DelayNoMore/battle_srv/models/room.go

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
)

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
	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
	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

	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

	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 {
	// Specifically when "renderFrameId < inputDelayFrames", the result is 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()

		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
			}

			if 0 == pR.RenderFrameId {
				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)
				}
				Logger.Info(fmt.Sprintf("In `battleMainLoop` for roomId=%v sent out kickoffFrame", pR.Id))
			}

			dynamicsDuration := int64(0)

			// Prefab and buffer backend inputFrameDownsync
			if pR.BackendDynamicsEnabled {
				unconfirmedMask, inputsBufferSnapshot := pR.doBattleMainLoopPerTickBackendDynamicsWithProperLocking(&dynamicsDuration)
				if 0 < unconfirmedMask {
					// Otherwise no need to downsync immediately
					Logger.Debug(fmt.Sprintf("roomId=%v, room.RenderFrameId=%v, room.CurDynamicsRenderFrameId=%v, room.LastAllConfirmedInputFrameId=%v, unconfirmedMask=%v", pR.Id, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, pR.LastAllConfirmedInputFrameId, unconfirmedMask))
					pR.downsyncToAllPlayers(pR.LastAllConfirmedInputFrameId, unconfirmedMask, inputsBufferSnapshot)
				}
			}

			pR.RenderFrameId++
			pR.LastRenderFrameIdTriggeredAt = utils.UnixtimeNano()
			elapsedInCalculation := (pR.LastRenderFrameIdTriggeredAt - stCalculation)
			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", pR.Id, pR.RenderFrameId, elapsedInCalculation, dynamicsDuration, pR.RollbackEstimatedDtNanos))
			}
			time.Sleep(time.Duration(pR.dilutedRollbackEstimatedDtNanos - elapsedInCalculation))
		}
	}

	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!
	// 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)

	newAllConfirmedCount := pR.markConfirmationIfApplicable(inputFrameUpsyncBatch, playerId, player)
	// [WARNING] By now "pR.InputsBufferLock" is unlocked
	if 0 < newAllConfirmedCount {
		// Downsync new all-confirmed inputFrames asap
		pR.downsyncToAllPlayers(pR.LastAllConfirmedInputFrameId, uint64(0), pR.createInputsBufferSnapshot())
	}
}

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)
	}
	// 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)
	}

	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)
	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 = 4
	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(59.5)
	pR.dilutedRollbackEstimatedDtNanos = pR.RollbackEstimatedDtNanos * int64(float64(pR.ServerFps)/dilutedServerFps)
	pR.BattleDurationFrames = 30 * 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(23.0),
			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.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:", 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))
		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
	}

	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))
	switch targetPlayer.BattleState {
	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 thatPlayerId, 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.
			*/
			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)
			}
		}
	case PlayerBattleStateIns.READDED_PENDING_BATTLE_COLLIDER_ACK:
		// only send resync info to the targetPlayer
		// i.e. implies that "targetPlayer.LastSentInputFrameId == MAGIC_LAST_SENT_INPUT_FRAME_ID_READDED"
		pR.downsyncToSinglePlayer(playerId, targetPlayer, pR.LastAllConfirmedInputFrameId, uint64(0), pR.createInputsBufferSnapshot())
	default:
	}

	targetPlayer.BattleState = PlayerBattleStateIns.ACTIVE
	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 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) {
	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(renderFrameId int32) bool {
	return ((renderFrameId & ((1 << pR.InputScaleFrames) - 1)) == 0)
}

func (pR *Room) prefabInputFrameDownsync(inputFrameId int32) *InputFrameDownsync {
	/*
	   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

