	}
	pR.OnDismissed()
}

func (pR *Room) OnDismissed() {

	// Always instantiates new HeapRAM blocks and let the old blocks die out due to not being retained by any root reference.
	pR.BulletBattleLocalIdCounter = 0
	pR.WorldToVirtualGridRatio = float64(1000)
	pR.VirtualGridToWorldRatio = float64(1.0) / pR.WorldToVirtualGridRatio // this is a one-off computation, should avoid division in iterations
	pR.SpAtkLookupFrames = 5
	pR.PlayerDefaultSpeed = int32(float64(1) * pR.WorldToVirtualGridRatio) // in virtual grids per frame
	pR.Players = make(map[int32]*Player)
	pR.PlayersArr = make([]*Player, pR.Capacity)
	pR.CollisionSysMap = make(map[int32]*resolv.Object)
	pR.PlayerDownsyncSessionDict = make(map[int32]*websocket.Conn)
	for _, oldChan := range pR.PlayerDownsyncChanDict {
		close(oldChan)
	}
	pR.PlayerDownsyncChanDict = make(map[int32](chan InputsBufferSnapshot))
	pR.PlayerSignalToCloseDict = make(map[int32]SignalToCloseConnCbType)
	pR.JoinIndexBooleanArr = make([]bool, pR.Capacity)
	pR.Barriers = make(map[int32]*Barrier)
	pR.RenderCacheSize = 1024
	pR.RenderFrameBuffer = NewRingBuffer(pR.RenderCacheSize)
	pR.InputsBuffer = NewRingBuffer((pR.RenderCacheSize >> 1) + 1)

	pR.LastAllConfirmedInputFrameId = -1
	pR.LastAllConfirmedInputFrameIdWithChange = -1
	pR.LastAllConfirmedInputList = make([]uint64, pR.Capacity)

	pR.RenderFrameId = 0
	pR.CurDynamicsRenderFrameId = 0
	pR.InputDelayFrames = 8
	pR.NstDelayFrames = 8
	pR.InputScaleFrames = uint32(2)
	pR.ServerFps = 60
	pR.RollbackEstimatedDtMillis = 16.667  // Use fixed-and-low-precision to mitigate the inconsistent floating-point-number issue between Golang and JavaScript
	pR.RollbackEstimatedDtNanos = 16666666 // A little smaller than the actual per frame time, just for logging FAST FRAME
	dilutedServerFps := float64(55.0)
	pR.dilutedRollbackEstimatedDtNanos = int64(float64(pR.RollbackEstimatedDtNanos) * float64(pR.ServerFps) / dilutedServerFps)
	pR.BattleDurationFrames = 60 * pR.ServerFps
	pR.BattleDurationNanos = int64(pR.BattleDurationFrames) * (pR.RollbackEstimatedDtNanos + 1)
	pR.InputFrameUpsyncDelayTolerance = 2
	pR.MaxChasingRenderFramesPerUpdate = 8

	pR.BackendDynamicsEnabled = true // [WARNING] When "false", recovery upon reconnection wouldn't work!
	punchSkillId := int32(1)
	pR.MeleeSkillConfig = make(map[int32]*MeleeBullet, 0)
	pR.MeleeSkillConfig[punchSkillId] = &MeleeBullet{
		// for offender
		StartupFrames:         int32(10),
		ActiveFrames:          int32(3),
		RecoveryFrames:        int32(34),
		RecoveryFramesOnBlock: int32(34),
		RecoveryFramesOnHit:   int32(34),
		Moveforward: &Vec2D{
			X: 0,
			Y: 0,
		},
		HitboxOffset: float64(12.0), // should be about the radius of the PlayerCollider
		HitboxSize: &Vec2D{
			X: float64(24.0),
			Y: float64(32.0),
		},

		// for defender
		HitStunFrames:      int32(18),
		BlockStunFrames:    int32(9),
		Pushback:           float64(8.0),
		ReleaseTriggerType: int32(1), // 1: rising-edge, 2: falling-edge
		Damage:             int32(5),
	}

	pR.SnapIntoPlatformOverlap = float64(0.1)
	pR.SnapIntoPlatformThreshold = float64(0.5)
	pR.JumpingInitVelY = int32(float64(7) * pR.WorldToVirtualGridRatio)
	pR.GravityX = 0
	pR.GravityY = -int32(float64(0.5) * pR.WorldToVirtualGridRatio) // makes all "playerCollider.Y" a multiple of 0.5 in all cases

	pR.ChooseStage()
	pR.EffectivePlayerCount = 0

	// [WARNING] It's deliberately ordered such that "pR.State = RoomBattleStateIns.IDLE" is put AFTER all the refreshing operations above.
	pR.State = RoomBattleStateIns.IDLE
	pR.updateScore()

	Logger.Info("The room is completely dismissed(all playerDownsyncChan closed):", zap.Any("roomId", pR.Id))
}

func (pR *Room) expelPlayerDuringGame(playerId int32) {
	if signalToCloseConnOfThisPlayer, existent := pR.PlayerSignalToCloseDict[playerId]; existent {
		signalToCloseConnOfThisPlayer(Constants.RetCode.UnknownError, "") // TODO: Specify an error code
	}
	pR.onPlayerExpelledDuringGame(playerId)
}

func (pR *Room) expelPlayerForDismissal(playerId int32) {
	if signalToCloseConnOfThisPlayer, existent := pR.PlayerSignalToCloseDict[playerId]; existent {
		signalToCloseConnOfThisPlayer(Constants.RetCode.UnknownError, "") // TODO: Specify an error code
	}
	pR.onPlayerExpelledForDismissal(playerId)
}

func (pR *Room) onPlayerExpelledDuringGame(playerId int32) {
	pR.onPlayerLost(playerId)
}

func (pR *Room) onPlayerExpelledForDismissal(playerId int32) {
	pR.onPlayerLost(playerId)

	Logger.Info("onPlayerExpelledForDismissal:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("nowRoomBattleState", pR.State), zap.Any("nowRoomEffectivePlayerCount", pR.EffectivePlayerCount))
}

func (pR *Room) OnPlayerDisconnected(playerId int32) {
	defer func() {
		if r := recover(); r != nil {
			Logger.Error("Room OnPlayerDisconnected, recovery spot#1, recovered from: ", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("panic", r))
		}
	}()

	if player, existent := pR.Players[playerId]; existent {
		thatPlayerBattleState := atomic.LoadInt32(&(player.BattleState))
		switch thatPlayerBattleState {
		case PlayerBattleStateIns.DISCONNECTED:
		case PlayerBattleStateIns.LOST:
		case PlayerBattleStateIns.EXPELLED_DURING_GAME:
		case PlayerBattleStateIns.EXPELLED_IN_DISMISSAL:
			Logger.Info("Room OnPlayerDisconnected[early return #1]:", zap.Any("playerId", playerId), zap.Any("playerBattleState", pR.Players[playerId].BattleState), zap.Any("roomId", pR.Id), zap.Any("nowRoomBattleState", pR.State), zap.Any("nowRoomEffectivePlayerCount", pR.EffectivePlayerCount))
			return
		}
	} else {
		// Not even the "pR.Players[playerId]" exists.
		Logger.Info("Room OnPlayerDisconnected[early return #2]:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("nowRoomBattleState", pR.State), zap.Any("nowRoomEffectivePlayerCount", pR.EffectivePlayerCount))
		return
	}

	currRoomBattleState := atomic.LoadInt32(&(pR.State))
	switch currRoomBattleState {
	case RoomBattleStateIns.WAITING:
		pR.onPlayerLost(playerId)
		delete(pR.Players, playerId) // Note that this statement MUST be put AFTER `pR.onPlayerLost(...)` to avoid nil pointer exception.
		if 0 == pR.EffectivePlayerCount {
			atomic.StoreInt32(&(pR.State), RoomBattleStateIns.IDLE)
		}
		pR.updateScore()
		Logger.Info("Player disconnected while room is at RoomBattleStateIns.WAITING:", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("nowRoomBattleState", pR.State), zap.Any("nowRoomEffectivePlayerCount", pR.EffectivePlayerCount))
	default:
		atomic.StoreInt32(&(pR.Players[playerId].BattleState), PlayerBattleStateIns.DISCONNECTED)
		pR.clearPlayerNetworkSession(playerId) // Still need clear the network session pointers, because "OnPlayerDisconnected" is only triggered from "signalToCloseConnOfThisPlayer" in "ws/serve.go", when the same player reconnects the network session pointers will be re-assigned
		Logger.Warn("OnPlayerDisconnected finished:", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("playerBattleState", pR.Players[playerId].BattleState), zap.Any("nowRoomBattleState", pR.State), zap.Any("nowRoomEffectivePlayerCount", pR.EffectivePlayerCount))
	}
}

func (pR *Room) onPlayerLost(playerId int32) {
	defer func() {
		if r := recover(); r != nil {
			Logger.Error("Room OnPlayerLost, recovery spot, recovered from: ", zap.Any("playerId", playerId), zap.Any("roomId", pR.Id), zap.Any("panic", r))
		}
	}()
	if player, existent := pR.Players[playerId]; existent {
		atomic.StoreInt32(&(player.BattleState), PlayerBattleStateIns.LOST)
		pR.clearPlayerNetworkSession(playerId)
		pR.EffectivePlayerCount--
		indiceInJoinIndexBooleanArr := int(player.JoinIndex - 1)
		if (0 <= indiceInJoinIndexBooleanArr) && (indiceInJoinIndexBooleanArr < len(pR.JoinIndexBooleanArr)) {
			pR.JoinIndexBooleanArr[indiceInJoinIndexBooleanArr] = false
		} else {
			Logger.Warn("Room OnPlayerLost, pR.JoinIndexBooleanArr is out of range: ", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("indiceInJoinIndexBooleanArr", indiceInJoinIndexBooleanArr), zap.Any("len(pR.JoinIndexBooleanArr)", len(pR.JoinIndexBooleanArr)))
		}
		player.JoinIndex = MAGIC_JOIN_INDEX_INVALID
		Logger.Warn("OnPlayerLost: ", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("resulted pR.JoinIndexBooleanArr", pR.JoinIndexBooleanArr))
	}
}

func (pR *Room) clearPlayerNetworkSession(playerId int32) {
	if _, y := pR.PlayerDownsyncSessionDict[playerId]; y {
		Logger.Debug("clearPlayerNetworkSession:", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId))
		// [WARNING] No need to close "pR.PlayerDownsyncChanDict[playerId]" immediately!
		delete(pR.PlayerDownsyncSessionDict, playerId)
		delete(pR.PlayerSignalToCloseDict, playerId)
	}
}

func (pR *Room) onPlayerAdded(playerId int32) {
	pR.EffectivePlayerCount++

	if 1 == pR.EffectivePlayerCount {
		pR.State = RoomBattleStateIns.WAITING
	}

	for index, value := range pR.JoinIndexBooleanArr {
		if false == value {
			pR.Players[playerId].JoinIndex = int32(index) + 1
			pR.JoinIndexBooleanArr[index] = true

