package battle

import (
	"math"
	"resolv"
)

const (
	MAX_FLOAT64                    = 1.7e+308
	MAX_INT32                      = int32(999999999)
	COLLISION_PLAYER_INDEX_PREFIX  = (1 << 17)
	COLLISION_BARRIER_INDEX_PREFIX = (1 << 16)
	COLLISION_BULLET_INDEX_PREFIX  = (1 << 15)

	PATTERN_ID_UNABLE_TO_OP = -2
	PATTERN_ID_NO_OP        = -1

	WORLD_TO_VIRTUAL_GRID_RATIO = float64(100)
	VIRTUAL_GRID_TO_WORLD_RATIO = float64(1.0) / WORLD_TO_VIRTUAL_GRID_RATIO

	GRAVITY_X = int32(0)
	GRAVITY_Y = -int32(float64(0.5) * WORLD_TO_VIRTUAL_GRID_RATIO) // makes all "playerCollider.Y" a multiple of 0.5 in all cases

	INPUT_DELAY_FRAMES = int32(8)  // in the count of render frames
	INPUT_SCALE_FRAMES = uint32(2) // inputDelayedAndScaledFrameId = ((originalFrameId - InputDelayFrames) >> InputScaleFrames)
	NST_DELAY_FRAMES   = int32(16) // network-single-trip delay in the count of render frames, proposed to be (InputDelayFrames >> 1) because we expect a round-trip delay to be exactly "InputDelayFrames"

	SP_ATK_LOOKUP_FRAMES = int32(5)

	SNAP_INTO_PLATFORM_OVERLAP   = float64(0.1)
	SNAP_INTO_PLATFORM_THRESHOLD = float64(0.5)

	NO_SKILL     = -1
	NO_SKILL_HIT = -1

	NO_LOCK_VEL = int32(-1)
)

// 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},
}

const (
	ATK_CHARACTER_STATE_IDLE1               = int32(0)
	ATK_CHARACTER_STATE_WALKING             = int32(1)
	ATK_CHARACTER_STATE_ATK1                = int32(2)
	ATK_CHARACTER_STATE_ATKED1              = int32(3)
	ATK_CHARACTER_STATE_INAIR_IDLE1_NO_JUMP = int32(4)
	ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP = int32(5)
	ATK_CHARACTER_STATE_INAIR_ATK1          = int32(6)
	ATK_CHARACTER_STATE_INAIR_ATKED1        = int32(7)
	ATK_CHARACTER_STATE_BLOWN_UP1           = int32(8)
	ATK_CHARACTER_STATE_LAY_DOWN1           = int32(9)
	ATK_CHARACTER_STATE_GET_UP1             = int32(10)

	ATK_CHARACTER_STATE_ATK2 = int32(11)
	ATK_CHARACTER_STATE_ATK3 = int32(12)
	ATK_CHARACTER_STATE_ATK4 = int32(13)
	ATK_CHARACTER_STATE_ATK5 = int32(14)
)

var inAirSet = map[int32]bool{
	ATK_CHARACTER_STATE_INAIR_IDLE1_NO_JUMP: true,
	ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP: true,
	ATK_CHARACTER_STATE_INAIR_ATK1:          true,
	ATK_CHARACTER_STATE_INAIR_ATKED1:        true,
	ATK_CHARACTER_STATE_BLOWN_UP1:           true,
}

var noOpSet = map[int32]bool{
	ATK_CHARACTER_STATE_ATKED1:       true,
	ATK_CHARACTER_STATE_INAIR_ATKED1: true,
	ATK_CHARACTER_STATE_BLOWN_UP1:    true,
	ATK_CHARACTER_STATE_LAY_DOWN1:    true,
	// During the invinsible frames of GET_UP1, the player is allowed to take any action
}

var invinsibleSet = map[int32]bool{
	ATK_CHARACTER_STATE_BLOWN_UP1: true,
	ATK_CHARACTER_STATE_LAY_DOWN1: true,
	ATK_CHARACTER_STATE_GET_UP1:   true,
}

var nonAttackingSet = map[int32]bool{
	ATK_CHARACTER_STATE_IDLE1:               true,
	ATK_CHARACTER_STATE_WALKING:             true,
	ATK_CHARACTER_STATE_INAIR_IDLE1_NO_JUMP: true,
	ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP: true,
	ATK_CHARACTER_STATE_ATKED1:              true,
	ATK_CHARACTER_STATE_INAIR_ATKED1:        true,
	ATK_CHARACTER_STATE_BLOWN_UP1:           true,
	ATK_CHARACTER_STATE_LAY_DOWN1:           true,
	ATK_CHARACTER_STATE_GET_UP1:             true,
}

func ShouldPrefabInputFrameDownsync(prevRenderFrameId int32, renderFrameId int32) (bool, int32) {
	for i := prevRenderFrameId + 1; i <= renderFrameId; i++ {
		if (0 <= i) && (0 == (i & ((1 << INPUT_SCALE_FRAMES) - 1))) {
			return true, i
		}
	}
	return false, -1
}

func ShouldGenerateInputFrameUpsync(renderFrameId int32) bool {
	return ((renderFrameId & ((1 << INPUT_SCALE_FRAMES) - 1)) == 0)
}

