package world

import (
	"encoding/binary"
	"errors"
	"fmt"
	"iter"
	"maps"
	"math/rand/v2"
	"slices"
	"sync"
	"sync/atomic"
	"time"

	"github.com/df-mc/dragonfly/server/block/cube"
	"github.com/df-mc/dragonfly/server/event"
	"github.com/df-mc/dragonfly/server/internal/sliceutil"
	"github.com/df-mc/dragonfly/server/world/chunk"
	"github.com/df-mc/dragonfly/server/world/redstone"
	"github.com/df-mc/goleveldb/leveldb"
	"github.com/go-gl/mathgl/mgl64"
	"github.com/google/uuid"
)

// World implements a Minecraft world. It manages all aspects of what players
// can see, such as blocks, entities and particles. World generally provides a
// synchronised state: All entities, blocks and players usually operate in this
// world, so World ensures that all its methods will always be safe for
// simultaneous calls. A nil *World is safe to use but not functional.
type World struct {
	conf Config
	ra   cube.Range

	queue        chan transaction
	queueClosing chan struct{}
	queueing     sync.WaitGroup

	// advance is a bool that specifies if this World should advance the current
	// tick, time and weather saved in the Settings struct held by the World.
	advance bool

	o sync.Once

	set     *Settings
	handler atomic.Pointer[Handler]

	weather

	closing chan struct{}
	running sync.WaitGroup

	// chunks holds a cache of chunks currently loaded. These chunks are cleared
	// from this map after some time of not being used.
	chunks map[ChunkPos]*Column

	// entities holds a map of entities currently loaded and the last ChunkPos
	// that the Entity was in. These are tracked so that a call to RemoveEntity
	// can find the correct Entity.
	entities map[*EntityHandle]ChunkPos

	r *rand.Rand

	// scheduledUpdates is a map of tick time values indexed by the block
	// position at which an update is scheduled. If the current tick exceeds the
	// tick value passed, the block update will be performed and the entry will
	// be removed from the map.
	scheduledUpdates *scheduledTickQueue
	neighbourUpdates []neighbourUpdate

	redstone redstone.State

	viewerMu sync.Mutex
	viewers  map[*Loader]Viewer
}

// transaction is a type that may be added to the transaction queue of a World.
// Its Run method is called when the transaction is taken out of the queue.
type transaction interface {
	Run(w *World)
}

// New creates a new initialised world. The world may be used right away, but
// it will not be saved or loaded from files until it has been given a
// different provider than the default. (NopProvider) By default, the name of
// the world will be 'World'.
func New() *World {
	var conf Config
	return conf.New()
}

// Name returns the display name of the world. Generally, this name is
// displayed at the top of the player list in the pause screen in-game. If a
// provider is set, the name will be updated according to the name that it
// provides.
func (w *World) Name() string {
	w.set.Lock()
	defer w.set.Unlock()
	return w.set.Name
}

// Dimension returns the Dimension assigned to the World in world.New. The sky
// colour and behaviour of a variety of world features differ based on the
// Dimension.
func (w *World) Dimension() Dimension {
	return w.conf.Dim
}

// Range returns the range in blocks of the World (min and max). It is
// equivalent to calling World.Dimension().Range().
func (w *World) Range() cube.Range {
	return w.ra
}

// BlockRegistry returns the BlockRegistry used by the World.
func (w *World) BlockRegistry() BlockRegistry {
	return w.conf.Blocks
}

// ExecFunc is a function that performs a synchronised transaction on a World.
type ExecFunc func(tx *Tx)

// Exec performs a synchronised transaction f on a World. Exec returns a channel
// that is closed once the transaction is complete. For Worlds created with
// Config.Synchronous set, the transaction is executed on the calling goroutine
// and the channel returned is closed when Exec returns. Awaiting a nested Exec
// from within a transaction deadlocks on non-synchronous Worlds.
func (w *World) Exec(f ExecFunc) <-chan struct{} {
	c := make(chan struct{})
	ntx := normalTransaction{c: c, f: f}
	if w.conf.Synchronous {
		ntx.Run(w)
		return c
	}
	w.queue <- ntx
	return c
}

func (w *World) weakExec(invalid *atomic.Bool, cond *sync.Cond, f ExecFunc) <-chan bool {
	c := make(chan bool, 1)
	if w.conf.Synchronous {
		valid := !invalid.Load()
		if valid {
			// As in weakTransaction.Run, f must not run under cond.L: it may
			// relock it, e.g. through RemoveEntity.
			cond.L.Unlock()
			tx := &Tx{w: w}
			f(tx)
			tx.close()
			cond.L.Lock()
		}
		c <- valid
		return c
	}
	w.queue <- weakTransaction{c: c, f: f, invalid: invalid, cond: cond}
	return c
}

// handleTransactions continuously reads transactions from the queue and runs
// them.
func (w *World) handleTransactions() {
	for {
		select {
		case tx := <-w.queue:
			tx.Run(w)
		case <-w.queueClosing:
			w.queueing.Done()
			return
		}
	}
}

// EntityRegistry returns the EntityRegistry that was passed to the World's
// Config upon construction.
func (w *World) EntityRegistry() EntityRegistry {
	return w.conf.Entities
}

// block reads a block from the position passed. If a chunk is not yet loaded
// at that position, the chunk is loaded, or generated if it could not be found
// in the world save, and the block returned.
func (w *World) block(pos cube.Pos) Block {
	return w.blockInChunk(w.chunk(chunkPosFromBlockPos(pos)), pos)
}

// blockInChunk reads a block from a chunk at the position passed. The block
// is assumed to be within the chunk passed.
func (w *World) blockInChunk(c *Column, pos cube.Pos) Block {
	if pos.OutOfBounds(w.ra) {
		// Fast way out.
		return w.conf.Blocks.Air()
	}
	rid := c.Block(uint8(pos[0]), int16(pos[1]), uint8(pos[2]), 0)
	if w.conf.Blocks.NBTBlock(rid) {
		// The block was also a block entity, so we look it up in the block entity map.
		if b, ok := c.BlockEntities[pos]; ok {
			return b
		}
		// Despite being a block with NBT, the block didn't actually have any
		// stored NBT yet. We add it here and update the block.
		nbtB := w.conf.Blocks.BlockByRuntimeIDOrAir(rid).(NBTer).DecodeNBT(map[string]any{}).(Block)
		c.BlockEntities[pos] = nbtB
		for _, v := range c.viewers {
			v.ViewBlockUpdate(pos, nbtB, 0)
		}
		return nbtB
	}
	return w.conf.Blocks.BlockByRuntimeIDOrAir(rid)
}