			// Lazily assign the initial position of "Player" for "RoomDownsyncFrame".
			playerPosList := *(pR.StrToVec2DListMap["PlayerStartingPos"])
			if index > len(playerPosList.Eles) {
				panic(fmt.Sprintf("onPlayerAdded error, index >= len(playerPosList), roomId=%v, playerId=%v, roomState=%v, roomEffectivePlayerCount=%v", pR.Id, playerId, pR.State, pR.EffectivePlayerCount))
			}
			playerPos := playerPosList.Eles[index]

			if nil == playerPos {
				panic(fmt.Sprintf("onPlayerAdded error, nil == playerPos, roomId=%v, playerId=%v, roomState=%v, roomEffectivePlayerCount=%v", pR.Id, playerId, pR.State, pR.EffectivePlayerCount))
			}
			pR.Players[playerId].VirtualGridX, pR.Players[playerId].VirtualGridY = WorldToVirtualGridPos(playerPos.X, playerPos.Y, pR.WorldToVirtualGridRatio)
			// Hardcoded initial character orientation/facing
			if 0 == (pR.Players[playerId].JoinIndex % 2) {
				pR.Players[playerId].DirX = -2
				pR.Players[playerId].DirY = 0
			} else {
				pR.Players[playerId].DirX = +2
				pR.Players[playerId].DirY = 0
			}

			break
		}
	}

	pR.updateScore()
	Logger.Info("onPlayerAdded:", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("playerBattleState", pR.Players[playerId].BattleState), zap.Any("joinIndex", pR.Players[playerId].JoinIndex), zap.Any("EffectivePlayerCount", pR.EffectivePlayerCount), zap.Any("resulted pR.JoinIndexBooleanArr", pR.JoinIndexBooleanArr), zap.Any("RoomBattleState", pR.State))
}

func (pR *Room) onPlayerReAdded(playerId int32) {
	/*
	 * [WARNING]
	 *
	 * If a player quits at "RoomBattleState.WAITING", then his/her re-joining will always invoke `AddPlayerIfPossible(...)`. Therefore, this
	 * function will only be invoked for players who quit the battle at ">RoomBattleState.WAITING" and re-join at "RoomBattleState.IN_BATTLE", during which the `pR.JoinIndexBooleanArr` doesn't change.
	 */
	Logger.Info("Room got `onPlayerReAdded` invoked,", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("resulted pR.JoinIndexBooleanArr", pR.JoinIndexBooleanArr))

	pR.updateScore()
}

func (pR *Room) OnPlayerBattleColliderAcked(playerId int32) bool {
	targetPlayer, existing := pR.Players[playerId]
	if false == existing {
		return false
	}

	shouldTryToStartBattle := true
	Logger.Debug(fmt.Sprintf("OnPlayerBattleColliderAcked-before: roomId=%v, roomState=%v, targetPlayerId=%v, targetPlayerBattleState=%v, capacity=%v, EffectivePlayerCount=%v", pR.Id, pR.State, targetPlayer.Id, targetPlayer.BattleState, pR.Capacity, pR.EffectivePlayerCount))
	targetPlayerBattleState := atomic.LoadInt32(&(targetPlayer.BattleState))
	switch targetPlayerBattleState {
	case PlayerBattleStateIns.ADDED_PENDING_BATTLE_COLLIDER_ACK:
		playerAckedFrame := &RoomDownsyncFrame{
			Id:      pR.RenderFrameId,
			Players: toPbPlayers(pR.Players, true),
		}

		// Broadcast normally added player info to all players in the same room
		for _, thatPlayer := range pR.Players {
			/*
			   [WARNING]
			   This `playerAckedFrame` is the first ever "RoomDownsyncFrame" for every "PersistentSessionClient on the frontend", and it goes right after each "BattleColliderInfo".

			   By making use of the sequential nature of each ws session, all later "RoomDownsyncFrame"s generated after `pRoom.StartBattle()` will be put behind this `playerAckedFrame`.

			   This function is triggered by an upsync message via WebSocket, thus downsync sending is also available by now.
			*/
			thatPlayerId := thatPlayer.Id
			thatPlayerBattleState := atomic.LoadInt32(&(thatPlayer.BattleState))
			Logger.Debug(fmt.Sprintf("OnPlayerBattleColliderAcked-middle: roomId=%v, roomState=%v, targetPlayerId=%v, targetPlayerBattleState=%v, thatPlayerId=%v, thatPlayerBattleState=%v", pR.Id, pR.State, targetPlayer.Id, targetPlayer.BattleState, thatPlayer.Id, thatPlayerBattleState))
			if thatPlayerId == targetPlayer.Id || (PlayerBattleStateIns.ADDED_PENDING_BATTLE_COLLIDER_ACK == thatPlayerBattleState || PlayerBattleStateIns.ACTIVE == thatPlayerBattleState) {
				Logger.Debug(fmt.Sprintf("OnPlayerBattleColliderAcked-sending DOWNSYNC_MSG_ACT_PLAYER_ADDED_AND_ACKED: roomId=%v, roomState=%v, targetPlayerId=%v, targetPlayerBattleState=%v, capacity=%v, EffectivePlayerCount=%v", pR.Id, pR.State, targetPlayer.Id, targetPlayer.BattleState, pR.Capacity, pR.EffectivePlayerCount))
				pR.sendSafely(playerAckedFrame, nil, DOWNSYNC_MSG_ACT_PLAYER_ADDED_AND_ACKED, thatPlayer.Id, true)
			}
		}
		atomic.StoreInt32(&(targetPlayer.BattleState), PlayerBattleStateIns.ACTIVE)
	case PlayerBattleStateIns.READDED_PENDING_BATTLE_COLLIDER_ACK:
		shouldTryToStartBattle = false
		atomic.StoreInt32(&(targetPlayer.BattleState), PlayerBattleStateIns.READDED_BATTLE_COLLIDER_ACKED)
	default:
	}

	Logger.Debug(fmt.Sprintf("OnPlayerBattleColliderAcked-post-downsync: roomId=%v, roomState=%v, targetPlayerId=%v, targetPlayerBattleState=%v, capacity=%v, EffectivePlayerCount=%v", pR.Id, pR.State, targetPlayer.Id, targetPlayer.BattleState, pR.Capacity, pR.EffectivePlayerCount))

	if shouldTryToStartBattle {
		if pR.Capacity == int(pR.EffectivePlayerCount) {
			allAcked := true
			for _, p := range pR.Players {
				if PlayerBattleStateIns.ACTIVE != p.BattleState {
					Logger.Warn("Unexpectedly got an inactive player", zap.Any("roomId", pR.Id), zap.Any("playerId", p.Id), zap.Any("battleState", p.BattleState))
					allAcked = false
					break
				}
			}
			if true == allAcked {
				pR.StartBattle() // WON'T run if the battle state is not in WAITING.
			}
		}

		pR.updateScore()
	}
	return true
}

func (pR *Room) sendSafely(roomDownsyncFrame *RoomDownsyncFrame, toSendInputFrameDownsyncs []*InputFrameDownsync, act int32, playerId int32, needLockExplicitly bool) {
	defer func() {
		if r := recover(); r != nil {
			Logger.Error("sendSafely, recovered from: ", zap.Any("roomId", pR.Id), zap.Any("playerId", playerId), zap.Any("panic", r))
		}
	}()

	if playerDownsyncSession, existent := pR.PlayerDownsyncSessionDict[playerId]; existent {
		pResp := &WsResp{
			Ret:                     int32(Constants.RetCode.Ok),
			Act:                     act,
			Rdf:                     roomDownsyncFrame,
			InputFrameDownsyncBatch: toSendInputFrameDownsyncs,
		}

		theBytes, marshalErr := proto.Marshal(pResp)
		if nil != marshalErr {
			panic(fmt.Sprintf("Error marshaling downsync message: roomId=%v, playerId=%v, roomState=%v, roomEffectivePlayerCount=%v", pR.Id, playerId, pR.State, pR.EffectivePlayerCount))
		}

		if err := playerDownsyncSession.WriteMessage(websocket.BinaryMessage, theBytes); nil != err {
			panic(fmt.Sprintf("Error sending downsync message: roomId=%v, playerId=%v, roomState=%v, roomEffectivePlayerCount=%v, err=%v", pR.Id, playerId, pR.State, pR.EffectivePlayerCount, err))
		}
	}
}

func (pR *Room) shouldPrefabInputFrameDownsync(prevRenderFrameId int32, renderFrameId int32) (bool, int32) {
	for i := prevRenderFrameId + 1; i <= renderFrameId; i++ {
		if (0 <= i) && (0 == (i & ((1 << pR.InputScaleFrames) - 1))) {
			return true, i
		}
	}
	return false, -1
}

func (pR *Room) prefabInputFrameDownsync(inputFrameId int32) *InputFrameDownsync {
	/*
	   [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked.