func ConvertToDelayedInputFrameId(renderFrameId int32) int32 {
	if renderFrameId < INPUT_DELAY_FRAMES {
		return 0
	}
	return ((renderFrameId - INPUT_DELAY_FRAMES) >> INPUT_SCALE_FRAMES)
}

func ConvertToNoDelayInputFrameId(renderFrameId int32) int32 {
	return (renderFrameId >> INPUT_SCALE_FRAMES)
}

func ConvertToFirstUsedRenderFrameId(inputFrameId int32) int32 {
	return ((inputFrameId << INPUT_SCALE_FRAMES) + INPUT_DELAY_FRAMES)
}

func ConvertToLastUsedRenderFrameId(inputFrameId int32) int32 {
	return ((inputFrameId << INPUT_SCALE_FRAMES) + INPUT_DELAY_FRAMES + (1 << INPUT_SCALE_FRAMES) - 1)
}

func 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,
	}
}

type SatResult struct {
	Overlap       float64
	OverlapX      float64
	OverlapY      float64
	AContainedInB bool
	BContainedInA bool
	Axis          resolv.Vector
}

func CalcPushbacks(oldDx, oldDy float64, playerShape, barrierShape *resolv.ConvexPolygon) (bool, float64, float64, *SatResult) {
	origX, origY := playerShape.Position()
	defer func() {
		playerShape.SetPosition(origX, origY)
	}()
	playerShape.SetPosition(origX+oldDx, origY+oldDy)

	overlapResult := &SatResult{
		Overlap:       0,
		OverlapX:      0,
		OverlapY:      0,
		AContainedInB: true,
		BContainedInA: true,
		Axis:          resolv.Vector{0, 0},
	}
	if overlapped := isPolygonPairOverlapped(playerShape, barrierShape, overlapResult); overlapped {
		pushbackX, pushbackY := overlapResult.Overlap*overlapResult.OverlapX, overlapResult.Overlap*overlapResult.OverlapY
		return true, pushbackX, pushbackY, overlapResult
	} else {
		return false, 0, 0, overlapResult
	}
}

func isPolygonPairOverlapped(a, b *resolv.ConvexPolygon, result *SatResult) bool {
	aCnt, bCnt := len(a.Points), len(b.Points)
	// Single point case
	if 1 == aCnt && 1 == bCnt {
		if nil != result {
			result.Overlap = 0
		}
		return a.Points[0][0] == b.Points[0][0] && a.Points[0][1] == b.Points[0][1]
	}

	if 1 < aCnt {
		for _, axis := range a.SATAxes() {
			if isPolygonPairSeparatedByDir(a, b, axis.Unit(), result) {
				return false
			}
		}
	}

	if 1 < bCnt {
		for _, axis := range b.SATAxes() {
			if isPolygonPairSeparatedByDir(a, b, axis.Unit(), result) {
				return false
			}
		}
	}

	return true
}

func isPolygonPairSeparatedByDir(a, b *resolv.ConvexPolygon, e resolv.Vector, result *SatResult) bool {
	/*
		[WARNING] This function is deliberately made private, it shouldn't be used alone (i.e. not along the norms of a polygon), otherwise the pushbacks calculated would be meaningless.

		Consider the following example
		a: {
			anchor: [1337.19 1696.74]
			points: [[0 0] [24 0] [24 24] [0 24]]
		},
		b: {
			anchor: [1277.72 1570.56]
			points: [[642.57 319.16] [0 319.16] [5.73 0] [643.75 0.90]]
		}

		e = (-2.98, 1.49).Unit()
	*/

	var aStart, aEnd, bStart, bEnd float64 = MAX_FLOAT64, -MAX_FLOAT64, MAX_FLOAT64, -MAX_FLOAT64
	for _, p := range a.Points {
		dot := (p[0]+a.X)*e[0] + (p[1]+a.Y)*e[1]

		if aStart > dot {
			aStart = dot
		}

		if aEnd < dot {
			aEnd = dot
		}
	}

	for _, p := range b.Points {
		dot := (p[0]+b.X)*e[0] + (p[1]+b.Y)*e[1]

		if bStart > dot {
			bStart = dot
		}

		if bEnd < dot {
			bEnd = dot
		}
	}

	if aStart > bEnd || aEnd < bStart {
		// Separated by unit vector "e"
		return true
	}

	if nil != result {
		overlap := float64(0)

		if aStart < bStart {
			result.AContainedInB = false

			if aEnd < bEnd {
				overlap = aEnd - bStart
				result.BContainedInA = false
			} else {
				option1 := aEnd - bStart
				option2 := bEnd - aStart
				if option1 < option2 {
					overlap = option1
				} else {
					overlap = -option2
				}
			}
		} else {
			result.BContainedInA = false

			if aEnd > bEnd {
				overlap = aStart - bEnd
				result.AContainedInB = false
			} else {
				option1 := aEnd - bStart
				option2 := bEnd - aStart
				if option1 < option2 {
					overlap = option1
				} else {
					overlap = -option2
				}
			}
		}

		currentOverlap := result.Overlap
		absoluteOverlap := overlap
		if overlap < 0 {
			absoluteOverlap = -overlap
		}

		if (0 == result.Axis[0] && 0 == result.Axis[1]) || currentOverlap > absoluteOverlap {
			var sign float64 = 1
			if overlap < 0 {
				sign = -1
			}

			result.Overlap = absoluteOverlap
			result.OverlapX = e[0] * sign
			result.OverlapY = e[1] * sign
		}