// biome reads the Biome at the position passed. If a chunk is not yet loaded
// at that position, the chunk is loaded, or generated if it could not be found
// in the world save, and the Biome returned.
func (w *World) biome(pos cube.Pos) Biome {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return ocean()
	}
	id := int(w.chunk(chunkPosFromBlockPos(pos)).Biome(uint8(pos[0]), int16(pos[1]), uint8(pos[2])))
	b, ok := BiomeByID(id)
	if !ok {
		w.conf.Log.Error("biome not found by ID", "ID", id)
	}
	return b
}

// HighestLightBlocker gets the Y value of the highest fully light blocking
// block at the x and z values passed in the World.
func (w *World) HighestLightBlocker(x, z int) int {
	return int(w.chunk(ChunkPos{int32(x >> 4), int32(z >> 4)}).HighestLightBlocker(uint8(x), uint8(z)))
}

// highestBlock looks up the highest non-air block in the World at a specific x
// and z The y value of the highest block is returned, or 0 if no blocks were
// present in the column.
func (w *World) highestBlock(x, z int) int {
	return int(w.chunk(ChunkPos{int32(x >> 4), int32(z >> 4)}).HighestBlock(uint8(x), uint8(z)))
}

// highestObstructingBlock returns the highest block in the World at a given x
// and z that has at least a solid top or bottom face.
func (w *World) highestObstructingBlock(x, z int) int {
	yHigh := w.highestBlock(x, z)
	src := worldSource{w: w}
	for y := yHigh; y >= w.Range()[0]; y-- {
		pos := cube.Pos{x, y, z}
		m := w.block(pos).Model()
		if m.FaceSolid(pos, cube.FaceUp, src) || m.FaceSolid(pos, cube.FaceDown, src) {
			return y
		}
	}
	return w.Range()[0]
}

// SetOpts holds several parameters that may be set to disable updates in the
// World of different kinds as a result of a call to SetBlock.
type SetOpts struct {
	// DisableBlockUpdates makes SetBlock not update any neighbouring blocks as
	// a result of the SetBlock call.
	DisableBlockUpdates bool
	// DisableLiquidDisplacement disables the displacement of liquid blocks to
	// the second layer (or back to the first layer, if it already was on the
	// second layer). Disabling this is not widely recommended unless
	// performance is very important, or where it is known no liquid can be
	// present anyway.
	DisableLiquidDisplacement bool
}

// setBlock writes a block to the position passed. If a chunk is not yet loaded
// at that position, the chunk is first loaded or generated if it could not be
// found in the world save. setBlock panics if the block passed has not yet
// been registered using RegisterBlock(). Nil may be passed as the block to set
// the block to air.
//
// A SetOpts struct may be passed to additionally modify behaviour of setBlock,
// specifically to improve performance under specific circumstances. Nil should
// be passed where performance is not essential, to make sure the world is
// updated adequately.
//
// setBlock should be avoided in situations where performance is critical when
// needing to set a lot of blocks to the world. BuildStructure may be used
// instead.
func (w *World) setBlock(pos cube.Pos, b Block, opts *SetOpts) {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return
	}
	if opts == nil {
		opts = &SetOpts{}
	}

	x, y, z := uint8(pos[0]), int16(pos[1]), uint8(pos[2])
	c := w.chunk(chunkPosFromBlockPos(pos))

	rid := w.conf.Blocks.BlockRuntimeID(b)

	var before uint32
	if rid != w.conf.Blocks.AirRuntimeID() && !opts.DisableLiquidDisplacement {
		before = c.Block(x, y, z, 0)
	}

	c.modified = true
	c.SetBlock(x, y, z, 0, rid)
	if w.conf.Blocks.NBTBlock(rid) {
		c.BlockEntities[pos] = b
	} else {
		delete(c.BlockEntities, pos)
	}

	viewers := slices.Clone(c.viewers)

	if !opts.DisableLiquidDisplacement {
		var secondLayer Block

		airRID := w.conf.Blocks.AirRuntimeID()
		if rid == airRID {
			if li := c.Block(x, y, z, 1); li != airRID {
				c.SetBlock(x, y, z, 0, li)
				c.SetBlock(x, y, z, 1, airRID)
				secondLayer = w.conf.Blocks.Air()
				b = w.conf.Blocks.BlockByRuntimeIDOrAir(li)
			}
		} else if w.conf.Blocks.LiquidDisplacingBlock(rid) {
			if w.conf.Blocks.LiquidBlock(before) {
				l := w.conf.Blocks.BlockByRuntimeIDOrAir(before)
				if b.(LiquidDisplacer).CanDisplace(l.(Liquid)) {
					c.SetBlock(x, y, z, 1, before)
					secondLayer = l
				}
			}
		} else if li := c.Block(x, y, z, 1); li != airRID {
			c.SetBlock(x, y, z, 1, airRID)
			secondLayer = w.conf.Blocks.Air()
		}

		if secondLayer != nil {
			for _, viewer := range viewers {
				viewer.ViewBlockUpdate(pos, secondLayer, 1)
			}
		}
	}

	for _, viewer := range viewers {
		viewer.ViewBlockUpdate(pos, b, 0)
	}

	if !opts.DisableBlockUpdates {
		w.doBlockUpdatesAround(pos)
	}
}

// setBiome sets the Biome at the position passed. If a chunk is not yet loaded
// at that position, the chunk is first loaded or generated if it could not be
// found in the world save.
func (w *World) setBiome(pos cube.Pos, b Biome) {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return
	}
	c := w.chunk(chunkPosFromBlockPos(pos))
	c.modified = true
	c.SetBiome(uint8(pos[0]), int16(pos[1]), uint8(pos[2]), uint32(b.EncodeBiome()))
}