	   Kindly note that on backend the prefab is much simpler than its frontend counterpart, because frontend will upsync its latest command immediately if there's any change w.r.t. its own prev cmd, thus if no upsync received from a frontend,
	   - EITHER it's due to local lag and bad network,
	   - OR there's no change w.r.t. to its prev cmd.
	*/

	var currInputFrameDownsync *InputFrameDownsync = nil
	tmp1 := pR.InputsBuffer.GetByFrameId(inputFrameId) // Would be nil if "pR.InputsBuffer.EdFrameId <= inputFrameId", else if "pR.InputsBuffer.EdFrameId > inputFrameId" is already met, then by now we can just return "tmp1.(*InputFrameDownsync)"
	if nil == tmp1 {
		for pR.InputsBuffer.EdFrameId <= inputFrameId {
			j := pR.InputsBuffer.EdFrameId
			currInputFrameDownsync = &InputFrameDownsync{
				InputFrameId:  j,
				InputList:     make([]uint64, pR.Capacity),
				ConfirmedList: uint64(0),
			}

			tmp2 := pR.InputsBuffer.GetByFrameId(j - 1) // There's no need for the backend to find the "lastAllConfirmed inputs" for prefabbing, either "BackendDynamicsEnabled" is true or false
			if nil != tmp2 {
				prevInputFrameDownsync := tmp2.(*InputFrameDownsync)
				for i, _ := range currInputFrameDownsync.InputList {
					currInputFrameDownsync.InputList[i] = (prevInputFrameDownsync.InputList[i] & uint64(15)) // Don't predict attack input!
				}
			}

			pR.InputsBuffer.Put(currInputFrameDownsync)
		}
	} else {
		currInputFrameDownsync = tmp1.(*InputFrameDownsync)
	}

	return currInputFrameDownsync
}

func (pR *Room) markConfirmationIfApplicable(inputFrameUpsyncBatch []*InputFrameUpsync, playerId int32, player *Player) *InputsBufferSnapshot {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	for _, inputFrameUpsync := range inputFrameUpsyncBatch {
		clientInputFrameId := inputFrameUpsync.InputFrameId
		if clientInputFrameId < pR.InputsBuffer.StFrameId {
			// The updates to "pR.InputsBuffer.StFrameId" is monotonically increasing, thus if "clientInputFrameId < pR.InputsBuffer.StFrameId" at any moment of time, it is obsolete in the future.
			Logger.Debug(fmt.Sprintf("Omitting obsolete inputFrameUpsync#1: roomId=%v, playerId=%v, clientInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.InputsBufferString(false)))
			continue
		}
		if clientInputFrameId < pR.LastAllConfirmedInputFrameId {
			Logger.Info(fmt.Sprintf("Omitting obsolete inputFrameUpsync#2: roomId=%v, playerId=%v, clientInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.InputsBufferString(false)))
			continue
		}
		if clientInputFrameId > pR.InputsBuffer.EdFrameId {
			Logger.Warn(fmt.Sprintf("Dropping too advanced inputFrameUpsync: roomId=%v, playerId=%v, clientInputFrameId=%v, InputsBuffer=%v; is this player cheating?", pR.Id, playerId, clientInputFrameId, pR.InputsBufferString(false)))
			continue
		}
		var targetInputFrameDownsync *InputFrameDownsync = nil
		if clientInputFrameId == pR.InputsBuffer.EdFrameId {
			targetInputFrameDownsync = pR.prefabInputFrameDownsync(clientInputFrameId)
			Logger.Debug(fmt.Sprintf("OnBattleCmdReceived-Prefabbed new inputFrameDownsync from inputFrameUpsync: roomId=%v, playerId=%v, clientInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.InputsBufferString(false)))
		} else {
			targetInputFrameDownsync = pR.InputsBuffer.GetByFrameId(clientInputFrameId).(*InputFrameDownsync)
			Logger.Debug(fmt.Sprintf("OnBattleCmdReceived-stuffing inputFrameDownsync from inputFrameUpsync: roomId=%v, playerId=%v, clientInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.InputsBufferString(false)))
		}
		targetInputFrameDownsync.InputList[player.JoinIndex-1] = inputFrameUpsync.Encoded
		targetInputFrameDownsync.ConfirmedList |= uint64(1 << uint32(player.JoinIndex-1))
	}

	newAllConfirmedCount := int32(0)
	inputFrameId1 := pR.LastAllConfirmedInputFrameId + 1
	totPlayerCnt := uint32(pR.Capacity)
	allConfirmedMask := uint64((1 << totPlayerCnt) - 1)

	for inputFrameId := inputFrameId1; inputFrameId < pR.InputsBuffer.EdFrameId; inputFrameId++ {
		tmp := pR.InputsBuffer.GetByFrameId(inputFrameId)
		if nil == tmp {
			panic(fmt.Sprintf("inputFrameId=%v doesn't exist for roomId=%v! InputsBuffer=%v", inputFrameId, pR.Id, pR.InputsBufferString(false)))
		}
		shouldBreakConfirmation := false
		inputFrameDownsync := tmp.(*InputFrameDownsync)

		if allConfirmedMask != inputFrameDownsync.ConfirmedList {
			for _, player := range pR.PlayersArr {
				thatPlayerBattleState := atomic.LoadInt32(&(player.BattleState))
				thatPlayerJoinMask := uint64(1 << uint32(player.JoinIndex-1))
				if 0 == (inputFrameDownsync.ConfirmedList & thatPlayerJoinMask) {
					if thatPlayerBattleState == PlayerBattleStateIns.ACTIVE {
						shouldBreakConfirmation = true
						break
					} else {
						Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable for roomId=%v, skipping UNCONFIRMED BUT INACTIVE player(id:%v, joinIndex:%v) while checking inputFrameId=[%v, %v): InputsBuffer=%v", pR.Id, player.Id, player.JoinIndex, inputFrameId1, pR.InputsBuffer.EdFrameId, pR.InputsBufferString(false)))
					}
				}
			}
		}

		if shouldBreakConfirmation {
			break
		} else {
			newAllConfirmedCount += 1
			pR.onInputFrameDownsyncAllConfirmed(inputFrameDownsync, -1)
		}
	}

	if 0 < newAllConfirmedCount {
		refRenderFrameIdIfNeeded := pR.CurDynamicsRenderFrameId - 1
		/*
		   [WARNING]

		   If "pR.InputsBufferLock" was previously held by "doBattleMainLoopPerTickBackendDynamicsWithProperLocking", then this value would be just (pR.LastAllConfirmedInputFrameId - newAllConfirmedCount).

		   However if "pR.InputsBufferLock" was previously held by another "OnBattleCmdReceived", this value might be smaller than (pR.LastAllConfirmedInputFrameId - newAllConfirmedCount)!
		*/
		snapshotStFrameId := pR.ConvertToInputFrameId(refRenderFrameIdIfNeeded, pR.InputDelayFrames)
		// Duplicate downsynced inputFrameIds will be filtered out by frontend.
		toSendInputFrameDownsyncs := pR.cloneInputsBuffer(snapshotStFrameId, pR.LastAllConfirmedInputFrameId+1)
		Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable for roomId=%v returning newAllConfirmedCount=%d: InputsBuffer=%v", pR.Id, newAllConfirmedCount, pR.InputsBufferString(false)))
		return &InputsBufferSnapshot{
			RefRenderFrameId:          refRenderFrameIdIfNeeded,
			UnconfirmedMask:           uint64(0),
			ToSendInputFrameDownsyncs: toSendInputFrameDownsyncs,
		}
	} else {
		return nil
	}
}

func (pR *Room) forceConfirmationIfApplicable(prevRenderFrameId int32) *InputsBufferSnapshot {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	// Force confirmation of non-all-confirmed inputFrame EXACTLY ONE AT A TIME, returns the non-confirmed mask of players, e.g. in a 4-player-battle returning 1001 means that players with JoinIndex=1 and JoinIndex=4 are non-confirmed for inputFrameId2

	renderFrameId1 := (pR.RenderFrameId - pR.NstDelayFrames) // the renderFrameId which should've been rendered on frontend
	if 0 > renderFrameId1 {
		return nil
	}
	ok := false
	renderFrameId2 := int32(-1)
	if ok, renderFrameId2 = pR.shouldPrefabInputFrameDownsync(prevRenderFrameId-pR.NstDelayFrames, renderFrameId1); !ok {
		/*
		   The backend "shouldPrefabInputFrameDownsync" shares the same rule as frontend "shouldGenerateInputFrameUpsync".