		result.Axis = e
	}

	// the specified unit vector "e" doesn't separate "a" and "b", overlap result is generated
	return false
}

func WorldToVirtualGridPos(wx, wy float64) (int32, int32) {
	// [WARNING] Introduces loss of precision!
	// In JavaScript floating numbers suffer from seemingly non-deterministic arithmetics, and even if certain libs solved this issue by approaches such as fixed-point-number, they might not be used in other libs -- e.g. the "collision libs" we're interested in -- thus couldn't kill all pains.
	var virtualGridX int32 = int32(math.Floor(wx * WORLD_TO_VIRTUAL_GRID_RATIO))
	var virtualGridY int32 = int32(math.Floor(wy * WORLD_TO_VIRTUAL_GRID_RATIO))
	return virtualGridX, virtualGridY
}

func VirtualGridToWorldPos(vx, vy int32) (float64, float64) {
	// No loss of precision
	var wx float64 = float64(vx) * VIRTUAL_GRID_TO_WORLD_RATIO
	var wy float64 = float64(vy) * VIRTUAL_GRID_TO_WORLD_RATIO
	return wx, wy
}

func WorldToPolygonColliderBLPos(wx, wy, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY float64) (float64, float64) {
	return wx - halfBoundingW - leftPadding + collisionSpaceOffsetX, wy - halfBoundingH - bottomPadding + collisionSpaceOffsetY
}

func PolygonColliderBLToWorldPos(cx, cy, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY float64) (float64, float64) {
	return cx + halfBoundingW + leftPadding - collisionSpaceOffsetX, cy + halfBoundingH + bottomPadding - collisionSpaceOffsetY
}

func PolygonColliderBLToVirtualGridPos(cx, cy, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY float64) (int32, int32) {
	wx, wy := PolygonColliderBLToWorldPos(cx, cy, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY)
	return WorldToVirtualGridPos(wx, wy)
}

func VirtualGridToPolygonColliderBLPos(vx, vy int32, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY float64) (float64, float64) {
	wx, wy := VirtualGridToWorldPos(vx, vy)
	return WorldToPolygonColliderBLPos(wx, wy, halfBoundingW, halfBoundingH, topPadding, bottomPadding, leftPadding, rightPadding, collisionSpaceOffsetX, collisionSpaceOffsetY)
}

func calcHardPushbacksNorms(joinIndex int32, 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
	}

	//playerColliderCenterX, playerColliderCenterY := playerCollider.Center()
	//fmt.Printf("joinIndex=%d calcHardPushbacksNorms has non-empty collision;playerColliderPos=(%.2f,%.2f)\n", joinIndex, playerColliderCenterX, playerColliderCenterY)
	for _, obj := range collision.Objects {
		isBarrier := false
		switch obj.Data.(type) {
		case *PlayerDownsync:
		case *MeleeBullet, *FireballBullet:
		default:
			// By default it's a regular barrier, even if data is nil, note that Golang syntax of switch-case is kind of confusing, this "default" condition is met only if "!*PlayerDownsync && !*MeleeBullet && !*FireballBullet".
			isBarrier = true
		}

		if !isBarrier {
			continue
		}
		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
		//fmt.Printf("joinIndex=%d calcHardPushbacksNorms found one hardpushback; immediatePushback=(%.2f,%.2f)\n", joinIndex, pushbackX, pushbackY)
	}
	return &ret
}

func deriveOpPattern(currPlayerDownsync, thatPlayerInNextFrame *PlayerDownsync, currRenderFrame *RoomDownsyncFrame, inputsBuffer *RingBuffer) (int, bool, int32, int32) {
	// returns (patternId, jumpedOrNot, effectiveDx, effectiveDy)
	delayedInputFrameId := ConvertToDelayedInputFrameId(currRenderFrame.Id)
	delayedInputFrameIdForPrevRdf := ConvertToDelayedInputFrameId(currRenderFrame.Id - 1)

	if 0 >= delayedInputFrameId {
		return PATTERN_ID_UNABLE_TO_OP, false, 0, 0
	}

	if _, existent := noOpSet[currPlayerDownsync.CharacterState]; existent {
		return PATTERN_ID_UNABLE_TO_OP, false, 0, 0
	}

	delayedInputList := inputsBuffer.GetByFrameId(delayedInputFrameId).(*InputFrameDownsync).InputList
	var delayedInputListForPrevRdf []uint64 = nil
	if 0 < delayedInputFrameIdForPrevRdf {
		delayedInputListForPrevRdf = inputsBuffer.GetByFrameId(delayedInputFrameIdForPrevRdf).(*InputFrameDownsync).InputList
	}

	jumpedOrNot := false
	joinIndex := currPlayerDownsync.JoinIndex
	decodedInput := decodeInput(delayedInputList[joinIndex-1])
	effDx, effDy := int32(0), int32(0)
	prevBtnALevel, prevBtnBLevel := int32(0), int32(0)
	if nil != delayedInputListForPrevRdf {
		prevDecodedInput := decodeInput(delayedInputListForPrevRdf[joinIndex-1])
		prevBtnALevel = prevDecodedInput.BtnALevel
		prevBtnBLevel = prevDecodedInput.BtnBLevel
	}