// buildStructure builds a Structure passed at a specific position in the
// world. Unlike setBlock, it takes a Structure implementation, which provides
// blocks to be placed at a specific location. buildStructure is specifically
// optimised to be able to process a large batch of chunks simultaneously and
// will do so within much less time than separate setBlock calls would. The
// method operates on a per-chunk basis, setting all blocks within a single
// chunk part of the Structure before moving on to the next chunk.
func (w *World) buildStructure(pos cube.Pos, s Structure) {
	dim := s.Dimensions()
	width, height, length := dim[0], dim[1], dim[2]
	maxX, maxY, maxZ := pos[0]+width, pos[1]+height, pos[2]+length
	f := func(x, y, z int) Block {
		return w.block(cube.Pos{pos[0] + x, pos[1] + y, pos[2] + z})
	}

	// We approach this on a per-chunk basis, so that we can keep only one chunk
	// in memory at a time while not needing to acquire a new chunk lock for
	// every block. This also allows us not to send block updates, but instead
	// send a single chunk update once.
	for chunkX := pos[0] >> 4; chunkX <= maxX>>4; chunkX++ {
		for chunkZ := pos[2] >> 4; chunkZ <= maxZ>>4; chunkZ++ {
			chunkPos := ChunkPos{int32(chunkX), int32(chunkZ)}
			c := w.chunk(chunkPos)

			baseX, baseZ := chunkX<<4, chunkZ<<4
			for i, sub := range c.Sub() {
				baseY := (i + (w.Range()[0] >> 4)) << 4
				if baseY>>4 < pos[1]>>4 {
					continue
				} else if baseY >= maxY {
					break
				}

				for localY := 0; localY < 16; localY++ {
					yOffset := baseY + localY
					if yOffset > w.Range()[1] || yOffset >= maxY {
						// We've hit the height limit for blocks.
						break
					} else if yOffset < w.Range()[0] || yOffset < pos[1] {
						// We've got a block below the minimum, but other blocks might still reach above
						// it, so don't break but continue.
						continue
					}
					for localX := 0; localX < 16; localX++ {
						xOffset := baseX + localX
						if xOffset < pos[0] || xOffset >= maxX {
							continue
						}
						for localZ := 0; localZ < 16; localZ++ {
							zOffset := baseZ + localZ
							if zOffset < pos[2] || zOffset >= maxZ {
								continue
							}
							b, liq := s.At(xOffset-pos[0], yOffset-pos[1], zOffset-pos[2], f)
							if b != nil {
								rid := w.conf.Blocks.BlockRuntimeID(b)
								sub.SetBlock(uint8(xOffset), uint8(yOffset), uint8(zOffset), 0, rid)

								nbtPos := cube.Pos{xOffset, yOffset, zOffset}
								if w.conf.Blocks.NBTBlock(rid) {
									c.BlockEntities[nbtPos] = b
								} else {
									delete(c.BlockEntities, nbtPos)
								}
							}
							if liq != nil {
								sub.SetBlock(uint8(xOffset), uint8(yOffset), uint8(zOffset), 1, w.conf.Blocks.BlockRuntimeID(liq))
							} else if len(sub.Layers()) > 1 {
								sub.SetBlock(uint8(xOffset), uint8(yOffset), uint8(zOffset), 1, w.conf.Blocks.AirRuntimeID())
							}
						}
					}
				}
			}
			c.SetBlock(0, 0, 0, 0, c.Block(0, 0, 0, 0)) // Make sure the heightmap is recalculated.
			c.modified = true

			// After setting all blocks of the structure within a single chunk,
			// we show the new chunk to all viewers once.
			for _, viewer := range c.viewers {
				viewer.ViewChunk(chunkPos, w.Dimension(), c.BlockEntities, c.Chunk)
			}
		}
	}
}

// liquid attempts to return a Liquid block at the position passed. This
// Liquid may be in the foreground or in any other layer. If found, the Liquid
// is returned. If not, the bool returned is false.
func (w *World) liquid(pos cube.Pos) (Liquid, bool) {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return nil, false
	}
	c := w.chunk(chunkPosFromBlockPos(pos))
	x, y, z := uint8(pos[0]), int16(pos[1]), uint8(pos[2])

	id := c.Block(x, y, z, 0)
	b, ok := w.conf.Blocks.BlockByRuntimeID(id)
	if !ok {
		w.conf.Log.Error("Liquid: no block with runtime ID", "ID", id)
		return nil, false
	}
	if liq, ok := b.(Liquid); ok {
		return liq, true
	}
	id = c.Block(x, y, z, 1)

	b, ok = w.conf.Blocks.BlockByRuntimeID(id)
	if !ok {
		w.conf.Log.Error("Liquid: no block with runtime ID", "ID", id)
		return nil, false
	}
	liq, ok := b.(Liquid)
	return liq, ok
}

// setLiquid sets a Liquid at a specific position in the World. Unlike
// setBlock, setLiquid will not necessarily overwrite any existing blocks. It
// will instead be in the same position as a block currently there, unless
// there already is a Liquid at that position, in which case it will be
// overwritten. If nil is passed for the Liquid, any Liquid currently present
// will be removed.
func (w *World) setLiquid(pos cube.Pos, b Liquid) {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return
	}
	chunkPos := chunkPosFromBlockPos(pos)
	c := w.chunk(chunkPos)
	if b == nil {
		w.removeLiquids(c, pos)
		w.doBlockUpdatesAround(pos)
		return
	}
	x, y, z := uint8(pos[0]), int16(pos[1]), uint8(pos[2])
	if !replaceable(w, c, pos, b) {
		if displacer, ok := w.blockInChunk(c, pos).(LiquidDisplacer); !ok || !displacer.CanDisplace(b) {
			return
		}
	}
	rid := w.conf.Blocks.BlockRuntimeID(b)
	if w.removeLiquids(c, pos) {
		c.SetBlock(x, y, z, 0, rid)
		for _, v := range c.viewers {
			v.ViewBlockUpdate(pos, b, 0)
		}
	} else {
		c.SetBlock(x, y, z, 1, rid)
		for _, v := range c.viewers {
			v.ViewBlockUpdate(pos, b, 1)
		}
	}
	c.modified = true