		   It's also important that "forceConfirmationIfApplicable" is NOT EXECUTED for every renderFrame, such that when a player is forced to resync, it has some time, i.e. (1 << InputScaleFrames) renderFrames, to upsync again.
		*/
		return nil
	}

	inputFrameId2 := pR.ConvertToInputFrameId(renderFrameId2, 0) // The inputFrame to force confirmation (if necessary)
	if inputFrameId2 < pR.LastAllConfirmedInputFrameId {
		// No need to force confirmation, the inputFrames already arrived
		Logger.Debug(fmt.Sprintf("inputFrameId2=%v is already all-confirmed for roomId=%v[type#1], no need to force confirmation of it", inputFrameId2, pR.Id))
		return nil
	}

	tmp := pR.InputsBuffer.GetByFrameId(inputFrameId2)
	if nil == tmp {
		panic(fmt.Sprintf("inputFrameId2=%v doesn't exist for roomId=%v, this is abnormal because the server should prefab inputFrameDownsync in a most advanced pace, check the prefab logic! InputsBuffer=%v", inputFrameId2, pR.Id, pR.InputsBufferString(false)))
	}

	totPlayerCnt := uint32(pR.Capacity)
	allConfirmedMask := uint64((1 << totPlayerCnt) - 1)

	// Force confirmation of "inputFrame2"
	inputFrame2 := tmp.(*InputFrameDownsync)
	oldConfirmedList := inputFrame2.ConfirmedList
	unconfirmedMask := (oldConfirmedList ^ allConfirmedMask)
	inputFrame2.ConfirmedList = allConfirmedMask
	pR.onInputFrameDownsyncAllConfirmed(inputFrame2, -1)

	if 0 < unconfirmedMask {
		// This condition should be rarely met!
		/*
		   Upon resynced on frontend, "refRenderFrameId" is now set to as advanced as possible, and it's the frontend's responsibility now to pave way for the "gap inputFrames"

		   If "NstDelayFrames" becomes larger, "pR.RenderFrameId - refRenderFrameId" possibly becomes larger because the force confirmation is delayed more.

		   Upon resync, it's still possible that "refRenderFrameId < frontend.chaserRenderFrameId" -- and this is allowed.
		*/
		refRenderFrameIdIfNeeded := pR.ConvertToLastUsedRenderFrameId(pR.LastAllConfirmedInputFrameId, pR.InputDelayFrames)
		if 0 > refRenderFrameIdIfNeeded {
			// Without a "refRenderFrame", there's no point to force confirmation, i.e. nothing to downsync to the "ACTIVE but slowly ticking frontend(s)"
			return nil
		}
		snapshotStFrameId := pR.ConvertToInputFrameId(refRenderFrameIdIfNeeded, pR.InputDelayFrames)
		toSendInputFrameDownsyncs := pR.cloneInputsBuffer(snapshotStFrameId, pR.LastAllConfirmedInputFrameId+1)
		return &InputsBufferSnapshot{
			RefRenderFrameId:          refRenderFrameIdIfNeeded,
			UnconfirmedMask:           unconfirmedMask,
			ToSendInputFrameDownsyncs: toSendInputFrameDownsyncs,
		}
	} else {
		return nil
	}
}

func (pR *Room) applyInputFrameDownsyncDynamics(fromRenderFrameId int32, toRenderFrameId int32, spaceOffsetX, spaceOffsetY float64) {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	if fromRenderFrameId >= toRenderFrameId {
		return
	}

	Logger.Debug(fmt.Sprintf("Applying inputFrame dynamics: roomId=%v, room.RenderFrameId=%v, fromRenderFrameId=%v, toRenderFrameId=%v", pR.Id, pR.RenderFrameId, fromRenderFrameId, toRenderFrameId))
	totPlayerCnt := uint32(pR.Capacity)
	allConfirmedMask := uint64((1 << totPlayerCnt) - 1)

	for collisionSysRenderFrameId := fromRenderFrameId; collisionSysRenderFrameId < toRenderFrameId; collisionSysRenderFrameId++ {
		currRenderFrameTmp := pR.RenderFrameBuffer.GetByFrameId(collisionSysRenderFrameId)
		if nil == currRenderFrameTmp {
			panic(fmt.Sprintf("collisionSysRenderFrameId=%v doesn't exist for roomId=%v, this is abnormal because it's to be used for applying dynamics to [fromRenderFrameId:%v, toRenderFrameId:%v)! RenderFrameBuffer=%v", collisionSysRenderFrameId, pR.Id, fromRenderFrameId, toRenderFrameId, pR.RenderFrameBufferString()))
		}
		currRenderFrame := currRenderFrameTmp.(*RoomDownsyncFrame)
		delayedInputFrameId := pR.ConvertToInputFrameId(collisionSysRenderFrameId, pR.InputDelayFrames)
		var delayedInputFrame *InputFrameDownsync = nil
		if 0 <= delayedInputFrameId {
			if delayedInputFrameId > pR.LastAllConfirmedInputFrameId {
				panic(fmt.Sprintf("delayedInputFrameId=%v is not yet all-confirmed for roomId=%v, this is abnormal because it's to be used for applying dynamics to [fromRenderFrameId:%v, toRenderFrameId:%v) @ collisionSysRenderFrameId=%v! InputsBuffer=%v", delayedInputFrameId, pR.Id, fromRenderFrameId, toRenderFrameId, collisionSysRenderFrameId, pR.InputsBufferString(false)))
			}
			tmp := pR.InputsBuffer.GetByFrameId(delayedInputFrameId)
			if nil == tmp {
				panic(fmt.Sprintf("delayedInputFrameId=%v doesn't exist for roomId=%v, this is abnormal because it's to be used for applying dynamics to [fromRenderFrameId:%v, toRenderFrameId:%v) @ collisionSysRenderFrameId=%v! InputsBuffer=%v", delayedInputFrameId, pR.Id, fromRenderFrameId, toRenderFrameId, collisionSysRenderFrameId, pR.InputsBufferString(false)))
			}
			delayedInputFrame = tmp.(*InputFrameDownsync)
			// [WARNING] It's possible that by now "allConfirmedMask != delayedInputFrame.ConfirmedList && delayedInputFrameId <= pR.LastAllConfirmedInputFrameId", we trust "pR.LastAllConfirmedInputFrameId" as the TOP AUTHORITY.
			delayedInputFrame.ConfirmedList = allConfirmedMask
		}

		nextRenderFrame := pR.applyInputFrameDownsyncDynamicsOnSingleRenderFrame(delayedInputFrame, currRenderFrame, pR.CollisionSysMap)
		pR.RenderFrameBuffer.Put(nextRenderFrame)
		pR.CurDynamicsRenderFrameId++
	}
}

// TODO: Write unit-test for this function to compare with its frontend counter part
func (pR *Room) applyInputFrameDownsyncDynamicsOnSingleRenderFrame(delayedInputFrame *InputFrameDownsync, currRenderFrame *RoomDownsyncFrame, collisionSysMap map[int32]*resolv.Object) *RoomDownsyncFrame {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	nextRenderFramePlayers := make(map[int32]*PlayerDownsync, pR.Capacity)
	// Make a copy first
	for playerId, currPlayerDownsync := range currRenderFrame.Players {
		nextRenderFramePlayers[playerId] = &PlayerDownsync{
			Id:              playerId,
			VirtualGridX:    currPlayerDownsync.VirtualGridX,
			VirtualGridY:    currPlayerDownsync.VirtualGridY,
			DirX:            currPlayerDownsync.DirX,
			DirY:            currPlayerDownsync.DirY,
			VelX:            currPlayerDownsync.VelX,
			VelY:            currPlayerDownsync.VelY,
			CharacterState:  currPlayerDownsync.CharacterState,
			InAir:           true,
			Speed:           currPlayerDownsync.Speed,
			BattleState:     currPlayerDownsync.BattleState,
			Score:           currPlayerDownsync.Score,
			Removed:         currPlayerDownsync.Removed,
			JoinIndex:       currPlayerDownsync.JoinIndex,
			FramesToRecover: currPlayerDownsync.FramesToRecover - 1,
			Hp:              currPlayerDownsync.Hp,
			MaxHp:           currPlayerDownsync.MaxHp,
		}
		if nextRenderFramePlayers[playerId].FramesToRecover < 0 {
			nextRenderFramePlayers[playerId].FramesToRecover = 0
		}
	}

	nextRenderFrameMeleeBullets := make([]*MeleeBullet, 0, len(currRenderFrame.MeleeBullets)) // Is there any better way to reduce malloc/free impact, e.g. smart prediction for fixed memory allocation?
	effPushbacks := make([]Vec2D, pR.Capacity)
	hardPushbackNorms := make([][]Vec2D, pR.Capacity)