	if 0 == currPlayerDownsync.FramesToRecover {
		// Jumping and moving are only allowed here
		effDx, effDy = decodedInput.Dx, decodedInput.Dy
		if decodedInput.BtnBLevel > prevBtnBLevel {
			if _, existent := inAirSet[currPlayerDownsync.CharacterState]; !existent {
				jumpedOrNot = true
			}
		}
	}

	patternId := PATTERN_ID_NO_OP
	if decodedInput.BtnALevel > prevBtnALevel {
		if 0 > effDy {
			patternId = 3
		} else if 0 < effDy {
			patternId = 2
		} else {
			patternId = 1
		}
	}

	return patternId, jumpedOrNot, effDx, effDy
}

// [WARNING] The params of this method is carefully tuned such that only "battle.RoomDownsyncFrame" is a necessary custom struct.
func ApplyInputFrameDownsyncDynamicsOnSingleRenderFrame(inputsBuffer *RingBuffer, currRenderFrame *RoomDownsyncFrame, collisionSys *resolv.Space, collisionSysMap map[int32]*resolv.Object, collisionSpaceOffsetX, collisionSpaceOffsetY float64, chConfigsOrderedByJoinIndex []*CharacterConfig) *RoomDownsyncFrame {
	// [WARNING] On backend this function MUST BE called while "InputsBufferLock" is locked!
	roomCapacity := len(currRenderFrame.PlayersArr)
	nextRenderFramePlayers := make([]*PlayerDownsync, roomCapacity)
	// Make a copy first
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		nextRenderFramePlayers[i] = &PlayerDownsync{
			Id:               currPlayerDownsync.Id,
			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,
			Hp:               currPlayerDownsync.Hp,
			MaxHp:            currPlayerDownsync.MaxHp,
			FramesToRecover:  currPlayerDownsync.FramesToRecover - 1,
			FramesInChState:  currPlayerDownsync.FramesInChState + 1,
			ActiveSkillId:    currPlayerDownsync.ActiveSkillId,
			ActiveSkillHit:   currPlayerDownsync.ActiveSkillHit,
			FramesInvinsible: currPlayerDownsync.FramesInvinsible - 1,
			ColliderRadius:   currPlayerDownsync.ColliderRadius,
		}
		if nextRenderFramePlayers[i].FramesToRecover < 0 {
			nextRenderFramePlayers[i].FramesToRecover = 0
		}
		if nextRenderFramePlayers[i].FramesInvinsible < 0 {
			nextRenderFramePlayers[i].FramesInvinsible = 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?
	nextRenderFrameFireballBullets := make([]*FireballBullet, 0, len(currRenderFrame.FireballBullets))
	effPushbacks := make([]Vec2D, roomCapacity)
	hardPushbackNorms := make([]*[]Vec2D, roomCapacity)
	jumpedOrNotList := make([]bool, roomCapacity)

	bulletLocalId := currRenderFrame.BulletLocalIdCounter
	// 1. Process player inputs
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		jumpedOrNotList[i] = false
		chConfig := chConfigsOrderedByJoinIndex[i]
		thatPlayerInNextFrame := nextRenderFramePlayers[i]
		patternId, jumpedOrNot, effDx, effDy := deriveOpPattern(currPlayerDownsync, thatPlayerInNextFrame, currRenderFrame, inputsBuffer)

		if jumpedOrNot {
			thatPlayerInNextFrame.VelY = int32(chConfig.JumpingInitVelY)
			jumpedOrNotList[i] = true
		}
		joinIndex := currPlayerDownsync.JoinIndex
		skillId := chConfig.SkillMapper(patternId, currPlayerDownsync)
		if skillConfig, existent := skills[skillId]; existent {
			thatPlayerInNextFrame.ActiveSkillId = int32(skillId)
			thatPlayerInNextFrame.ActiveSkillHit = 0
			thatPlayerInNextFrame.FramesToRecover = skillConfig.RecoveryFrames
			xfac := int32(1)
			if 0 > thatPlayerInNextFrame.DirX {
				xfac = -xfac
			}
			hasLockVel := false

			// Hardcoded to use only the first hit for now
			switch v := skillConfig.Hits[thatPlayerInNextFrame.ActiveSkillHit].(type) {
			case *MeleeBullet:
				var newBullet MeleeBullet = *v // Copied primitive fields into an onstack variable
				newBullet.BulletLocalId = bulletLocalId
				bulletLocalId++
				newBullet.OriginatedRenderFrameId = currRenderFrame.Id
				newBullet.OffenderJoinIndex = joinIndex
				nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, &newBullet)
				if NO_LOCK_VEL != v.SelfLockVelX {
					hasLockVel = true
					thatPlayerInNextFrame.VelX = xfac * v.SelfLockVelX
				}
				if NO_LOCK_VEL != v.SelfLockVelY {
					hasLockVel = true
					thatPlayerInNextFrame.VelY = v.SelfLockVelY
				}
			case *FireballBullet:
				var newBullet FireballBullet = *v // Copied primitive fields into an onstack variable
				newBullet.BulletLocalId = bulletLocalId
				bulletLocalId++
				newBullet.VirtualGridX, newBullet.VirtualGridY = currPlayerDownsync.VirtualGridX+xfac*newBullet.HitboxOffsetX, currPlayerDownsync.VirtualGridY+newBullet.HitboxOffsetY
				newBullet.OriginatedRenderFrameId = currRenderFrame.Id
				newBullet.OffenderJoinIndex = joinIndex
				newBullet.DirX = xfac
				newBullet.DirY = 0
				newBullet.VelX = newBullet.Speed * xfac
				newBullet.VelY = 0
				nextRenderFrameFireballBullets = append(nextRenderFrameFireballBullets, &newBullet)
				if NO_LOCK_VEL != v.SelfLockVelX {
					hasLockVel = true
					thatPlayerInNextFrame.VelX = xfac * v.SelfLockVelX
				}
				if NO_LOCK_VEL != v.SelfLockVelY {
					hasLockVel = true
					thatPlayerInNextFrame.VelY = v.SelfLockVelY
				}
			}