	w.doBlockUpdatesAround(pos)
}

// removeLiquids removes any liquid blocks that may be present at a specific
// block position in the chunk passed. The bool returned specifies if no blocks
// were left on the foreground layer.
func (w *World) removeLiquids(c *Column, pos cube.Pos) bool {
	x, y, z := uint8(pos[0]), int16(pos[1]), uint8(pos[2])
	air := w.conf.Blocks.Air()

	noneLeft := false
	if noLeft, changed := w.removeLiquidOnLayer(c.Chunk, x, y, z, 0); noLeft {
		if changed {
			for _, v := range c.viewers {
				v.ViewBlockUpdate(pos, air, 0)
			}
		}
		noneLeft = true
	}
	if _, changed := w.removeLiquidOnLayer(c.Chunk, x, y, z, 1); changed {
		for _, v := range c.viewers {
			v.ViewBlockUpdate(pos, air, 1)
		}
	}
	return noneLeft
}

// removeLiquidOnLayer removes a liquid block from a specific layer in the
// chunk passed, returning true if successful.
func (w *World) removeLiquidOnLayer(c *chunk.Chunk, x uint8, y int16, z, layer uint8) (bool, bool) {
	id := c.Block(x, y, z, layer)
	airRID := w.conf.Blocks.AirRuntimeID()

	b, ok := w.conf.Blocks.BlockByRuntimeID(id)
	if !ok {
		w.conf.Log.Error("removeLiquidOnLayer: no block with runtime ID", "ID", id)
		return false, false
	}
	if _, ok := b.(Liquid); ok {
		c.SetBlock(x, y, z, layer, airRID)
		return true, true
	}
	return id == airRID, false
}

// additionalLiquid checks if the block at a position has additional liquid on
// another layer and returns the liquid if so.
func (w *World) additionalLiquid(pos cube.Pos) (Liquid, bool) {
	if pos.OutOfBounds(w.Range()) {
		// Fast way out.
		return nil, false
	}
	c := w.chunk(chunkPosFromBlockPos(pos))
	id := c.Block(uint8(pos[0]), int16(pos[1]), uint8(pos[2]), 1)

	b, ok := w.conf.Blocks.BlockByRuntimeID(id)
	if !ok {
		w.conf.Log.Error("additionalLiquid: no block with runtime ID", "ID", id)
		return nil, false
	}
	liq, ok := b.(Liquid)
	return liq, ok
}

// light returns the light level at the position passed. This is the highest of
// the sky and block light. The light value returned is a value in the range
// 0-15, where 0 means there is no light present, whereas 15 means the block is
// fully lit.
func (w *World) light(pos cube.Pos) uint8 {
	if pos[1] < w.ra[0] {
		// Fast way out.
		return 0
	}
	if pos[1] > w.ra[1] {
		// Above the rest of the world, so full skylight.
		return 15
	}
	return w.chunk(chunkPosFromBlockPos(pos)).Light(uint8(pos[0]), int16(pos[1]), uint8(pos[2]))
}

// skyLight returns the skylight level at the position passed. This light level
// is not influenced by blocks that emit light, such as torches. The light
// value, similarly to light, is a value in the range 0-15, where 0 means no
// light is present.
func (w *World) skyLight(pos cube.Pos) uint8 {
	if pos[1] < w.ra[0] {
		// Fast way out.
		return 0
	}
	if pos[1] > w.ra[1] {
		// Above the rest of the world, so full skylight.
		return 15
	}
	return w.chunk(chunkPosFromBlockPos(pos)).SkyLight(uint8(pos[0]), int16(pos[1]), uint8(pos[2]))
}

// Time returns the current time of the world. The time is incremented every
// 1/20th of a second, unless World.StopTime() is called.
func (w *World) Time() int {
	if w == nil {
		return 0
	}
	w.set.Lock()
	defer w.set.Unlock()
	return int(w.set.Time)
}

// SetTime sets the new time of the world. SetTime will always work, regardless
// of whether the time is stopped or not.
func (w *World) SetTime(new int) {
	if w == nil {
		return
	}
	w.set.Lock()
	w.set.Time = int64(new)
	w.set.Unlock()

	viewers, _ := w.allViewers()
	for _, viewer := range viewers {
		viewer.ViewTime(new)
	}
}

// StopTime stops the time in the world. When called, the time will no longer
// cycle and the world will remain at the time when StopTime is called. The
// time may be restarted by calling World.StartTime().
func (w *World) StopTime() {
	w.enableTimeCycle(false)
}

// StartTime restarts the time in the world. When called, the time will start
// cycling again and the day/night cycle will continue. The time may be stopped
// again by calling World.StopTime().
func (w *World) StartTime() {
	w.enableTimeCycle(true)
}

// TimeCycle returns whether time cycle is enabled.
func (w *World) TimeCycle() bool {
	if w == nil {
		return false
	}
	w.set.Lock()
	defer w.set.Unlock()
	return w.set.TimeCycle
}

// enableTimeCycle enables or disables the time cycling of the World.
func (w *World) enableTimeCycle(v bool) {
	if w == nil {
		return
	}
	w.set.Lock()
	defer w.set.Unlock()
	w.set.TimeCycle = v
	viewers, _ := w.allViewers()
	for _, viewer := range viewers {
		viewer.ViewTimeCycle(v)
	}
}

// temperature returns the temperature in the World at a specific position.
// Higher altitudes and different biomes influence the temperature returned.
func (w *World) temperature(pos cube.Pos) float64 {
	const (
		tempDrop = 1.0 / 600
		seaLevel = 64
	)
	diff := max(pos[1]-seaLevel, 0)
	return w.biome(pos).Temperature() - float64(diff)*tempDrop
}

// addParticle spawns a Particle at a given position in the World. Viewers that
// are viewing the chunk will be shown the particle.
func (w *World) addParticle(pos mgl64.Vec3, p Particle) {
	p.Spawn(w, pos)
	for _, viewer := range w.viewersOf(pos) {
		viewer.ViewParticle(pos, p)
	}
}