	// 1. Process player inputs
	if nil != delayedInputFrame {
		var delayedInputFrameForPrevRenderFrame *InputFrameDownsync = nil
		tmp := pR.InputsBuffer.GetByFrameId(pR.ConvertToInputFrameId(currRenderFrame.Id-1, pR.InputDelayFrames))
		if nil != tmp {
			delayedInputFrameForPrevRenderFrame = tmp.(*InputFrameDownsync)
		}
		inputList := delayedInputFrame.InputList
		for _, player := range pR.PlayersArr {
			playerId := player.Id
			joinIndex := player.JoinIndex
			currPlayerDownsync, thatPlayerInNextFrame := currRenderFrame.Players[playerId], nextRenderFramePlayers[playerId]
			if 0 < thatPlayerInNextFrame.FramesToRecover {
				continue
			}
			decodedInput := pR.decodeInput(inputList[joinIndex-1])
			prevBtnALevel, prevBtnBLevel := int32(0), int32(0)
			if nil != delayedInputFrameForPrevRenderFrame {
				prevDecodedInput := pR.decodeInput(delayedInputFrameForPrevRenderFrame.InputList[joinIndex-1])
				prevBtnALevel = prevDecodedInput.BtnALevel
				prevBtnBLevel = prevDecodedInput.BtnBLevel
			}

			if decodedInput.BtnBLevel > prevBtnBLevel {
				characStateAlreadyInAir := false
				if ATK_CHARACTER_STATE_INAIR_IDLE1 == thatPlayerInNextFrame.CharacterState || ATK_CHARACTER_STATE_INAIR_ATK1 == thatPlayerInNextFrame.CharacterState || ATK_CHARACTER_STATE_INAIR_ATKED1 == thatPlayerInNextFrame.CharacterState {
					characStateAlreadyInAir = true
				}
				characStateIsInterruptWaivable := false
				if ATK_CHARACTER_STATE_IDLE1 == thatPlayerInNextFrame.CharacterState || ATK_CHARACTER_STATE_WALKING == thatPlayerInNextFrame.CharacterState || ATK_CHARACTER_STATE_INAIR_IDLE1 == thatPlayerInNextFrame.CharacterState {
					characStateIsInterruptWaivable = true
				}
				if !characStateAlreadyInAir && characStateIsInterruptWaivable {
					thatPlayerInNextFrame.VelY = pR.JumpingInitVelY
					if 1 == currPlayerDownsync.JoinIndex {
						Logger.Info(fmt.Sprintf("playerId=%v, joinIndex=%v jumped at {renderFrame.id: %d, virtualX: %d, virtualY: %d, nextVelX: %d, nextVelY: %d}, delayedInputFrame.id=%d", playerId, joinIndex, currRenderFrame.Id, currPlayerDownsync.VirtualGridX, currPlayerDownsync.VirtualGridY, thatPlayerInNextFrame.VelX, thatPlayerInNextFrame.VelY, delayedInputFrame.InputFrameId))
					}
				}
			}

			if decodedInput.BtnALevel > prevBtnALevel {
				punchSkillId := int32(1)
				punchConfig := pR.MeleeSkillConfig[punchSkillId]
				var newMeleeBullet MeleeBullet = *punchConfig
				newMeleeBullet.BattleLocalId = pR.BulletBattleLocalIdCounter
				pR.BulletBattleLocalIdCounter += 1
				newMeleeBullet.OffenderJoinIndex = joinIndex
				newMeleeBullet.OffenderPlayerId = playerId
				newMeleeBullet.OriginatedRenderFrameId = currRenderFrame.Id
				nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, &newMeleeBullet)
				thatPlayerInNextFrame.FramesToRecover = newMeleeBullet.RecoveryFrames
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_ATK1
				if false == currPlayerDownsync.InAir {
					thatPlayerInNextFrame.VelX = 0
				}
				Logger.Debug(fmt.Sprintf("roomId=%v, playerId=%v triggered a rising-edge of btnA at currRenderFrame.id=%v, delayedInputFrame.id=%v", pR.Id, playerId, currRenderFrame.Id, delayedInputFrame.InputFrameId))

			} else if decodedInput.BtnALevel < prevBtnALevel {
				Logger.Debug(fmt.Sprintf("roomId=%v, playerId=%v triggered a falling-edge of btnA at currRenderFrame.id=%v, delayedInputFrame.id=%v", pR.Id, playerId, currRenderFrame.Id, delayedInputFrame.InputFrameId))
			} else {
				// No bullet trigger, process movement inputs
				// Note that by now "0 == thatPlayerInNextFrame.FramesToRecover", we should change "CharacterState" to "WALKING" or "IDLE" depending on player inputs
				if 0 != decodedInput.Dx || 0 != decodedInput.Dy {
					thatPlayerInNextFrame.DirX = decodedInput.Dx
					thatPlayerInNextFrame.DirY = decodedInput.Dy
					thatPlayerInNextFrame.VelX = decodedInput.Dx * currPlayerDownsync.Speed
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_WALKING
				} else {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
					thatPlayerInNextFrame.VelX = 0
				}
			}
		}
	}

	// 2. Process player movement
	for _, player := range pR.PlayersArr {
		playerId := player.Id
		joinIndex := player.JoinIndex
		effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y = float64(0), float64(0)
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		currPlayerDownsync, thatPlayerInNextFrame := currRenderFrame.Players[playerId], nextRenderFramePlayers[playerId]
		// Reset playerCollider position from the "virtual grid position"
		newVx, newVy := currPlayerDownsync.VirtualGridX+currPlayerDownsync.VelX, currPlayerDownsync.VirtualGridY+currPlayerDownsync.VelY

		colliderWidth, colliderHeight := player.ColliderRadius*2, player.ColliderRadius*4
		playerCollider.X, playerCollider.Y = VirtualGridToPolygonColliderBLPos(newVx, newVy, colliderWidth*0.5, colliderHeight*0.5, pR.SnapIntoPlatformOverlap, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, pR.VirtualGridToWorldRatio)
		// Update in the collision system
		playerCollider.Update()

		if currPlayerDownsync.InAir {
			thatPlayerInNextFrame.VelX += pR.GravityX
			thatPlayerInNextFrame.VelY += pR.GravityY
		}
	}

	// 3. Add bullet colliders into collision system
	bulletColliders := make(map[int32]*resolv.Object, 0) // Will all be removed at the end of `applyInputFrameDownsyncDynamicsOnSingleRenderFrame` due to the need for being rollback-compatible
	removedBulletsAtCurrFrame := make(map[int32]int32, 0)
	for _, meleeBullet := range currRenderFrame.MeleeBullets {
		if (meleeBullet.OriginatedRenderFrameId+meleeBullet.StartupFrames <= currRenderFrame.Id) && (meleeBullet.OriginatedRenderFrameId+meleeBullet.StartupFrames+meleeBullet.ActiveFrames > currRenderFrame.Id) {
			collisionBulletIndex := COLLISION_BULLET_INDEX_PREFIX + meleeBullet.BattleLocalId
			collisionOffenderIndex := COLLISION_PLAYER_INDEX_PREFIX + meleeBullet.OffenderJoinIndex
			offenderCollider := collisionSysMap[collisionOffenderIndex]
			offender := currRenderFrame.Players[meleeBullet.OffenderPlayerId]

			xfac := float64(1.0) // By now, straight Punch offset doesn't respect "y-axis"
			if 0 > offender.DirX {
				xfac = float64(-1.0)
			}
			offenderWx, offenderWy := VirtualGridToWorldPos(offender.VirtualGridX, offender.VirtualGridY, pR.VirtualGridToWorldRatio)
			bulletWx, bulletWy := offenderWx+xfac*meleeBullet.HitboxOffset, offenderWy

			newBulletCollider := GenerateRectCollider(bulletWx, bulletWy, meleeBullet.HitboxSize.X, meleeBullet.HitboxSize.Y, 0, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, "MeleeBullet")
			newBulletCollider.Data = meleeBullet
			pR.Space.Add(newBulletCollider)
			collisionSysMap[collisionBulletIndex] = newBulletCollider
			bulletColliders[collisionBulletIndex] = newBulletCollider

			Logger.Debug(fmt.Sprintf("roomId=%v, a meleeBullet is added to collisionSys at currRenderFrame.id=%v as start-up frames ended and active frame is not yet ended: %v, from offenderCollider=%v, xfac=%v", pR.Id, currRenderFrame.Id, ConvexPolygonStr(newBulletCollider.Shape.(*resolv.ConvexPolygon)), ConvexPolygonStr(offenderCollider.Shape.(*resolv.ConvexPolygon)), xfac))
		}
	}

	// 4. Invoke collision system stepping (no-op for backend collision lib)