			if false == hasLockVel && false == currPlayerDownsync.InAir {
				thatPlayerInNextFrame.VelX = 0
			}
			thatPlayerInNextFrame.CharacterState = skillConfig.BoundChState
			continue // Don't allow movement if skill is used
		}

		if 0 == currPlayerDownsync.FramesToRecover {
			if 0 != effDx || 0 != effDy {
				if 0 != effDx {
					thatPlayerInNextFrame.DirX = effDx
				}
				thatPlayerInNextFrame.DirY = effDy
				thatPlayerInNextFrame.VelX = effDx * currPlayerDownsync.Speed
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_WALKING
			} else {
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
				thatPlayerInNextFrame.VelX = 0
			}
		}
	}

	// 2. Process player movement
	playerColliders := make([]*resolv.Object, len(currRenderFrame.PlayersArr), len(currRenderFrame.PlayersArr)) // Will all be removed at the end of this function due to the need for being rollback-compatible
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y = float64(0), float64(0)

		chConfig := chConfigsOrderedByJoinIndex[i]
		// Reset playerCollider position from the "virtual grid position"
		newVx, newVy := currPlayerDownsync.VirtualGridX+currPlayerDownsync.VelX, currPlayerDownsync.VirtualGridY+currPlayerDownsync.VelY
		if jumpedOrNotList[i] {
			newVy += chConfig.JumpingInitVelY // Immediately gets out of any snapping
		}

		wx, wy := VirtualGridToWorldPos(newVx, newVy)
		colliderWidth, colliderHeight := currPlayerDownsync.ColliderRadius*2, currPlayerDownsync.ColliderRadius*4
		switch currPlayerDownsync.CharacterState {
		case ATK_CHARACTER_STATE_LAY_DOWN1:
			colliderWidth, colliderHeight = currPlayerDownsync.ColliderRadius*4, currPlayerDownsync.ColliderRadius*2
		case ATK_CHARACTER_STATE_BLOWN_UP1, ATK_CHARACTER_STATE_INAIR_IDLE1_NO_JUMP, ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP:
			colliderWidth, colliderHeight = currPlayerDownsync.ColliderRadius*2, currPlayerDownsync.ColliderRadius*2
		}

		colliderWorldWidth, colliderWorldHeight := VirtualGridToWorldPos(colliderWidth, colliderHeight)

		playerCollider := GenerateRectCollider(wx, wy, colliderWorldWidth, colliderWorldHeight, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, collisionSpaceOffsetX, collisionSpaceOffsetY, currPlayerDownsync, "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
		playerColliders[i] = playerCollider

		// Add to collision system
		collisionSys.Add(playerCollider)

		thatPlayerInNextFrame := nextRenderFramePlayers[i]
		if currPlayerDownsync.InAir {
			thatPlayerInNextFrame.VelX += GRAVITY_X
			thatPlayerInNextFrame.VelY += GRAVITY_Y
		}
	}

	// 3. Add bullet colliders into collision system
	bulletColliders := make([]*resolv.Object, 0, len(currRenderFrame.MeleeBullets)) // Will all be removed at the end of this function due to the need for being rollback-compatible
	for _, meleeBullet := range currRenderFrame.MeleeBullets {
		if (meleeBullet.OriginatedRenderFrameId+meleeBullet.StartupFrames <= currRenderFrame.Id) && (meleeBullet.OriginatedRenderFrameId+meleeBullet.StartupFrames+meleeBullet.ActiveFrames > currRenderFrame.Id) {
			offender := currRenderFrame.PlayersArr[meleeBullet.OffenderJoinIndex-1]

			xfac := int32(1) // By now, straight Punch offset doesn't respect "y-axis"
			if 0 > offender.DirX {
				xfac = -xfac
			}
			bulletWx, bulletWy := VirtualGridToWorldPos(offender.VirtualGridX+xfac*meleeBullet.HitboxOffsetX, offender.VirtualGridY)
			hitboxSizeWx, hitboxSizeWy := VirtualGridToWorldPos(meleeBullet.HitboxSizeX, meleeBullet.HitboxSizeY)
			newBulletCollider := GenerateRectCollider(bulletWx, bulletWy, hitboxSizeWx, hitboxSizeWy, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, collisionSpaceOffsetX, collisionSpaceOffsetY, meleeBullet, "MeleeBullet")
			collisionSys.Add(newBulletCollider)
			bulletColliders = append(bulletColliders, newBulletCollider)
		} else if meleeBullet.OriginatedRenderFrameId+meleeBullet.StartupFrames+meleeBullet.ActiveFrames > currRenderFrame.Id {
			nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, meleeBullet)
		}
	}

