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"

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

	NO_SKILL     = -1
	NO_SKILL_HIT = -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)
)

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{}

func ConvertToInputFrameId(renderFrameId int32, inputDelayFrames int32, inputScaleFrames uint32) int32 {
	if renderFrameId < inputDelayFrames {
		return 0
	}
	return ((renderFrameId - inputDelayFrames) >> inputScaleFrames)
}

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:
		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".
			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, inputDelayFrames int32, inputScaleFrames uint32) (int, bool, int32, int32) {
	// returns (patternId, jumpedOrNot, effectiveDx, effectiveDy)
	delayedInputFrameId := ConvertToInputFrameId(currRenderFrame.Id, inputDelayFrames, inputScaleFrames)
	delayedInputFrameIdForPrevRdf := ConvertToInputFrameId(currRenderFrame.Id-1, inputDelayFrames, inputScaleFrames)

	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 {
		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,
		}
		if nextRenderFramePlayers[i].FramesToRecover < 0 {
			nextRenderFramePlayers[i].FramesToRecover = 0
		}
	}

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

	// 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, INPUT_DELAY_FRAMES, INPUT_SCALE_FRAMES)

		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

			// TODO: Respect non-zero "selfLockVel"

			// 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.OriginatedRenderFrameId = currRenderFrame.Id
				newBullet.OffenderJoinIndex = joinIndex
				nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, &newBullet)
				thatPlayerInNextFrame.FramesToRecover = skillConfig.RecoveryFrames
			}

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

		if 0 == currPlayerDownsync.FramesToRecover {
			if 0 != effDx || 0 != effDy {
				thatPlayerInNextFrame.DirX, thatPlayerInNextFrame.DirY = effDx, 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
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		effPushbacks[joinIndex-1].X, effPushbacks[joinIndex-1].Y = float64(0), float64(0)
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		thatPlayerInNextFrame := nextRenderFramePlayers[i]
		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
		}

		playerCollider.X, playerCollider.Y = VirtualGridToPolygonColliderBLPos(newVx, newVy, playerCollider.W*0.5, playerCollider.H*0.5, 0, 0, 0, 0, collisionSpaceOffsetX, collisionSpaceOffsetY)
		// Update in the collision system
		playerCollider.Update()

		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 {
			nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, meleeBullet)
		}
	}

	// 4. Calc pushbacks for each player (after its movement) w/o bullets
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		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:
					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.VelX = 0
				thatPlayerInNextFrame.VelY = 0
				if ATK_CHARACTER_STATE_BLOWN_UP1 == thatPlayerInNextFrame.CharacterState {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_LAY_DOWN1
					thatPlayerInNextFrame.FramesToRecover = chConfig.LayDownFramesToRecover
				} else {
					thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
					thatPlayerInNextFrame.FramesToRecover = 0
				}
			} else {
				// 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 thatPlayerInNextFrame.FramesInChState == chConfig.GetUpFrames {
						// [WARNING] Before reaching here, the player had 3 invinsible frames to either attack or jump, if it ever took any action then this condition wouldn't have been met, thus we hereby only transit it back to IDLE as it took no action
						thatPlayerInNextFrame.CharacterState = ATK_CHARACTER_STATE_IDLE1
					}
				}
			}
		}
	}

	// 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
		switch v := bulletCollider.Data.(type) {
		case *MeleeBullet:
			if nil == collision {
				nextRenderFrameMeleeBullets = append(nextRenderFrameMeleeBullets, v)
				continue
			}
			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
					}
					if _, existent := invinsibleSet[t.CharacterState]; existent {
						continue
					}
					overlapped, _, _, _ := CalcPushbacks(0, 0, bulletShape, defenderShape)
					if !overlapped {
						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:
				}
			}
		}
	}

	// 6. Get players out of stuck barriers if there's any
	for i, currPlayerDownsync := range currRenderFrame.PlayersArr {
		joinIndex := currPlayerDownsync.JoinIndex
		collisionPlayerIndex := COLLISION_PLAYER_INDEX_PREFIX + joinIndex
		playerCollider := collisionSysMap[collisionPlayerIndex]
		// 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)
		}
	}

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

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
}