	// 5. Calc pushbacks for each player (after its movement) w/o bullets
	for _, player := range pR.PlayersArr {
		joinIndex := player.JoinIndex
		playerId := player.Id
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		playerShape := playerCollider.Shape.(*resolv.ConvexPolygon)
		hardPushbackNorms[joinIndex-1] = pR.calcHardPushbacksNorms(playerCollider, playerShape, pR.SnapIntoPlatformOverlap, &(effPushbacks[joinIndex-1]))
		if 0 < len(hardPushbackNorms[joinIndex-1]) {
			Logger.Debug(fmt.Sprintf("playerId=%d, joinIndex=%d got %d non-empty hardPushbacks at renderFrame.id=%d", playerId, joinIndex, len(hardPushbackNorms), currRenderFrame.Id))
		}
		currPlayerDownsync, thatPlayerInNextFrame := currRenderFrame.Players[playerId], nextRenderFramePlayers[playerId]
		fallStopping := false
		possiblyFallStoppedOnAnotherPlayer := false
		collision := playerCollider.Check(0, 0)
		if nil == collision {
			continue
		}
		for _, obj := range collision.Objects {
			isBarrier, isAnotherPlayer, isBullet := false, false, false
			switch obj.Data.(type) {
			case *Barrier:
				isBarrier = true
			case *Player:
				isAnotherPlayer = true
			case *MeleeBullet:
				isBullet = true
			}
			if isBullet {
				// ignore bullets for this step
				continue
			}
			bShape := obj.Shape.(*resolv.ConvexPolygon)
			overlapped, pushbackX, pushbackY, overlapResult := CalcPushbacks(0, 0, playerShape, bShape)
			if !overlapped {
				continue
			}
			normAlignmentWithGravity := (overlapResult.OverlapX*float64(0) + overlapResult.OverlapY*float64(-1.0))
			landedOnGravityPushback := (pR.SnapIntoPlatformThreshold < normAlignmentWithGravity) // prevents false snapping on the lateral sides
			if landedOnGravityPushback {
				// kindly note that one player might land on top of another player, and snapping is also required in such case
				pushbackX, pushbackY = (overlapResult.Overlap-pR.SnapIntoPlatformOverlap)*overlapResult.OverlapX, (overlapResult.Overlap-pR.SnapIntoPlatformOverlap)*overlapResult.OverlapY
				thatPlayerInNextFrame.InAir = false
			}
			for _, hardPushbackNorm := range hardPushbackNorms[joinIndex-1] {
				projectedMagnitude := pushbackX*hardPushbackNorm.X + pushbackY*hardPushbackNorm.Y
				if isBarrier || (isAnotherPlayer && 0 > projectedMagnitude) {
					pushbackX -= projectedMagnitude * hardPushbackNorm.X
					pushbackY -= projectedMagnitude * hardPushbackNorm.Y
				}
			}
			effPushbacks[joinIndex-1].X += pushbackX
			effPushbacks[joinIndex-1].Y += pushbackY
			if currPlayerDownsync.InAir && landedOnGravityPushback {
				fallStopping = true
				if isAnotherPlayer {
					possiblyFallStoppedOnAnotherPlayer = true
				}
				if 1 == thatPlayerInNextFrame.JoinIndex {
					Logger.Info(fmt.Sprintf("playerId=%d, joinIndex=%d fallStopping#1 at {renderFrame.id: %d, virtualX: %d, virtualY: %d, velX: %d, velY: %d} with effPushback={%.3f, %.3f}, overlapMag=%.4f, possiblyFallStoppedOnAnotherPlayer=%v", playerId, joinIndex, currRenderFrame.Id, currPlayerDownsync.VirtualGridX, currPlayerDownsync.VirtualGridY, currPlayerDownsync.VelX, currPlayerDownsync.VelY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y, overlapResult.Overlap, possiblyFallStoppedOnAnotherPlayer))
				}
			}
			if 1 == joinIndex && currPlayerDownsync.InAir && isBarrier && !landedOnGravityPushback {
				Logger.Warn(fmt.Sprintf("playerId=%d, joinIndex=%d inAir & pushed back by barrier & not landed at {renderFrame.id: %d, virtualX: %d, virtualY: %d, velX: %d, velY: %d} with effPushback={%.3f, %.3f}, playerColliderPos={%.3f, %.3f}, barrierPos={%.3f, %.3f}, overlapMag=%.4f, len(hardPushbackNorms)=%d", playerId, joinIndex, currRenderFrame.Id, currPlayerDownsync.VirtualGridX, currPlayerDownsync.VirtualGridY, currPlayerDownsync.VelX, currPlayerDownsync.VelY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y, playerCollider.X-pR.collisionSpaceOffsetX, playerCollider.Y-pR.collisionSpaceOffsetY, bShape.X-pR.collisionSpaceOffsetX, bShape.Y-pR.collisionSpaceOffsetY, overlapResult.Overlap, len(hardPushbackNorms)))
			}
			if 1 == joinIndex && currPlayerDownsync.InAir && isAnotherPlayer {
				Logger.Warn(fmt.Sprintf("playerId=%d, joinIndex=%d inAir & pushed back by another player at {renderFrame.id: %d, virtualX: %d, virtualY: %d, velX: %d, velY: %d} with effPushback={%.3f, %.3f}, landedOnGravityPushback=%v, fallStopping=%v, playerColliderPos={%.3f, %.3f}, anotherPlayerColliderPos={%.3f, %.3f}, overlapMag=%.4f, len(hardPushbackNorms)=%d", playerId, joinIndex, currRenderFrame.Id, currPlayerDownsync.VirtualGridX, currPlayerDownsync.VirtualGridY, currPlayerDownsync.VelX, currPlayerDownsync.VelY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y, landedOnGravityPushback, fallStopping, playerCollider.X-pR.collisionSpaceOffsetX, playerCollider.Y-pR.collisionSpaceOffsetY, bShape.X-pR.collisionSpaceOffsetX, bShape.Y-pR.collisionSpaceOffsetY, overlapResult.Overlap, len(hardPushbackNorms)))
			}
		}
		if fallStopping {
			thatPlayerInNextFrame.VelX = 0
			thatPlayerInNextFrame.VelY = 0
			thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
			thatPlayerInNextFrame.FramesToRecover = 0
		}
		if currPlayerDownsync.InAir {
			switch thatPlayerInNextFrame.CharacterState {
			case ATK_CHARACTER_STATE_IDLE1:
			case ATK_CHARACTER_STATE_WALKING:
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_IDLE1
			case ATK_CHARACTER_STATE_ATK1:
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_ATK1
			case ATK_CHARACTER_STATE_ATKED1:
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_ATKED1
			default:
			}
		}
	}

	// 6. Check bullet-anything collisions
	for _, bulletCollider := range bulletColliders {
		shouldRemove := false
		meleeBullet := bulletCollider.Data.(*MeleeBullet)
		collisionBulletIndex := COLLISION_BULLET_INDEX_PREFIX + meleeBullet.BattleLocalId
		bulletShape := bulletCollider.Shape.(*resolv.ConvexPolygon)
		if collision := bulletCollider.Check(0, 0); collision != nil {
			offender := currRenderFrame.Players[meleeBullet.OffenderPlayerId]
			for _, obj := range collision.Objects {
				defenderShape := obj.Shape.(*resolv.ConvexPolygon)
				switch t := obj.Data.(type) {
				case *Player:
					if meleeBullet.OffenderPlayerId != t.Id {
						if overlapped, _, _, _ := CalcPushbacks(0, 0, bulletShape, defenderShape); overlapped {
							joinIndex := t.JoinIndex
							xfac := float64(1.0) // By now, straight Punch offset doesn't respect "y-axis"
							if 0 > offender.DirX {
								xfac = float64(-1.0)
							}
							pushbackX, pushbackY := -xfac*meleeBullet.Pushback, float64(0)

							for _, hardPushbackNorm := range hardPushbackNorms[joinIndex-1] {
								projectedMagnitude := pushbackX*hardPushbackNorm.X + pushbackY*hardPushbackNorm.Y
								if 0 > projectedMagnitude {
									Logger.Info(fmt.Sprintf("defenderPlayerId=%d, joinIndex=%d reducing bullet pushback={%.3f, %.3f} by {%.3f, %.3f} where hardPushbackNorm={%.3f, %.3f}, projectedMagnitude=%.3f at renderFrame.id=%d", t.Id, joinIndex, pushbackX, pushbackY, projectedMagnitude*hardPushbackNorm.X, projectedMagnitude*hardPushbackNorm.Y, hardPushbackNorm.X, hardPushbackNorm.Y, projectedMagnitude, currRenderFrame.Id))
									pushbackX -= projectedMagnitude * hardPushbackNorm.X
									pushbackY -= projectedMagnitude * hardPushbackNorm.Y
								}
							}

							effPushbacks[joinIndex-1].X += pushbackX
							effPushbacks[joinIndex-1].Y += pushbackY
							atkedPlayerInCurFrame, atkedPlayerInNextFrame := currRenderFrame.Players[t.Id], nextRenderFramePlayers[t.Id]
							atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_ATKED1
							if atkedPlayerInCurFrame.InAir {
								atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_ATKED1
							}
							oldFramesToRecover := nextRenderFramePlayers[t.Id].FramesToRecover
							if meleeBullet.HitStunFrames > oldFramesToRecover {
								atkedPlayerInNextFrame.FramesToRecover = meleeBullet.HitStunFrames
							}
							Logger.Debug(fmt.Sprintf("roomId=%v, a meleeBullet collides w/ player at currRenderFrame.id=%v: b=%v, p=%v", pR.Id, currRenderFrame.Id, ConvexPolygonStr(bulletShape), ConvexPolygonStr(defenderShape)))
						}
					}
				default:
					Logger.Debug(fmt.Sprintf("Bullet %v collided with non-player %v: roomId=%v, currRenderFrame.Id=%v, delayedInputFrame.Id=%v, objDataType=%t, objData=%v", ConvexPolygonStr(bulletShape), ConvexPolygonStr(defenderShape), pR.Id, currRenderFrame.Id, delayedInputFrame.InputFrameId, obj.Data, obj.Data))
				}
			}
			shouldRemove = true
		}
		if shouldRemove {
			removedBulletsAtCurrFrame[collisionBulletIndex] = 1
		}
	}

	// [WARNING] Remove bullets from collisionSys ANYWAY for the convenience of rollback
	for _, meleeBullet := range currRenderFrame.MeleeBullets {
		collisionBulletIndex := COLLISION_BULLET_INDEX_PREFIX + meleeBullet.BattleLocalId
		if bulletCollider, existent := collisionSysMap[collisionBulletIndex]; existent {
			bulletCollider.Space.Remove(bulletCollider)
			delete(collisionSysMap, collisionBulletIndex)
		}
		if _, existent := removedBulletsAtCurrFrame[collisionBulletIndex]; existent {
			continue
		}
		nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, meleeBullet)
	}