	if 0 == inputFrameId && 0 == pR.InputsBuffer.Cnt {
		currInputFrameDownsync = &InputFrameDownsync{
			InputFrameId:  0,
			InputList:     make([]uint64, pR.Capacity),
			ConfirmedList: uint64(0),
		}
	} else {
		tmp := pR.InputsBuffer.GetByFrameId(inputFrameId - 1) // There's no need for the backend to find the "lastAllConfirmed inputs" for prefabbing, either "BackendDynamicsEnabled" is true or false
		if nil == tmp {
			panic(fmt.Sprintf("Error prefabbing inputFrameDownsync: roomId=%v, InputsBuffer=%v", pR.Id, pR.InputsBufferString(false)))
		}
		prevInputFrameDownsync := tmp.(*InputFrameDownsync)
		currInputList := make([]uint64, pR.Capacity) // Would be a clone of the values
		for i, _ := range currInputList {
			currInputList[i] = (prevInputFrameDownsync.InputList[i] & uint64(15)) // Don't predict attack input!
		}
		currInputFrameDownsync = &InputFrameDownsync{
			InputFrameId:  inputFrameId,
			InputList:     currInputList,
			ConfirmedList: uint64(0),
		}
	}

	pR.InputsBuffer.Put(currInputFrameDownsync)
	return currInputFrameDownsync
}

func (pR *Room) markConfirmationIfApplicable(inputFrameUpsyncBatch []*InputFrameUpsync, playerId int32, player *Player) int {
	Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable-InputsBufferLock about to lock: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	pR.InputsBufferLock.Lock()
	Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable-InputsBufferLock locked: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	defer func() {
		pR.InputsBufferLock.Unlock()
		Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable-InputsBufferLock unlocked: roomId=%v, fromPlayerId=%v", pR.Id, playerId))
	}()

	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, lastAllConfirmedInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.LastAllConfirmedInputFrameId, pR.InputsBufferString(false)))
			continue
		}
		if clientInputFrameId < pR.LastAllConfirmedInputFrameId {
			Logger.Info(fmt.Sprintf("Omitting obsolete inputFrameUpsync#2: roomId=%v, playerId=%v, clientInputFrameId=%v, lastAllConfirmedInputFrameId=%v, InputsBuffer=%v", pR.Id, playerId, clientInputFrameId, pR.LastAllConfirmedInputFrameId, pR.InputsBufferString(false)))
			continue
		}
		if clientInputFrameId > pR.InputsBuffer.EdFrameId {
			Logger.Warn(fmt.Sprintf("Dropping too advanced inputFrameUpsync: roomId=%v, playerId=%v, clientInputFrameId=%v, lastAllConfirmedInputFrameId=%v, InputsBuffer=%v; is this player cheating?", pR.Id, playerId, clientInputFrameId, pR.LastAllConfirmedInputFrameId, 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 := 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)
		}
	}

	Logger.Debug(fmt.Sprintf("markConfirmationIfApplicable checking inputFrameId=[%v, %v) for roomId=%v, newAllConfirmedCount=%d: InputsBuffer=%v", inputFrameId1, pR.InputsBuffer.EdFrameId, pR.Id, newAllConfirmedCount, pR.InputsBufferString(false)))
	return newAllConfirmedCount
}

func (pR *Room) forceConfirmationIfApplicable() uint64 {
	// [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 || !pR.shouldPrefabInputFrameDownsync(renderFrameId1) {
		/*
		   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 0
	}

	inputFrameId2 := pR.ConvertToInputFrameId(renderFrameId1, 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 0
	}

	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)

	return unconfirmedMask
}

func (pR *Room) applyInputFrameDownsyncDynamics(fromRenderFrameId int32, toRenderFrameId int32, spaceOffsetX, spaceOffsetY float64) {
	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)

	Logger.Debug(fmt.Sprintf("applyInputFrameDownsyncDynamics-InputsBufferLock about to lock: roomId=%v", pR.Id))
	pR.InputsBufferLock.Lock()
	Logger.Debug(fmt.Sprintf("applyInputFrameDownsyncDynamics-InputsBufferLock locked: roomId=%v", pR.Id))

	defer func() {
		pR.InputsBufferLock.Unlock()
		Logger.Debug(fmt.Sprintf("applyInputFrameDownsyncDynamics-InputsBufferLock unlocked: roomId=%v", pR.Id))
	}()
	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 {
	// TODO: Derive "nextRenderFramePlayers[*].CharacterState" as the frontend counter-part!
	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,
			CharacterState:  currPlayerDownsync.CharacterState,
			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
		}
	}