// playSound plays a sound at a specific position in the World. Viewers of that
// position will be able to hear the sound if they are close enough.
func (w *World) playSound(tx *Tx, pos mgl64.Vec3, s Sound) {
	ctx := event.C(tx)
	if w.Handler().HandleSound(ctx, s, pos); ctx.Cancelled() {
		return
	}
	s.Play(w, pos)
	for _, viewer := range w.viewersOf(pos) {
		viewer.ViewSound(pos, s)
	}
}

// addEntity adds an EntityHandle to a World. The Entity will be visible to all
// viewers of the World that have the chunk at the EntityHandle's position. If
// the chunk that the EntityHandle is in is not yet loaded, it will first be
// loaded. addEntity panics if the EntityHandle is already in a world.
// addEntity returns the Entity created by the EntityHandle.
func (w *World) addEntity(tx *Tx, handle *EntityHandle) Entity {
	handle.setAndUnlockWorld(w)
	pos := chunkPosFromVec3(handle.data.Pos)
	w.entities[handle] = pos

	c := w.chunk(pos)
	c.Entities, c.modified = append(c.Entities, handle), true

	e := handle.mustEntity(tx)
	for _, v := range c.viewers {
		// Show the entity to all viewers in the chunk of the entity.
		showEntity(e, v)
	}
	w.Handler().HandleEntitySpawn(tx, e)
	return e
}

// removeEntity removes an Entity from the World that is currently present in
// it. Any viewers of the Entity will no longer be able to see it.
// removeEntity returns the EntityHandle of the Entity. After removing an Entity
// from the World, the Entity is no longer usable.
func (w *World) removeEntity(e Entity, tx *Tx) *EntityHandle {
	handle := e.H()
	pos, found := w.entities[handle]
	if !found {
		// The entity currently isn't in this world.
		return nil
	}
	w.Handler().HandleEntityDespawn(tx, e)

	c := w.chunk(pos)
	c.Entities, c.modified = sliceutil.DeleteVal(c.Entities, handle), true

	w.removeEntityFromViewLayers(e)
	for _, v := range c.viewers {
		v.HideEntity(e)
	}
	delete(w.entities, handle)
	handle.unsetAndLockWorld()
	return handle
}

// removeEntityFromViewLayers removes stale overrides for despawned entities. Entities that own a ViewLayer,
// such as players, are skipped because they may be removed temporarily when respawning or changing worlds.
func (w *World) removeEntityFromViewLayers(e Entity) {
	if _, ok := e.(viewLayerViewer); ok {
		return
	}
	viewers, _ := w.allViewers()
	for _, viewer := range viewers {
		v, ok := viewer.(viewLayerViewer)
		if !ok || v.ViewLayer() == nil {
			continue
		}
		v.ViewLayer().remove(e)
	}
}

// entitiesWithin returns an iterator that yields all entities contained within
// the cube.BBox passed.
func (w *World) entitiesWithin(tx *Tx, box cube.BBox) iter.Seq[Entity] {
	return func(yield func(Entity) bool) {
		minPos, maxPos := chunkPosFromVec3(box.Min()), chunkPosFromVec3(box.Max())

		for x := minPos[0]; x <= maxPos[0]; x++ {
			for z := minPos[1]; z <= maxPos[1]; z++ {
				c, ok := w.chunks[ChunkPos{x, z}]
				if !ok {
					// The chunk wasn't loaded, so there are no entities here.
					continue
				}
				for _, handle := range slices.Clone(c.Entities) {
					if !box.Vec3Within(handle.data.Pos) {
						continue
					}
					ent, ok := handle.Entity(tx)
					if ok && !yield(ent) {
						return
					}
				}
			}
		}
	}
}

// allEntities returns an iterator that yields all entities in the World.
func (w *World) allEntities(tx *Tx) iter.Seq[Entity] {
	return func(yield func(Entity) bool) {
		for e := range w.entities {
			if ent := e.mustEntity(tx); !yield(ent) {
				return
			}
		}
	}
}

// allPlayers returns an iterator that yields all player entities in the World.
func (w *World) allPlayers(tx *Tx) iter.Seq[Entity] {
	return func(yield func(Entity) bool) {
		for e := range w.entities {
			if e.t.EncodeEntity() == "minecraft:player" {
				if ent := e.mustEntity(tx); !yield(ent) {
					return
				}
			}
		}
	}
}

// Spawn returns the spawn of the world. Every new player will by default spawn
// on this position in the world when joining.
func (w *World) Spawn() cube.Pos {
	if w == nil {
		return cube.Pos{}
	}

	if w.Dimension() == End {
		return cube.Pos{100, 50}
	} else if w.Dimension() == Nether {
		return cube.Pos{}
	}

	w.set.Lock()
	defer w.set.Unlock()
	return w.set.Spawn
}

// SetSpawn sets the spawn of the world to a different position. The player
// will be spawned in the centre of this position when newly joining.
func (w *World) SetSpawn(pos cube.Pos) {
	if w == nil {
		return
	}

	// nether has no spawn point and end spawn point is always 100 50 0.
	if w.Dimension() == Nether || w.Dimension() == End {
		return
	}

	w.set.Lock()
	w.set.Spawn = pos
	w.set.Unlock()

	viewers, _ := w.allViewers()
	for _, viewer := range viewers {
		viewer.ViewWorldSpawn(pos)
	}
}

// PlayerSpawn returns the spawn position of a player with a UUID in this World.
func (w *World) PlayerSpawn(id uuid.UUID) cube.Pos {
	if w == nil {
		return cube.Pos{}
	}
	pos, exist, err := w.conf.Provider.LoadPlayerSpawnPosition(id)
	if err != nil {
		w.conf.Log.Error("load player spawn: "+err.Error(), "ID", id)
		return w.Spawn()
	}
	if !exist {
		return w.Spawn()
	}
	return pos
}