	// 7. Get players out of stuck barriers if there's any
	for _, player := range pR.PlayersArr {
		joinIndex := player.JoinIndex
		playerId := player.Id
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		playerShape := playerCollider.Shape.(*resolv.ConvexPolygon)
		// Update "virtual grid position"
		currPlayerDownsync, thatPlayerInNextFrame := currRenderFrame.Players[playerId], nextRenderFramePlayers[playerId]
		colliderWidth, colliderHeight := player.ColliderRadius*2, player.ColliderRadius*4
		thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY = PolygonColliderBLToVirtualGridPos(playerCollider.X-effPushbacks[joinIndex-1].X, playerCollider.Y-effPushbacks[joinIndex-1].Y, colliderWidth*0.5, colliderHeight*0.5, pR.SnapIntoPlatformOverlap, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, pR.WorldToVirtualGridRatio)

		if 1 == thatPlayerInNextFrame.JoinIndex {
			if thatPlayerInNextFrame.InAir && (0 != thatPlayerInNextFrame.VelY) {
				// Logger.Info(fmt.Sprintf("playerId=%d, joinIndex=%d inAir trajectory: {nextRenderFrame.id: %d, nextVirtualX: %d, nextVirtualY: %d, nextVelX: %d, nextVelY: %d}, with playerColliderPos={%.3f, %.3f}, effPushback={%.3f, %.3f}", playerId, joinIndex, currRenderFrame.Id+1, thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY, thatPlayerInNextFrame.VelX, thatPlayerInNextFrame.VelY, playerShape.X-pR.collisionSpaceOffsetX, playerShape.Y-pR.collisionSpaceOffsetY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y))
			}
			if currPlayerDownsync.InAir && !thatPlayerInNextFrame.InAir {
				Logger.Warn(fmt.Sprintf("playerId=%d, joinIndex=%d fallStopping#2 at {nextRenderFrame.id: %d, nextVirtualX: %d, nextVirtualY: %d, nextVelX: %d, nextVelY: %d}, with playerColliderPos={%.3f, %.3f}, effPushback={%.3f, %.3f}", playerId, joinIndex, currRenderFrame.Id+1, thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY, thatPlayerInNextFrame.VelX, thatPlayerInNextFrame.VelY, playerShape.X-pR.collisionSpaceOffsetX, playerShape.Y-pR.collisionSpaceOffsetY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y))
			}
			if !currPlayerDownsync.InAir && thatPlayerInNextFrame.InAir {
				Logger.Warn(fmt.Sprintf("playerId=%d, joinIndex=%d took off at {nextRenderFrame.id: %d, nextVirtualX: %d, nextVirtualY: %d, nextVelX: %d, nextVelY: %d}, with playerColliderPos={%.3f, %.3f}, effPushback={%.3f, %.3f}", playerId, joinIndex, currRenderFrame.Id+1, thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY, thatPlayerInNextFrame.VelX, thatPlayerInNextFrame.VelY, playerShape.X-pR.collisionSpaceOffsetX, playerShape.Y-pR.collisionSpaceOffsetY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y))
			}
		}
	}

	return &RoomDownsyncFrame{
		Id:             currRenderFrame.Id + 1,
		Players:        nextRenderFramePlayers,
		MeleeBullets:   nextRenderFrameMeleeBullets,
		CountdownNanos: (pR.BattleDurationNanos - int64(currRenderFrame.Id)*pR.RollbackEstimatedDtNanos),
	}
}

func (pR *Room) decodeInput(encodedInput uint64) *InputFrameDecoded {
	encodedDirection := (encodedInput & uint64(15))
	btnALevel := int32((encodedInput >> 4) & 1)
	btnBLevel := int32((encodedInput >> 5) & 1)
	return &InputFrameDecoded{
		Dx:        DIRECTION_DECODER[encodedDirection][0],
		Dy:        DIRECTION_DECODER[encodedDirection][1],
		BtnALevel: btnALevel,
		BtnBLevel: btnBLevel,
	}
}

func (pR *Room) inputFrameIdDebuggable(inputFrameId int32) bool {
	return 0 == (inputFrameId % 10)
}

func (pR *Room) refreshColliders(spaceW, spaceH int32) {
	// Kindly note that by now, we've already got all the shapes in the tmx file into "pR.(Players | Barriers)" from "ParseTmxLayersAndGroups"

	minStep := (int(float64(pR.PlayerDefaultSpeed)*pR.VirtualGridToWorldRatio) << 1) // the approx minimum distance a player can move per frame in world coordinate
	pR.Space = resolv.NewSpace(int(spaceW), int(spaceH), minStep, minStep)           // allocate a new collision space everytime after a battle is settled
	for _, player := range pR.Players {
		wx, wy := VirtualGridToWorldPos(player.VirtualGridX, player.VirtualGridY, pR.VirtualGridToWorldRatio)
		colliderWidth, colliderHeight := player.ColliderRadius*2, player.ColliderRadius*4
		playerCollider := GenerateRectCollider(wx, wy, colliderWidth, colliderHeight, pR.SnapIntoPlatformOverlap, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, "Player") // the coords of all barrier boundaries are multiples of tileWidth(i.e. 16), by adding snapping y-padding when "landedOnGravityPushback" all "playerCollider.Y" would be a multiple of 1.0
		playerCollider.Data = player
		pR.Space.Add(playerCollider)
		// Keep track of the collider in "pR.CollisionSysMap"
		joinIndex := player.JoinIndex
		pR.PlayersArr[joinIndex-1] = player
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		pR.CollisionSysMap[collisionPlayerIndex] = playerCollider
	}

	for _, barrier := range pR.Barriers {
		boundaryUnaligned := barrier.Boundary
		barrierCollider := GenerateConvexPolygonCollider(boundaryUnaligned, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, "Barrier")
		barrierCollider.Data = barrier
		pR.Space.Add(barrierCollider)
	}
}

func (pR *Room) printBarrier(barrierCollider *resolv.Object) {
	Logger.Info(fmt.Sprintf("Barrier in roomId=%v: w=%v, h=%v, shape=%v", pR.Id, barrierCollider.W, barrierCollider.H, barrierCollider.Shape))
}

func (pR *Room) doBattleMainLoopPerTickBackendDynamicsWithProperLocking(prevRenderFrameId int32, pDynamicsDuration *int64) {
	Logger.Debug(fmt.Sprintf("doBattleMainLoopPerTickBackendDynamicsWithProperLocking-InputsBufferLock to about lock: roomId=%v", pR.Id))
	pR.InputsBufferLock.Lock()
	Logger.Debug(fmt.Sprintf("doBattleMainLoopPerTickBackendDynamicsWithProperLocking-InputsBufferLock locked: roomId=%v", pR.Id))

	defer func() {
		pR.InputsBufferLock.Unlock()
		Logger.Debug(fmt.Sprintf("doBattleMainLoopPerTickBackendDynamicsWithProperLocking-InputsBufferLock unlocked: roomId=%v", pR.Id))
	}()

	if ok, thatRenderFrameId := pR.shouldPrefabInputFrameDownsync(prevRenderFrameId, pR.RenderFrameId); ok {
		noDelayInputFrameId := pR.ConvertToInputFrameId(thatRenderFrameId, 0)
		if existingInputFrame := pR.InputsBuffer.GetByFrameId(noDelayInputFrameId); nil == existingInputFrame {
			pR.prefabInputFrameDownsync(noDelayInputFrameId)
		}
	}

	// Force setting all-confirmed of buffered inputFrames periodically, kindly note that if "pR.BackendDynamicsEnabled", what we want to achieve is "recovery upon reconnection", which certainly requires "forceConfirmationIfApplicable" to move "pR.LastAllConfirmedInputFrameId" forward as much as possible
	inputsBufferSnapshot := pR.forceConfirmationIfApplicable(prevRenderFrameId)

	if 0 <= pR.LastAllConfirmedInputFrameId {
		dynamicsStartedAt := utils.UnixtimeNano()
		// Apply "all-confirmed inputFrames" to move forward "pR.CurDynamicsRenderFrameId"
		nextDynamicsRenderFrameId := pR.ConvertToLastUsedRenderFrameId(pR.LastAllConfirmedInputFrameId, pR.InputDelayFrames) + 1
		Logger.Debug(fmt.Sprintf("roomId=%v, room.RenderFrameId=%v, room.CurDynamicsRenderFrameId=%v, LastAllConfirmedInputFrameId=%v, InputDelayFrames=%v, nextDynamicsRenderFrameId=%v", pR.Id, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, pR.LastAllConfirmedInputFrameId, pR.InputDelayFrames, nextDynamicsRenderFrameId))
		pR.applyInputFrameDownsyncDynamics(pR.CurDynamicsRenderFrameId, nextDynamicsRenderFrameId, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY)
		*pDynamicsDuration = utils.UnixtimeNano() - dynamicsStartedAt
	}

	if nil != inputsBufferSnapshot {
		// Logger.Warn(fmt.Sprintf("roomId=%v, room.RenderFrameId=%v, room.CurDynamicsRenderFrameId=%v, room.LastAllConfirmedInputFrameId=%v, unconfirmedMask=%v", pR.Id, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, pR.LastAllConfirmedInputFrameId, inputsBufferSnapshot.UnconfirmedMask))
		pR.downsyncToAllPlayers(inputsBufferSnapshot)
	}
}

func (pR *Room) downsyncToAllPlayers(inputsBufferSnapshot *InputsBufferSnapshot) {
	/*
		       [WARNING] This function MUST BE called while "pR.InputsBufferLock" is LOCKED to **preserve the order of generation of "inputsBufferSnapshot" for sending** -- see comments in "OnBattleCmdReceived" and [this issue](https://github.com/genxium/DelayNoMore/issues/12).

		       Actually if each player session were both intrinsically thread-safe & non-blocking for writing (like Java NIO), I could've just called "playerSession.WriteMessage" while holding "pR.InputsBufferLock" -- but the ws session provided by Gorilla library is neither thread-safe nor non-blocking for writing, which is fine because it creates a chance for the users to solve an interesting problem :)

			   Moreover, we're downsyncing a same "inputsBufferSnapshot" for all players in the same battle and this is by design, i.e. not respecting "player.LastSentInputFrameId" because "new all-confirmed inputFrameDownsyncs" are the same for all players and ws is TCP-based (no loss of consecutive packets except for reconnection -- which is already handled by READDED_BATTLE_COLLIDER_ACKED)

		       Lastly noting just for fun, if in "OnBattleCmdReceived" we need downsync to a single specific player (keeping **the order of generation of "inputsBufferSnapshot" preserved for sending** of course), in theory it's better to do it by the following order.
		       1. lock "InputsBuffer";
		       2. generate downsync msg;
		       3. lock "pR.PlayerDownsyncChanDict[playerId]";
		       4. put downsync msg to "pR.PlayerDownsyncChanDict[playerId]";
		       5. unlock "InputsBuffer";
		       6. now other threads are allowed to lock "inputsBuffer", and we can do "other things" on "pR.PlayerDownsyncChanDict[playerId]";
		       7. unlock "pR.PlayerDownsyncChanDict[playerId]".