	toRet := &RoomDownsyncFrame{
		Id:             currRenderFrame.Id + 1,
		Players:        nextRenderFramePlayers,
		CountdownNanos: (pR.BattleDurationNanos - int64(currRenderFrame.Id)*pR.RollbackEstimatedDtNanos),
		MeleeBullets:   make([]*MeleeBullet, 0), // Is there any better way to reduce malloc/free impact, e.g. smart prediction for fixed memory allocation?
	}

	bulletPushbacks := make([]Vec2D, pR.Capacity) // Guaranteed determinism regardless of traversal order
	effPushbacks := make([]Vec2D, pR.Capacity)    // Guaranteed determinism regardless of traversal order

	// Reset playerCollider position from the "virtual grid position"
	for _, player := range pR.PlayersArr {
		playerId := player.Id
		joinIndex := player.JoinIndex
		bulletPushbacks[joinIndex-1].X, bulletPushbacks[joinIndex-1].Y = float64(0), float64(0)
		effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y = float64(0), float64(0)
		currPlayerDownsync := currRenderFrame.Players[playerId]
		newVx, newVy := currPlayerDownsync.VirtualGridX, currPlayerDownsync.VirtualGridY
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		playerCollider.X, playerCollider.Y = VirtualGridToPolygonColliderAnchorPos(newVx, newVy, player.ColliderRadius, player.ColliderRadius, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, pR.VirtualGridToWorldRatio)
	}

	// Check bullet-anything collisions first, because the pushbacks caused by bullets might later be reverted by player-barrier collision
	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, 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))
		}
	}

	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 {
							xfac := float64(1.0) // By now, straight Punch offset doesn't respect "y-axis"
							if 0 > offender.DirX {
								xfac = float64(-1.0)
							}
							bulletPushbacks[t.JoinIndex-1].X += xfac * meleeBullet.Pushback
							nextRenderFramePlayers[t.Id].CharacterState = ATK_CHARACTER_STATE_ATKED1
							oldFramesToRecover := nextRenderFramePlayers[t.Id].FramesToRecover
							if meleeBullet.HitStunFrames > oldFramesToRecover {
								nextRenderFramePlayers[t.Id].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
		}
	}

	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
		}
		toRet.MeleeBullets = append(toRet.MeleeBullets, meleeBullet)
	}

	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
		// Process player inputs
		for _, player := range pR.PlayersArr {
			playerId := player.Id
			joinIndex := player.JoinIndex
			collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
			playerCollider := collisionSysMap[collisionPlayerIndex]
			thatPlayerInNextFrame := nextRenderFramePlayers[playerId]
			if 0 < thatPlayerInNextFrame.FramesToRecover {
				// No need to process inputs for this player, but there might be bullet pushbacks on this player
				// Also note that in this case we keep "CharacterState" of this player from last render frame
				playerCollider.X += bulletPushbacks[joinIndex-1].X
				playerCollider.Y += bulletPushbacks[joinIndex-1].Y
				// Update in the collision system
				playerCollider.Update()
				if 0 != bulletPushbacks[joinIndex-1].X || 0 != bulletPushbacks[joinIndex-1].Y {
					Logger.Debug(fmt.Sprintf("roomId=%v, playerId=%v is pushed back by (%.2f, %.2f) by bullet impacts, now its framesToRecover is %d at currRenderFrame.id=%v", pR.Id, playerId, bulletPushbacks[joinIndex-1].X, bulletPushbacks[joinIndex-1].Y, thatPlayerInNextFrame.FramesToRecover, currRenderFrame.Id))
				}
				continue
			}
			currPlayerDownsync := currRenderFrame.Players[playerId]
			decodedInput := pR.decodeInput(inputList[joinIndex-1])
			prevBtnALevel := int32(0)
			if nil != delayedInputFrameForPrevRenderFrame {
				prevDecodedInput := pR.decodeInput(delayedInputFrameForPrevRenderFrame.InputList[joinIndex-1])
				prevBtnALevel = prevDecodedInput.BtnALevel
			}

