feat: Add EntityArgumentType for minecraft:entity commands (PR minekube#78)

- Add EntityArgumentType with SingleEntity and OnlyPlayers flags
- Implement EntityArgumentPropertyCodec for proper serialization/deserialization
- Update registry to use EntityArgumentType instead of simple byte codec
- Add PlayerArgument constant for easy player-only entity selection
- Add comprehensive documentation referencing wiki.vg/Command_Data
- Add test command demonstrating EntityArgumentType usage

This provides proper minecraft:entity argument type support, enabling better
command argument handling and auto-completion for player/entity selection.

Based on contribution by @gmax79 in PR minekube#78.
Co-authored-by: gmax79 <gmax79@users.noreply.github.com>

Author: robinbraemer

pkg/edition/java/proto/packet/brigadier/codecs.go

@@ -409,6 +409,31 @@ var (
 			}, nil
 		},
 	}
+
+	EntityArgumentPropertyCodec ArgumentPropertyCodec = &ArgumentPropertyCodecFuncs{
+		EncodeFn: func(wr io.Writer, v any, protocol proto.Protocol) error {
+			i, ok := v.(*EntityArgumentType)
+			if !ok {
+				return fmt.Errorf("expected *EntityArgumentType but got %T", v)
+			}
+			var b byte
+			if i.SingleEntity {
+				b = b | 0x1
+			}
+			if i.OnlyPlayers {
+				b = b | 0x2
+			}
+			return util.WriteByte(wr, b)
+		},
+		DecodeFn: func(rd io.Reader, protocol proto.Protocol) (any, error) {
+			b, err := util.ReadByte(rd)
+			if err != nil {
+				return nil, err
+			}
+
+			return &EntityArgumentType{SingleEntity: b&0x1 != 0, OnlyPlayers: b&0x2 != 0}, nil
+		},
+	}
 )
 
 const (

pkg/edition/java/proto/packet/brigadier/registry.go

@@ -181,7 +181,7 @@ func init() {
 	register(id("brigadier:string", mapSet(Minecraft_1_19, 5)), brigodier.String, StringArgumentPropertyCodec)
 
 	// Minecraft argument types
-	emptyWithCodec(id("minecraft:entity", mapSet(Minecraft_1_19, 6)), ByteArgumentPropertyCodec)
+	register(id("minecraft:entity", mapSet(Minecraft_1_19, 6)), PlayerArgument, EntityArgumentPropertyCodec)
 	empty(id("minecraft:game_profile", mapSet(Minecraft_1_19, 7)))
 	empty(id("minecraft:block_pos", mapSet(Minecraft_1_19, 8)))
 	empty(id("minecraft:column_pos", mapSet(Minecraft_1_19, 9)))

pkg/edition/java/proto/packet/brigadier/types.go

@@ -4,7 +4,10 @@ import (
 	"go.minekube.com/brigodier"
 )
 
-var RegistryKeyArgument brigodier.ArgumentType = &RegistryKeyArgumentType{}
+var (
+	RegistryKeyArgument brigodier.ArgumentType = &RegistryKeyArgumentType{}
+	PlayerArgument      brigodier.ArgumentType = &EntityArgumentType{SingleEntity: true, OnlyPlayers: true}
+)
 
 type RegistryKeyArgumentType struct {
 	Identifier string
@@ -55,3 +58,16 @@ func (r *ResourceSelectorArgumentType) Parse(rd *brigodier.StringReader) (any, e
 }
 
 func (r *ResourceSelectorArgumentType) String() string { return "resource_selector_argument" }
+
+// EntityArgumentType represents the minecraft:entity argument type.
+// See https://wiki.vg/Command_Data (minecraft:entity)
+// It provides auto-substitution of online player names in commands.
+type EntityArgumentType struct {
+	SingleEntity bool // Only select one entity
+	OnlyPlayers  bool // Only select players (not other entities)
+}
+
+func (t *EntityArgumentType) String() string { return "entity" }
+func (t *EntityArgumentType) Parse(rd *brigodier.StringReader) (any, error) {
+	return rd.ReadString()
+}

pkg/edition/java/proxy/builtin_commands.go

@@ -2,14 +2,17 @@ package proxy
 
 import (
 	"go.minekube.com/brigodier"
+	. "go.minekube.com/common/minecraft/component"
 	"go.minekube.com/gate/pkg/command"
+	"go.minekube.com/gate/pkg/edition/java/proto/packet/brigadier"
 )
 
 func (p *Proxy) registerBuiltinCommands() []string {
 	return []string{
 		p.command.Register(newServerCmd(p)).Name(),
 		p.command.Register(newGlistCmd(p)).Name(),
 		p.command.Register(newSendCmd(p)).Name(),
+		p.command.Register(newTestEntityCmd(p)).Name(),
 	}
 }
 
@@ -18,3 +21,14 @@ func hasCmdPerm(proxy *Proxy, perm string) brigodier.RequireFn {
 		return !proxy.cfg.RequireBuiltinCommandPermissions || c.Source.HasPermission(perm)
 	})
 }
+
+func newTestEntityCmd(proxy *Proxy) brigodier.LiteralNodeBuilder {
+	return brigodier.Literal("testentity").
+		Then(brigodier.Argument("player", brigadier.PlayerArgument).
+			Executes(command.Command(func(c *command.Context) error {
+				playerName := c.String("player")
+				return c.SendMessage(&Text{
+					Content: "EntityArgumentType: You specified player '" + playerName + "' - this uses proper minecraft:entity argument type!",
+				})
+			})))
+}