	for _, fireballBullet := range currRenderFrame.FireballBullets {
		if (fireballBullet.OriginatedRenderFrameId+fireballBullet.StartupFrames < currRenderFrame.Id) && (fireballBullet.OriginatedRenderFrameId+fireballBullet.StartupFrames+fireballBullet.ActiveFrames > currRenderFrame.Id) {
			bulletWx, bulletWy := VirtualGridToWorldPos(fireballBullet.VirtualGridX, fireballBullet.VirtualGridY)
			hitboxSizeWx, hitboxSizeWy := VirtualGridToWorldPos(fireballBullet.HitboxSizeX, fireballBullet.HitboxSizeY)
			newBulletCollider := GenerateRectCollider(bulletWx, bulletWy, hitboxSizeWx, hitboxSizeWy, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, SNAP_INTO_PLATFORM_OVERLAP, collisionSpaceOffsetX, collisionSpaceOffsetY, fireballBullet, "FireballBullet")
			collisionSys.Add(newBulletCollider)
			bulletColliders = append(bulletColliders, newBulletCollider)
		} else if fireballBullet.OriginatedRenderFrameId+fireballBullet.StartupFrames+fireballBullet.ActiveFrames > currRenderFrame.Id {
			nextRenderFrameFireballBullets = append(nextRenderFrameFireballBullets, fireballBullet)
		}
	}

	// 4. Calc pushbacks for each player (after its movement) w/o bullets
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		playerCollider := playerColliders[i]
		playerShape := playerCollider.Shape.(*resolv.ConvexPolygon)
		hardPushbackNorms[joinIndex-1] = calcHardPushbacksNorms(joinIndex, playerCollider, playerShape, SNAP_INTO_PLATFORM_OVERLAP, &(effPushbacks[joinIndex-1]))
		thatPlayerInNextFrame := nextRenderFramePlayers[i]
		chConfig := chConfigsOrderedByJoinIndex[i]
		landedOnGravityPushback := false

		if collision := playerCollider.Check(0, 0); nil != collision {
			for _, obj := range collision.Objects {
				isBarrier, isAnotherPlayer, isBullet := false, false, false
				switch obj.Data.(type) {
				case *PlayerDownsync:
					isAnotherPlayer = true
				case *MeleeBullet, *FireballBullet:
					isBullet = true
				default:
					// By default it's a regular barrier, even if data is nil
					isBarrier = 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))
				if isAnotherPlayer {
					// [WARNING] The "zero overlap collision" might be randomly detected/missed on either frontend or backend, to have deterministic result we added paddings to all sides of a playerCollider. As each velocity component of (velX, velY) being a multiple of 0.5 at any renderFrame, each position component of (x, y) can only be a multiple of 0.5 too, thus whenever a 1-dimensional collision happens between players from [player#1: i*0.5, player#2: j*0.5, not collided yet] to [player#1: (i+k)*0.5, player#2: j*0.5, collided], the overlap becomes (i+k-j)*0.5+2*s, and after snapping subtraction the effPushback magnitude for each player is (i+k-j)*0.5, resulting in 0.5-multiples-position for the next renderFrame.
					pushbackX, pushbackY = (overlapResult.Overlap-SNAP_INTO_PLATFORM_OVERLAP*2)*overlapResult.OverlapX, (overlapResult.Overlap-SNAP_INTO_PLATFORM_OVERLAP*2)*overlapResult.OverlapY
				}
				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 SNAP_INTO_PLATFORM_THRESHOLD < normAlignmentWithGravity {
					landedOnGravityPushback = true
					//playerColliderCenterX, playerColliderCenterY := playerCollider.Center()
					//fmt.Printf("joinIndex=%d landedOnGravityPushback\n{renderFrame.id: %d, isBarrier: %v, isAnotherPlayer: %v}\nhardPushbackNormsOfThisPlayer=%v, playerColliderPos=(%.2f,%.2f), immediatePushback={%.3f, %.3f}, effPushback={%.3f, %.3f}, overlapMag=%.4f\n", joinIndex, currRenderFrame.Id, isBarrier, isAnotherPlayer, *hardPushbackNorms[joinIndex-1], playerColliderCenterX, playerColliderCenterY, pushbackX, pushbackY, effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y, overlapResult.Overlap)
				}
			}
		}
		if landedOnGravityPushback {
			thatPlayerInNextFrame.InAir = false
			if currPlayerDownsync.InAir && 0 >= currPlayerDownsync.VelY {
				// fallStopping
				thatPlayerInNextFrame.VelY = 0
				thatPlayerInNextFrame.VelX = 0
				if _, existent := nonAttackingSet[thatPlayerInNextFrame.CharacterState]; existent {
					if ATK_CHARACTER_STATE_BLOWN_UP1 == thatPlayerInNextFrame.CharacterState {
						thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_LAY_DOWN1
						thatPlayerInNextFrame.FramesToRecover = chConfig.LayDownFramesToRecover
					} else {
						halfColliderWidthDiff, halfColliderHeightDiff := int32(0), currPlayerDownsync.ColliderRadius
						_, halfColliderWorldHeightDiff := VirtualGridToWorldPos(halfColliderWidthDiff, halfColliderHeightDiff)
						effPushbacks[joinIndex-1].Y -= halfColliderWorldHeightDiff // To prevent bouncing due to abrupt change of collider shape
						thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
					}
				}
			} else {
				if _, existent := nonAttackingSet[thatPlayerInNextFrame.CharacterState]; existent {
					// not fallStopping, could be in LayDown or GetUp
					if ATK_CHARACTER_STATE_LAY_DOWN1 == thatPlayerInNextFrame.CharacterState {
						if 0 == thatPlayerInNextFrame.FramesToRecover {
							thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_GET_UP1
							thatPlayerInNextFrame.FramesToRecover = chConfig.GetUpFramesToRecover
						}
					} else if ATK_CHARACTER_STATE_GET_UP1 == thatPlayerInNextFrame.CharacterState {
						if 0 == thatPlayerInNextFrame.FramesToRecover {
							thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
							thatPlayerInNextFrame.FramesInvinsible = chConfig.GetUpInvinsibleFrames
						}
					}
				}
			}
		}
	}