			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
				toRet.MeleeBullets = append(toRet.MeleeBullets, &newMeleeBullet)
				thatPlayerInNextFrame.FramesToRecover = newMeleeBullet.RecoveryFrames
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_ATK1
				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.CharacterState = ATK_CHARACTER_STATE_WALKING
				} else {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
				}
			}

			movementX, movementY := VirtualGridToWorldPos(decodedInput.Dx+decodedInput.Dx*currPlayerDownsync.Speed, decodedInput.Dy+decodedInput.Dy*currPlayerDownsync.Speed, pR.VirtualGridToWorldRatio)
			playerCollider.X += movementX
			playerCollider.Y += movementY

			// Update in the collision system
			playerCollider.Update()
		}

		// handle pushbacks upon collision after all movements treated as simultaneous
		for _, player := range pR.PlayersArr {
			joinIndex := player.JoinIndex
			collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
			playerCollider := collisionSysMap[collisionPlayerIndex]
			if collision := playerCollider.Check(0, 0); collision != nil {
				playerShape := playerCollider.Shape.(*resolv.ConvexPolygon)
				for _, obj := range collision.Objects {
					barrierShape := obj.Shape.(*resolv.ConvexPolygon)
					if overlapped, pushbackX, pushbackY, overlapResult := CalcPushbacks(0, 0, playerShape, barrierShape); overlapped {
						Logger.Debug(fmt.Sprintf("Overlapped: a=%v, b=%v, pushbackX=%v, pushbackY=%v", ConvexPolygonStr(playerShape), ConvexPolygonStr(barrierShape), pushbackX, pushbackY))
						effPushbacks[joinIndex-1].X += pushbackX
						effPushbacks[joinIndex-1].Y += pushbackY
					} else {
						Logger.Debug(fmt.Sprintf("Collided BUT not overlapped: a=%v, b=%v, overlapResult=%v", ConvexPolygonStr(playerShape), ConvexPolygonStr(barrierShape), overlapResult))
					}
				}
			}
		}

		for _, player := range pR.PlayersArr {
			playerId := player.Id
			joinIndex := player.JoinIndex
			collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
			playerCollider := collisionSysMap[collisionPlayerIndex]

			// Update "virtual grid position"
			newVx, newVy := PolygonColliderAnchorToVirtualGridPos(playerCollider.X-effPushbacks[joinIndex-1].X, playerCollider.Y-effPushbacks[joinIndex-1].Y, player.ColliderRadius, player.ColliderRadius, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, pR.WorldToVirtualGridRatio)
			thatPlayerInNextFrame := nextRenderFramePlayers[playerId]
			thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY = newVx, newVy
		}

		Logger.Debug(fmt.Sprintf("After applyInputFrameDownsyncDynamicsOnSingleRenderFrame: currRenderFrame.Id=%v, inputList=%v, currRenderFrame.Players=%v, nextRenderFramePlayers=%v", currRenderFrame.Id, inputList, currRenderFrame.Players, nextRenderFramePlayers))
	}

	return toRet
}

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

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*3
		playerCollider := GenerateRectCollider(wx, wy, colliderWidth, colliderHeight, pR.collisionSpaceOffsetX, pR.collisionSpaceOffsetY, "Player")
		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")
		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(pDynamicsDuration *int64) (uint64, *RingBuffer) {
	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() {
		// [WARNING] Unlock before calling "downsyncToAllPlayers -> downsyncToSinglePlayer" to avoid unlocking lags due to blocking network I/O
		pR.InputsBufferLock.Unlock()
		Logger.Debug(fmt.Sprintf("doBattleMainLoopPerTickBackendDynamicsWithProperLocking-InputsBufferLock unlocked: roomId=%v", pR.Id))
	}()