		       The difference from our current approach is that the "pR.PlayerDownsyncChanDict[playerId]" in use is a Golang channel, i.e. when executing #4 it automatically executes #3 (before) & #7 (after) as well, thus we couldn't do #5 & #6 in between.
	*/
	for playerId, playerDownsyncChan := range pR.PlayerDownsyncChanDict {
		/*
		   [WARNING] While the order of generation of "inputsBufferSnapshot" is preserved for sending, the underlying network I/O blocking action is dispatched to "downsyncLoop of each player" such that "markConfirmationIfApplicable & forceConfirmationIfApplicable" can re-hold "pR.InputsBufferLock" asap and proceed with more inputFrameUpsyncs.

		   The use of "downsyncLoop of each player" also waives the need of guarding each "pR.PlayerDownsyncSessionDict[playerId]" from multithread-access (e.g. by a "pR.PlayerDownsyncSessionMutexDict[playerId]"), i.e. Gorilla v1.2.0 "conn.WriteMessage" isn't thread-safe https://github.com/gorilla/websocket/blob/v1.2.0/conn.go#L585.
		*/
		playerDownsyncChan <- (*inputsBufferSnapshot)
		Logger.Debug(fmt.Sprintf("Sent inputsBufferSnapshot(refRenderFrameId:%d, unconfirmedMask:%v) to for (roomId: %d, playerId:%d, playerDownsyncChan:%p)#1", inputsBufferSnapshot.RefRenderFrameId, inputsBufferSnapshot.UnconfirmedMask, pR.Id, playerId, playerDownsyncChan))
	}
}

func (pR *Room) downsyncToSinglePlayer(playerId int32, player *Player, refRenderFrameId int32, unconfirmedMask uint64, toSendInputFrameDownsyncsSnapshot []*InputFrameDownsync) {
	/*
	   [WARNING] This function MUST BE called while "pR.InputsBufferLock" is unlocked -- otherwise the network I/O blocking of "sendSafely" might cause significant lag for "markConfirmationIfApplicable & forceConfirmationIfApplicable"!

	   We hereby assume that Golang runtime allocates & frees small amount of RAM quickly enough compared to either network I/O blocking in worst cases or the high frequency "per inputFrameDownsync*player" locking (though "OnBattleCmdReceived" locks at the same frequency but it's inevitable).
	*/

	playerJoinIndex := player.JoinIndex - 1
	playerBattleState := atomic.LoadInt32(&(player.BattleState))
	switch playerBattleState {
	case PlayerBattleStateIns.DISCONNECTED:
	case PlayerBattleStateIns.LOST:
	case PlayerBattleStateIns.EXPELLED_DURING_GAME:
	case PlayerBattleStateIns.EXPELLED_IN_DISMISSAL:
	case PlayerBattleStateIns.ADDED_PENDING_BATTLE_COLLIDER_ACK:
	case PlayerBattleStateIns.READDED_PENDING_BATTLE_COLLIDER_ACK:
		return
	}

	shouldResync1 := (PlayerBattleStateIns.READDED_BATTLE_COLLIDER_ACKED == playerBattleState) // i.e. implies that "MAGIC_LAST_SENT_INPUT_FRAME_ID_READDED == player.LastSentInputFrameId"
	shouldResync2 := (0 < (unconfirmedMask & uint64(1<<uint32(playerJoinIndex))))              // This condition is critical, if we don't send resync upon this condition, the "reconnected or slowly-clocking player" might never get its input synced
	// shouldResync2 := (0 < unconfirmedMask) // An easier version of the above, might keep sending "refRenderFrame"s to still connected players when any player is disconnected
	shouldResyncOverall := (shouldResync1 || shouldResync2)

	if shouldResyncOverall {
		// A rejoined player, should guarantee that when it resyncs to "refRenderFrameId" a matching inputFrame to apply exists
		Logger.Debug(fmt.Sprintf("Resyncing player: roomId=%v, playerId=%v, playerJoinIndex=%v, renderFrameId=%v, curDynamicsRenderFrameId=%v, playerLastSentInputFrameId=%v, refRenderFrameId=%v", pR.Id, playerId, player.JoinIndex, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, player.LastSentInputFrameId, refRenderFrameId))
	}

	/*
	   Resync helps
	   1. when player with a slower frontend clock lags significantly behind and thus wouldn't get its inputUpsync recognized due to faster "forceConfirmation"
	   2. reconnection
	*/

	//toSendInputFrameIdSt, toSendInputFrameIdEd := toSendInputFrameDownsyncsSnapshot[0].InputFrameId, toSendInputFrameDownsyncsSnapshot[len(toSendInputFrameDownsyncsSnapshot)-1].InputFrameId+1
	_, toSendInputFrameIdEd := toSendInputFrameDownsyncsSnapshot[0].InputFrameId, toSendInputFrameDownsyncsSnapshot[len(toSendInputFrameDownsyncsSnapshot)-1].InputFrameId+1
	if pR.BackendDynamicsEnabled && shouldResyncOverall {
		tmp := pR.RenderFrameBuffer.GetByFrameId(refRenderFrameId)
		if nil == tmp {
			panic(fmt.Sprintf("Required refRenderFrameId=%v for (roomId=%v, renderFrameId=%v, playerId=%v, playerLastSentInputFrameId=%v) doesn't exist! InputsBuffer=%v, RenderFrameBuffer=%v", refRenderFrameId, pR.Id, pR.RenderFrameId, playerId, player.LastSentInputFrameId, pR.InputsBufferString(false), pR.RenderFrameBufferString()))
		}

		refRenderFrame := tmp.(*RoomDownsyncFrame)
		for _, player := range pR.PlayersArr {
			refRenderFrame.Players[player.Id].ColliderRadius = player.ColliderRadius // hardcoded for now
		}
		refRenderFrame.BackendUnconfirmedMask = unconfirmedMask
		pR.sendSafely(refRenderFrame, toSendInputFrameDownsyncsSnapshot, DOWNSYNC_MSG_ACT_FORCED_RESYNC, playerId, false)
		// Logger.Warn(fmt.Sprintf("Sent refRenderFrameId=%v & inputFrameIds [%d, %d), for roomId=%v, playerId=%d, playerJoinIndex=%d, renderFrameId=%d, curDynamicsRenderFrameId=%d, playerLastSentInputFrameId=%d: InputsBuffer=%v", refRenderFrameId, toSendInputFrameIdSt, toSendInputFrameIdEd, pR.Id, playerId, player.JoinIndex, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, player.LastSentInputFrameId, pR.InputsBufferString(false)))
	} else {
		pR.sendSafely(nil, toSendInputFrameDownsyncsSnapshot, DOWNSYNC_MSG_ACT_INPUT_BATCH, playerId, false)
	}
	player.LastSentInputFrameId = toSendInputFrameIdEd - 1
	if shouldResync1 {
		atomic.StoreInt32(&(player.BattleState), PlayerBattleStateIns.ACTIVE)
	}
}

func (pR *Room) cloneInputsBuffer(stFrameId, edFrameId int32) []*InputFrameDownsync {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	cloned := make([]*InputFrameDownsync, 0, edFrameId-stFrameId)
	prevFrameFound := false
	j := stFrameId
	for j < edFrameId {
		tmp := pR.InputsBuffer.GetByFrameId(j)
		if nil == tmp {
			if false == prevFrameFound {
				j++
				continue // allowed to keep not finding the requested inputFrames at the beginning
			} else {
				break // The "id"s are always consecutive
			}
		}
		prevFrameFound = true
		foo := tmp.(*InputFrameDownsync)

		bar := &InputFrameDownsync{
			InputFrameId:  foo.InputFrameId,
			InputList:     make([]uint64, len(foo.InputList)),
			ConfirmedList: foo.ConfirmedList,
		}
		for i, input := range foo.InputList {
			bar.InputList[i] = input
		}
		cloned = append(cloned, bar)
		j++
	}

	return cloned
}

func (pR *Room) calcHardPushbacksNorms(playerCollider *resolv.Object, playerShape *resolv.ConvexPolygon, snapIntoPlatformOverlap float64, pEffPushback *Vec2D) []Vec2D {
	ret := make([]Vec2D, 0, 10) // no one would simultaneously have more than 5 hardPushbacks
	collision := playerCollider.Check(0, 0)
	if nil == collision {
		return ret
	}
	for _, obj := range collision.Objects {
		switch obj.Data.(type) {
		case *Barrier:
			barrierShape := obj.Shape.(*resolv.ConvexPolygon)
			overlapped, pushbackX, pushbackY, overlapResult := CalcPushbacks(0, 0, playerShape, barrierShape)
			if !overlapped {
				continue
			}
			// ALWAY snap into hardPushbacks!
			// [OverlapX, OverlapY] is the unit vector that points into the platform
			pushbackX, pushbackY = (overlapResult.Overlap-snapIntoPlatformOverlap)*overlapResult.OverlapX, (overlapResult.Overlap-snapIntoPlatformOverlap)*overlapResult.OverlapY
			ret = append(ret, Vec2D{X: overlapResult.OverlapX, Y: overlapResult.OverlapY})
			pEffPushback.X += pushbackX
			pEffPushback.Y += pushbackY
		default:
		}
	}
	return ret
}