	// 5. Check bullet-anything collisions
	for _, bulletCollider := range bulletColliders {
		collision := bulletCollider.Check(0, 0)
		bulletCollider.Space.Remove(bulletCollider) // Make sure that the bulletCollider is always removed for each renderFrame
		addToNextRenderFrame := true
		if nil != collision {
			switch v := bulletCollider.Data.(type) {
			case *MeleeBullet:
				bulletShape := bulletCollider.Shape.(*resolv.ConvexPolygon)
				offender := currRenderFrame.PlayersArr[v.OffenderJoinIndex-1]
				for _, obj := range collision.Objects {
					defenderShape := obj.Shape.(*resolv.ConvexPolygon)
					switch t := obj.Data.(type) {
					case *PlayerDownsync:
						if v.OffenderJoinIndex == t.JoinIndex {
							continue
						}
						overlapped, _, _, _ := CalcPushbacks(0, 0, bulletShape, defenderShape)
						if !overlapped {
							continue
						}
						addToNextRenderFrame = false
						if _, existent := invinsibleSet[t.CharacterState]; existent {
							continue
						}
						if 0 < t.FramesInvinsible {
							continue
						}
						xfac := int32(1) // By now, straight Punch offset doesn't respect "y-axis"
						if 0 > offender.DirX {
							xfac = -xfac
						}
						pushbackVelX, pushbackVelY := xfac*v.PushbackVelX, v.PushbackVelY
						atkedPlayerInNextFrame := nextRenderFramePlayers[t.JoinIndex-1]
						atkedPlayerInNextFrame.VelX = pushbackVelX
						atkedPlayerInNextFrame.VelY = pushbackVelY
						if v.BlowUp {
							atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_BLOWN_UP1
						} else {
							atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_ATKED1
						}
						oldFramesToRecover := nextRenderFramePlayers[t.JoinIndex-1].FramesToRecover
						if v.HitStunFrames > oldFramesToRecover {
							atkedPlayerInNextFrame.FramesToRecover = v.HitStunFrames
						}
					default:
						addToNextRenderFrame = false
					}
				}
			case *FireballBullet:
				bulletShape := bulletCollider.Shape.(*resolv.ConvexPolygon)
				offender := currRenderFrame.PlayersArr[v.OffenderJoinIndex-1]
				for _, obj := range collision.Objects {
					defenderShape := obj.Shape.(*resolv.ConvexPolygon)
					switch t := obj.Data.(type) {
					case *PlayerDownsync:
						if v.OffenderJoinIndex == t.JoinIndex {
							continue
						}
						overlapped, _, _, _ := CalcPushbacks(0, 0, bulletShape, defenderShape)
						if !overlapped {
							continue
						}
						addToNextRenderFrame = false
						if _, existent := invinsibleSet[t.CharacterState]; existent {
							continue
						}
						if 0 < t.FramesInvinsible {
							continue
						}
						xfac := int32(1) // By now, straight Punch offset doesn't respect "y-axis"
						if 0 > offender.DirX {
							xfac = -xfac
						}
						pushbackVelX, pushbackVelY := xfac*v.PushbackVelX, v.PushbackVelY
						atkedPlayerInNextFrame := nextRenderFramePlayers[t.JoinIndex-1]
						atkedPlayerInNextFrame.VelX = pushbackVelX
						atkedPlayerInNextFrame.VelY = pushbackVelY
						if v.BlowUp {
							atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_BLOWN_UP1
						} else {
							atkedPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_ATKED1
						}
						oldFramesToRecover := nextRenderFramePlayers[t.JoinIndex-1].FramesToRecover
						if v.HitStunFrames > oldFramesToRecover {
							atkedPlayerInNextFrame.FramesToRecover = v.HitStunFrames
						}
					default:
						addToNextRenderFrame = false
					}
				}
			}
		}
		if addToNextRenderFrame {
			switch v := bulletCollider.Data.(type) {
			case *MeleeBullet:
				nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, v)
			case *FireballBullet:
				v.VirtualGridX, v.VirtualGridY = v.VirtualGridX+v.VelX, v.VirtualGridY+v.VelY
				nextRenderFrameFireballBullets = append(nextRenderFrameFireballBullets, v)
			}
		}
	}

	// 6. Get players out of stuck barriers if there's any
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		playerCollider := playerColliders[i]
		// Update "virtual grid position"
		thatPlayerInNextFrame := nextRenderFramePlayers[i]
		thatPlayerInNextFrame.VirtualGridX, thatPlayerInNextFrame.VirtualGridY = PolygonColliderBLToVirtualGridPos(playerCollider.X-effPushbacks[joinIndex-1].X, playerCollider.Y-effPushbacks[joinIndex-1].Y, playerCollider.W*0.5, playerCollider.H*0.5, 0, 0, 0, 0, collisionSpaceOffsetX, collisionSpaceOffsetY)