	if pR.shouldPrefabInputFrameDownsync(pR.RenderFrameId) {
		noDelayInputFrameId := pR.ConvertToInputFrameId(pR.RenderFrameId, 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
	unconfirmedMask := pR.forceConfirmationIfApplicable()

	if 0 <= pR.LastAllConfirmedInputFrameId {
		dynamicsStartedAt := utils.UnixtimeNano()
		// Apply "all-confirmed inputFrames" to move forward "pR.CurDynamicsRenderFrameId"
		nextDynamicsRenderFrameId := pR.ConvertToLastUsedRenderFrameId(pR.LastAllConfirmedInputFrameId, pR.InputDelayFrames)
		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
	}

	var inputsBufferSnapshot *RingBuffer = nil
	if 0 < unconfirmedMask {
		// [WARNING] This condition should be rarely met!
		// Otherwise there's no need to create the inputsBufferSnapshot
		inputsBufferSnapshot = pR.createInputsBufferSnapshot()
	}
	return unconfirmedMask, inputsBufferSnapshot
}

func (pR *Room) downsyncToAllPlayers(upperToSendInputFrameId int32, unconfirmedMask uint64, inputsBufferSnapshot *RingBuffer) {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is unlocked -- otherwise the blocking "sendSafely" might cause significant lag!
	for _, player := range pR.PlayersArr {
		thatPlayerBattleState := atomic.LoadInt32(&(player.BattleState))
		switch thatPlayerBattleState {
		case PlayerBattleStateIns.DISCONNECTED:
		case PlayerBattleStateIns.LOST:
		case PlayerBattleStateIns.EXPELLED_DURING_GAME:
		case PlayerBattleStateIns.EXPELLED_IN_DISMISSAL:
		case PlayerBattleStateIns.READDED_PENDING_BATTLE_COLLIDER_ACK: // This is the reason why battleState filter is put at "downsyncToAllPlayers" instead of "downsyncToSinglePlayer"
			continue
		}
		pR.downsyncToSinglePlayer(player.Id, player, pR.LastAllConfirmedInputFrameId, unconfirmedMask, inputsBufferSnapshot)
	}
}

func (pR *Room) downsyncToSinglePlayer(playerId int32, player *Player, upperToSendInputFrameId int32, unconfirmedMask uint64, inputsBufferSnapshot *RingBuffer) {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is unlocked -- otherwise the blocking "sendSafely" might cause significant lag!
	// [WARNING] Websocket is TCP-based, thus no need to re-send a previously sent inputFrame to a same player!
	lowerToSentInputFrameId := player.LastSentInputFrameId + 1
	/*
			   [WARNING]
			   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.
	*/
	refRenderFrameId := pR.CurDynamicsRenderFrameId - 1

	shouldResync1 := (MAGIC_LAST_SENT_INPUT_FRAME_ID_READDED == player.LastSentInputFrameId)
	shouldResync2 := (0 < (unconfirmedMask & uint64(1<<uint32(player.JoinIndex-1)))) // 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
		lowerToSentInputFrameId = pR.ConvertToInputFrameId(refRenderFrameId, pR.InputDelayFrames)
		Logger.Info(fmt.Sprintf("Resyncing player: roomId=%v, playerId=%v, playerJoinIndex=%v, renderFrameId=%v, curDynamicsRenderFrameId=%v, playerLastSentInputFrameId=%v, refRenderFrameId=%v, lowerToSentInputFrameId=%v, upperToSendInputFrameId=%v", pR.Id, playerId, player.JoinIndex, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, player.LastSentInputFrameId, refRenderFrameId, lowerToSentInputFrameId, upperToSendInputFrameId))
		// TODO: What if "lowerToSentInputFrameId > pR.InputsBuffer.StFrameId" now?
	}

	if lowerToSentInputFrameId > upperToSendInputFrameId {
		Logger.Debug(fmt.Sprintf("Not sending due to potentially empty toSendInputFrameDownsyncs: roomId=%v, playerId=%v, playerLastSentInputFrameId=%v, refRenderFrameId=%v, lowerToSentInputFrameId=%v, upperToSendInputFrameId=%v, lastSentInputFrameId=%v, playerAckingInputFrameId=%v", pR.Id, playerId, player.LastSentInputFrameId, refRenderFrameId, lowerToSentInputFrameId, upperToSendInputFrameId, player.LastSentInputFrameId, player.AckingInputFrameId))
		return
	}