// SetPlayerSpawn sets the spawn position of a player with a UUID in this
// World. If the player has a spawn in the world, the player will be teleported
// to this location on respawn.
func (w *World) SetPlayerSpawn(id uuid.UUID, pos cube.Pos) {
	if w == nil {
		return
	}
	if err := w.conf.Provider.SavePlayerSpawnPosition(id, pos); err != nil {
		w.conf.Log.Error("save player spawn: "+err.Error(), "ID", id)
	}
}

// SetRequiredSleepDuration sets the duration of time players in the world must sleep for, in order to advance to the
// next day.
func (w *World) SetRequiredSleepDuration(duration time.Duration) {
	if w == nil {
		return
	}
	w.set.Lock()
	defer w.set.Unlock()
	w.set.RequiredSleepTicks = duration.Milliseconds() / 50
}

// DefaultGameMode returns the default game mode of the world. When players
// join, they are given this game mode. The default game mode may be changed
// using SetDefaultGameMode().
func (w *World) DefaultGameMode() GameMode {
	if w == nil {
		return GameModeSurvival
	}
	w.set.Lock()
	defer w.set.Unlock()
	return w.set.DefaultGameMode
}

// SetTickRange sets the range in chunks around each Viewer that will have the
// chunks (their blocks and entities) ticked when the World is ticked.
func (w *World) SetTickRange(v int) {
	if w == nil {
		return
	}
	w.set.Lock()
	defer w.set.Unlock()
	w.set.TickRange = int32(v)
}

// tickRange returns the tick range around each Viewer.
func (w *World) tickRange() int {
	w.set.Lock()
	defer w.set.Unlock()
	return int(w.set.TickRange)
}

// SetDefaultGameMode changes the default game mode of the world. When players
// join, they are then given that game mode.
func (w *World) SetDefaultGameMode(mode GameMode) {
	if w == nil {
		return
	}
	w.set.Lock()
	defer w.set.Unlock()
	w.set.DefaultGameMode = mode
}

// Difficulty returns the difficulty of the world. Properties of mobs in the
// world and the player's hunger will depend on this difficulty.
func (w *World) Difficulty() Difficulty {
	if w == nil {
		return DifficultyNormal
	}
	w.set.Lock()
	defer w.set.Unlock()
	return w.set.Difficulty
}

// SetDifficulty changes the difficulty of a world.
func (w *World) SetDifficulty(d Difficulty) {
	if w == nil {
		return
	}
	w.set.Lock()
	defer w.set.Unlock()
	w.set.Difficulty = d
}

// scheduleBlockUpdate schedules a block update at the position passed for the
// block type passed after a specific delay. If the block at that position does
// not handle block updates, nothing will happen.
// Block updates are both block and position specific. A block update is only
// scheduled if no block update with the same position and block type is
// already scheduled at a later time than the newly scheduled update.
func (w *World) scheduleBlockUpdate(pos cube.Pos, b Block, delay time.Duration) {
	if pos.OutOfBounds(w.Range()) {
		return
	}
	w.scheduledUpdates.schedule(w.conf.Blocks, pos, b, delay)
}

// doBlockUpdatesAround schedules block updates directly around and on the
// position passed.
func (w *World) doBlockUpdatesAround(pos cube.Pos) {
	if w == nil || pos.OutOfBounds(w.Range()) {
		return
	}
	changed := pos

	w.updateNeighbour(pos, changed)
	pos.Neighbours(func(pos cube.Pos) {
		w.updateNeighbour(pos, changed)
	}, w.Range())
}

// neighbourUpdate represents a position that needs to be updated because of a
// neighbour that changed.
type neighbourUpdate struct {
	pos, neighbour cube.Pos
}

// updateNeighbour ticks the position passed as a result of the neighbour
// passed being updated.
func (w *World) updateNeighbour(pos, changedNeighbour cube.Pos) {
	w.neighbourUpdates = append(w.neighbourUpdates, neighbourUpdate{pos: pos, neighbour: changedNeighbour})
}

// Handle changes the current Handler of the world. As a result, events called
// by the world will call the methods of the Handler passed. Handle sets the
// world's Handler to NopHandler if nil is passed.
func (w *World) Handle(h Handler) {
	if w == nil {
		return
	}
	if h == nil {
		h = NopHandler{}
	}
	w.handler.Store(&h)
}

// viewersOf returns all viewers viewing the position passed.
func (w *World) viewersOf(pos mgl64.Vec3) []Viewer {
	c, ok := w.chunks[chunkPosFromVec3(pos)]
	if !ok {
		return nil
	}
	return c.viewers
}

// PortalDestination returns the destination World for a portal of a specific
// Dimension. If no destination World could be found, the current World is
// returned. Calling PortalDestination(Nether) on an Overworld World returns
// Nether, while calling PortalDestination(Nether) on a Nether World will
// return the Overworld, for instance.
func (w *World) PortalDestination(dim Dimension) *World {
	if w.conf.PortalDestination == nil {
		return w
	}
	if res := w.conf.PortalDestination(dim); res != nil {
		return res
	}
	return w
}

// Save saves the World to the provider.
func (w *World) Save() {
	<-w.Exec(w.save(w.saveChunk))
}

// save saves all loaded chunks to the World's provider.
func (w *World) save(f func(*Tx, ChunkPos, *Column)) ExecFunc {
	return func(tx *Tx) {
		if w.conf.ReadOnly {
			return
		}
		w.conf.Log.Debug("Saving chunks in memory to disk...")
		for pos, c := range w.chunks {
			f(tx, pos, c)
		}
		w.conf.Log.Debug("Updating level.dat values...")
		w.conf.Provider.SaveSettings(w.set)
	}
}

// saveChunk saves a chunk and its entities to disk after compacting the chunk.
func (w *World) saveChunk(_ *Tx, pos ChunkPos, c *Column) {
	if !w.conf.ReadOnly && c.modified {
		c.Compact()
		if err := w.conf.Provider.StoreColumn(pos, w.conf.Dim, w.columnTo(c, pos)); err != nil {
			w.conf.Log.Error("save chunk: "+err.Error(), "X", pos[0], "Z", pos[1])
		}
	}
}