		// Update "CharacterState"
		if thatPlayerInNextFrame.InAir {
			oldNextCharacterState := thatPlayerInNextFrame.CharacterState
			switch oldNextCharacterState {
			case ATK_CHARACTER_STATE_IDLE1, ATK_CHARACTER_STATE_WALKING:
				if jumpedOrNotList[i] || ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP == currPlayerDownsync.CharacterState {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_IDLE1_BY_JUMP
				} else {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_IDLE1_NO_JUMP
				}
			case ATK_CHARACTER_STATE_ATK1:
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_ATK1
				// No inAir transition for ATK2/ATK3 for now
			case ATK_CHARACTER_STATE_ATKED1:
				thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_INAIR_ATKED1
			}
		}

		// Reset "FramesInChState" if "CharacterState" is changed
		if thatPlayerInNextFrame.CharacterState != currPlayerDownsync.CharacterState {
			thatPlayerInNextFrame.FramesInChState = 0
		}

		// Remove any active skill if not attacking
		if _, existent := nonAttackingSet[thatPlayerInNextFrame.CharacterState]; existent {
			thatPlayerInNextFrame.ActiveSkillId = int32(NO_SKILL)
			thatPlayerInNextFrame.ActiveSkillHit = int32(NO_SKILL_HIT)
		}
	}

	for _, playerCollider := range playerColliders {
		playerCollider.Space.Remove(playerCollider)
	}

	return &RoomDownsyncFrame{
		Id:                   currRenderFrame.Id + 1,
		PlayersArr:           nextRenderFramePlayers,
		BulletLocalIdCounter: bulletLocalId,
		MeleeBullets:         nextRenderFrameMeleeBullets,
		FireballBullets:      nextRenderFrameFireballBullets,
	}
}

func GenerateRectCollider(wx, wy, w, h, topPadding, bottomPadding, leftPadding, rightPadding, spaceOffsetX, spaceOffsetY float64, data interface{}, tag string) *resolv.Object {
	blX, blY := WorldToPolygonColliderBLPos(wx, wy, w*0.5, h*0.5, topPadding, bottomPadding, leftPadding, rightPadding, spaceOffsetX, spaceOffsetY)
	return generateRectColliderInCollisionSpace(blX, blY, leftPadding+w+rightPadding, bottomPadding+h+topPadding, data, tag)
}

func generateRectColliderInCollisionSpace(blX, blY, w, h float64, data interface{}, tag string) *resolv.Object {
	collider := resolv.NewObject(blX, blY, w, h, tag) // Unlike its frontend counter part, the position of a "resolv.Object" must be specified by "bottom-left point" because "w" and "h" must be positive, see "resolv.Object.BoundsToSpace" for details
	shape := resolv.NewRectangle(0, 0, w, h)
	collider.SetShape(shape)
	collider.Data = data
	return collider
}

func GenerateConvexPolygonCollider(unalignedSrc *Polygon2D, spaceOffsetX, spaceOffsetY float64, data interface{}, tag string) *resolv.Object {
	aligned := AlignPolygon2DToBoundingBox(unalignedSrc)
	var w, h float64 = 0, 0

	shape := resolv.NewConvexPolygon()
	for i, pi := range aligned.Points {
		for j, pj := range aligned.Points {
			if i == j {
				continue
			}
			if math.Abs(pj.X-pi.X) > w {
				w = math.Abs(pj.X - pi.X)
			}
			if math.Abs(pj.Y-pi.Y) > h {
				h = math.Abs(pj.Y - pi.Y)
			}
		}
	}

	for i := 0; i < len(aligned.Points); i++ {
		p := aligned.Points[i]
		shape.AddPoints(p.X, p.Y)
	}

	collider := resolv.NewObject(aligned.Anchor.X+spaceOffsetX, aligned.Anchor.Y+spaceOffsetY, w, h, tag)
	collider.SetShape(shape)
	collider.Data = data

	return collider
}

func AlignPolygon2DToBoundingBox(input *Polygon2D) *Polygon2D {
	// Transform again to put "anchor" at the "bottom-left point (w.r.t. world space)" of the bounding box for "resolv"
	boundingBoxBL := &Vec2D{
		X: MAX_FLOAT64,
		Y: MAX_FLOAT64,
	}
	for _, p := range input.Points {
		if p.X < boundingBoxBL.X {
			boundingBoxBL.X = p.X
		}
		if p.Y < boundingBoxBL.Y {
			boundingBoxBL.Y = p.Y
		}
	}

	// Now "input.Anchor" should move to "input.Anchor+boundingBoxBL", thus "boundingBoxBL" is also the value of the negative diff for all "input.Points"
	output := &Polygon2D{
		Anchor: &Vec2D{
			X: input.Anchor.X + boundingBoxBL.X,
			Y: input.Anchor.Y + boundingBoxBL.Y,
		},
		Points: make([]*Vec2D, len(input.Points)),
	}

	for i, p := range input.Points {
		output.Points[i] = &Vec2D{
			X: p.X - boundingBoxBL.X,
			Y: p.Y - boundingBoxBL.Y,
		}
	}

	return output
}