	// [WARNING] EDGE CASE HERE: Upon initialization, all of "lastAllConfirmedInputFrameId", "lastAllConfirmedInputFrameIdWithChange" and "anchorInputFrameId" are "-1", thus "j" starts with "0", however "inputFrameId: 0" might not have been all confirmed!
	// [WARNING] Clone to deliberately avoid holding "pR.InputsBufferLock" while using network I/O
	j, toSendInputFrameDownsyncs := pR.cloneInputFrameDownsyncsByFrameIdRange(lowerToSentInputFrameId, upperToSendInputFrameId+1, inputsBufferSnapshot)

	if 0 >= len(toSendInputFrameDownsyncs) {
		Logger.Debug(fmt.Sprintf("Not sending due to actually empty toSendInputFrameDownsyncs: roomId=%v, playerId=%v, playerLastSentInputFrameId=%v, refRenderFrameId=%v, lowerToSentInputFrameId=%v, upperToSendInputFrameId=%v, j=%v, lastSentInputFrameId=%v, playerAckingInputFrameId=%v", pR.Id, playerId, player.LastSentInputFrameId, refRenderFrameId, lowerToSentInputFrameId, upperToSendInputFrameId, j, player.LastSentInputFrameId, player.AckingInputFrameId))
		return
	}

	/*
	   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
	*/
	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, j=%v doesn't exist! InputsBuffer=%v, RenderFrameBuffer=%v", refRenderFrameId, pR.Id, pR.RenderFrameId, playerId, player.LastSentInputFrameId, j, 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
		Logger.Warn(fmt.Sprintf("Sending refRenderFrameId=%v for roomId=%v, , playerId=%v, playerJoinIndex=%v, renderFrameId=%v, curDynamicsRenderFrameId=%v, playerLastSentInputFrameId=%v, lowerToSentInputFrameId=%v, upperToSendInputFrameId=%v, j=%v: InputsBuffer=%v", refRenderFrameId, pR.Id, playerId, player.JoinIndex, pR.RenderFrameId, pR.CurDynamicsRenderFrameId, player.LastSentInputFrameId, lowerToSentInputFrameId, upperToSendInputFrameId, j, pR.InputsBufferString(false)))
		pR.sendSafely(refRenderFrame, toSendInputFrameDownsyncs, DOWNSYNC_MSG_ACT_FORCED_RESYNC, playerId)
	} else {
		pR.sendSafely(nil, toSendInputFrameDownsyncs, DOWNSYNC_MSG_ACT_INPUT_BATCH, playerId)
	}
	player.LastSentInputFrameId = j - 1
}

func (pR *Room) createInputsBufferSnapshot() *RingBuffer {
	// [WARNING] This function MUST BE called while "pR.InputsBufferLock" is locked!
	snapshot := NewRingBuffer(pR.InputsBuffer.N)
	for j := pR.InputsBuffer.StFrameId; j < pR.InputsBuffer.Ed; j++ {
		tmp := pR.InputsBuffer.GetByFrameId(j)
		snapshot.Put(tmp)
	}

	return snapshot
}

func (pR *Room) cloneInputFrameDownsyncsByFrameIdRange(stFrameId, edFrameId int32, src *RingBuffer) (int32, []*InputFrameDownsync) {
	dst := make([]*InputFrameDownsync, 0, src.Cnt)
	prevFrameFound := true
	j := stFrameId
	for j < edFrameId {
		tmp := src.GetByFrameId(j)
		if nil == tmp {
			if false == prevFrameFound {
				// The "id"s are always consecutive
				break
			} else {
				prevFrameFound = false
				continue
			}
		}
		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
		}
		dst = append(dst, bar)
		j++
	}

	return j, dst
}