// closeChunk saves a chunk and its entities to disk after compacting the chunk.
// Afterwards, scheduled updates from that chunk are removed and all entities
// in it are closed.
func (w *World) closeChunk(tx *Tx, pos ChunkPos, c *Column) {
	w.saveChunk(tx, pos, c)
	w.scheduledUpdates.removeChunk(pos)
	// Note: We close c.Entities here because some entities may remove
	// themselves from the world in their Close method, which can lead to
	// unexpected conditions.
	for _, e := range slices.Clone(c.Entities) {
		_ = e.mustEntity(tx).Close()
	}
	clear(c.Entities)
	delete(w.chunks, pos)
}

// Close closes the world and saves all chunks currently loaded.
func (w *World) Close() error {
	w.o.Do(w.close)
	return nil
}

// close stops the World from ticking, saves all chunks to the Provider and
// updates the world's settings.
func (w *World) close() {
	<-w.Exec(func(tx *Tx) {
		// Let user code run anything that needs to be finished before closing.
		w.Handler().HandleClose(tx)
		w.Handle(NopHandler{})

		w.save(w.closeChunk)(tx)
	})

	close(w.closing)
	w.running.Wait()

	close(w.queueClosing)
	w.queueing.Wait()

	if w.set.ref.Add(-1); !w.advance {
		return
	}
	w.conf.Log.Debug("Closing provider...")
	if err := w.conf.Provider.Close(); err != nil {
		w.conf.Log.Error("close world provider: " + err.Error())
	}
}

// allViewers returns all viewers and loaders, regardless of where in the world
// they are viewing.
func (w *World) allViewers() ([]Viewer, []*Loader) {
	w.viewerMu.Lock()
	defer w.viewerMu.Unlock()

	viewers, loaders := make([]Viewer, 0, len(w.viewers)), make([]*Loader, 0, len(w.viewers))
	for k, v := range w.viewers {
		viewers = append(viewers, v)
		loaders = append(loaders, k)
	}
	return viewers, loaders
}

// addWorldViewer adds a viewer to the world. Should only be used while the
// viewer isn't viewing any chunks.
func (w *World) addWorldViewer(l *Loader) {
	w.viewerMu.Lock()
	w.viewers[l] = l.viewer
	w.viewerMu.Unlock()

	l.viewer.ViewTime(w.Time())
	l.viewer.ViewTimeCycle(w.TimeCycle())
	w.set.Lock()
	raining, thundering := w.set.Raining, w.set.Raining && w.set.Thundering
	w.set.Unlock()
	l.viewer.ViewWeather(raining, thundering)
	l.viewer.ViewWorldSpawn(w.Spawn())
}

// addViewer adds a viewer to the World at a given position. Any events that
// happen in the chunk at that position, such as block and entity changes, will
// be sent to the viewer.
func (w *World) addViewer(tx *Tx, c *Column, loader *Loader) {
	c.viewers = append(c.viewers, loader.viewer)
	c.loaders = append(c.loaders, loader)

	for _, entity := range c.Entities {
		showEntity(entity.mustEntity(tx), loader.viewer)
	}
}

// removeViewer removes a viewer from a chunk position. All entities will be
// hidden from the viewer and no more calls will be made when events in the
// chunk happen.
func (w *World) removeViewer(tx *Tx, pos ChunkPos, loader *Loader) {
	if w == nil {
		return
	}
	c, ok := w.chunks[pos]
	if !ok {
		return
	}
	if i := slices.Index(c.loaders, loader); i != -1 {
		c.viewers = slices.Delete(c.viewers, i, i+1)
		c.loaders = slices.Delete(c.loaders, i, i+1)
	}

	// Hide all entities in the chunk from the viewer.
	for _, entity := range c.Entities {
		loader.viewer.HideEntity(entity.mustEntity(tx))
	}
}

// Handler returns the Handler of the world.
func (w *World) Handler() Handler {
	if w == nil {
		return NopHandler{}
	}
	return *w.handler.Load()
}

// showEntity shows an Entity to a viewer of the world. It makes sure
// everything of the Entity, including the items held, is shown.
func showEntity(e Entity, viewer Viewer) {
	viewer.ViewEntity(e)
	viewer.ViewEntityItems(e)
	viewer.ViewEntityArmour(e)
}

// chunk reads a chunk from the position passed. If a chunk at that position is
// not yet loaded, the chunk is loaded from the provider, or generated if it
// did not yet exist. Additionally, chunks newly loaded have the light in them
// calculated before they are returned.
func (w *World) chunk(pos ChunkPos) *Column {
	c, ok := w.chunks[pos]
	if ok {
		return c
	}
	c, err := w.loadChunk(pos)
	chunk.LightArea([]*chunk.Chunk{c.Chunk}, int(pos[0]), int(pos[1])).Fill()
	if err != nil {
		w.conf.Log.Error("load chunk: "+err.Error(), "X", pos[0], "Z", pos[1])
		return c
	}
	w.calculateLight(pos)
	return c
}

// loadChunk attempts to load a chunk from the provider, or generates a chunk
// if one doesn't currently exist.
func (w *World) loadChunk(pos ChunkPos) (*Column, error) {
	column, err := w.conf.Provider.LoadColumn(pos, w.conf.Dim)
	switch {
	case err == nil:
		col := w.columnFrom(column, pos)
		w.chunks[pos] = col
		for _, e := range col.Entities {
			w.entities[e] = pos
			e.w = w
		}
		return col, nil
	case errors.Is(err, leveldb.ErrNotFound):
		// The provider doesn't have a chunk saved at this position, so we generate a new one.
		col := newColumn(chunk.New(w.conf.Blocks, w.Range()))
		w.chunks[pos] = col

		w.conf.Generator.GenerateChunk(pos, col.Chunk)
		return col, nil
	default:
		return newColumn(chunk.New(w.conf.Blocks, w.Range())), err
	}
}

// calculateLight calculates the light in the chunk passed and spreads the
// light of any surrounding neighbours if they have all chunks loaded around it
// as a result of the one passed.
func (w *World) calculateLight(centre ChunkPos) {
	for x := int32(-1); x <= 1; x++ {
		for z := int32(-1); z <= 1; z++ {
			// For all the neighbours of this chunk, if they exist, check if all
			// neighbours of that chunk now exist because of this one.
			pos := ChunkPos{centre[0] + x, centre[1] + z}
			if _, ok := w.chunks[pos]; ok {
				// Attempt to spread the light of all neighbours into the
				// surrounding ones.
				w.spreadLight(pos)
			}
		}
	}
}

// spreadLight spreads the light from the chunk passed at the position passed
// to all neighbours if each of them is loaded.
func (w *World) spreadLight(pos ChunkPos) {
	c := make([]*chunk.Chunk, 0, 9)
	for z := int32(-1); z <= 1; z++ {
		for x := int32(-1); x <= 1; x++ {
			neighbour, ok := w.chunks[ChunkPos{pos[0] + x, pos[1] + z}]
			if !ok {
				// Not all surrounding chunks existed: Stop spreading light.
				return
			}
			c = append(c, neighbour.Chunk)
		}
	}
	// All chunks surrounding the current one are present, so we can spread.
	chunk.LightArea(c, int(pos[0])-1, int(pos[1])-1).Spread()
}

// autoSave runs until the world is running, saving and removing chunks that
// are no longer in use.
func (w *World) autoSave() {
	save := &time.Ticker{C: make(<-chan time.Time)}
	if w.conf.SaveInterval > 0 {
		save = time.NewTicker(w.conf.SaveInterval)
		defer save.Stop()
	}
	closeUnused := time.NewTicker(w.conf.ChunkUnloadInterval)
	defer closeUnused.Stop()

	for {
		select {
		case <-closeUnused.C:
			<-w.Exec(w.closeUnusedChunks)
		case <-save.C:
			w.Save()
		case <-w.closing:
			w.running.Done()
			return
		}
	}
}

// closeUnusedChunk closes all chunks currently not in use by any viewer.
func (w *World) closeUnusedChunks(tx *Tx) {
	for pos, c := range w.chunks {
		if len(c.viewers) == 0 {
			w.closeChunk(tx, pos, c)
		}
	}
}

// Column represents the data of a chunk including the (block) entities and
// viewers and loaders.
type Column struct {
	modified bool

	*chunk.Chunk
	Entities      []*EntityHandle
	BlockEntities map[cube.Pos]Block

	viewers []Viewer
	loaders []*Loader
}

// newColumn returns a new Column wrapper around the chunk.Chunk passed.
func newColumn(c *chunk.Chunk) *Column {
	return &Column{Chunk: c, BlockEntities: map[cube.Pos]Block{}}
}

// columnTo converts a Column to a chunk.Column so that it can be written to
// a provider.
func (w *World) columnTo(col *Column, pos ChunkPos) *chunk.Column {
	scheduled := w.scheduledUpdates.fromChunk(pos)
	c := &chunk.Column{
		Chunk:           col.Chunk,
		Entities:        make([]chunk.Entity, 0, len(col.Entities)),
		BlockEntities:   make([]chunk.BlockEntity, 0, len(col.BlockEntities)),
		ScheduledBlocks: make([]chunk.ScheduledBlockUpdate, 0, len(scheduled)),
		Tick:            w.scheduledUpdates.currentTick,
	}
	for _, e := range col.Entities {
		data := e.encodeNBT()
		maps.Copy(data, e.t.EncodeNBT(&e.data))
		data["identifier"] = e.t.EncodeEntity()
		c.Entities = append(c.Entities, chunk.Entity{ID: int64(binary.LittleEndian.Uint64(e.id[8:])), Data: data})
	}
	for pos, be := range col.BlockEntities {
		c.BlockEntities = append(c.BlockEntities, chunk.BlockEntity{Pos: pos, Data: be.(NBTer).EncodeNBT()})
	}
	for _, t := range scheduled {
		c.ScheduledBlocks = append(c.ScheduledBlocks, chunk.ScheduledBlockUpdate{Pos: t.pos, Block: w.conf.Blocks.BlockRuntimeID(t.b), Tick: t.t})
	}
	return c
}

// columnFrom converts a chunk.Column to a Column after reading it from a
// provider.
func (w *World) columnFrom(c *chunk.Column, _ ChunkPos) *Column {
	col := &Column{
		Chunk:         c.Chunk,
		Entities:      make([]*EntityHandle, 0, len(c.Entities)),
		BlockEntities: make(map[cube.Pos]Block, len(c.BlockEntities)),
	}
	for _, e := range c.Entities {
		eid, ok := e.Data["identifier"].(string)
		if !ok {
			w.conf.Log.Error("read column: entity without identifier field", "ID", e.ID)
			continue
		}
		t, ok := w.conf.Entities.Lookup(eid)
		if !ok {
			w.conf.Log.Error("read column: unknown entity type", "ID", e.ID, "type", eid)
			continue
		}
		col.Entities = append(col.Entities, entityFromData(t, e.ID, e.Data))
	}
	for _, be := range c.BlockEntities {
		rid := c.Chunk.Block(uint8(be.Pos[0]), int16(be.Pos[1]), uint8(be.Pos[2]), 0)
		b, ok := w.conf.Blocks.BlockByRuntimeID(rid)
		if !ok {
			w.conf.Log.Error("read column: no block with runtime ID", "ID", rid)
			continue
		}
		nb, ok := b.(NBTer)
		if !ok {
			w.conf.Log.Error("read column: block with nbt does not implement NBTer", "block", fmt.Sprintf("%#v", b))
			continue
		}
		col.BlockEntities[be.Pos] = nb.DecodeNBT(be.Data).(Block)
	}
	scheduled, savedTick := make([]scheduledTick, 0, len(c.ScheduledBlocks)), c.Tick
	for _, t := range c.ScheduledBlocks {
		bl := w.conf.Blocks.BlockByRuntimeIDOrAir(t.Block)
		scheduled = append(scheduled, scheduledTick{
			pos:   t.Pos,
			b:     bl,
			bhash: w.conf.Blocks.BlockHash(bl),
			t:     w.scheduledUpdates.currentTick + (t.Tick - savedTick),
		})
	}
	w.scheduledUpdates.add(scheduled)
	return col
}