add bounds checking for ChunkAt and add TimeAt function to ChunkHeader

cmd/strom/extract_mca/main.go

@@ -0,0 +1,52 @@
+package extract_mca
+
+import (
+	"flag"
+	"fmt"
+	"log/slog"
+	"os"
+
+	"github.com/admin-else/strom/level/anvil"
+	"github.com/admin-else/strom/nbt"
+)
+
+var (
+	cmd        = flag.NewFlagSet("print-nbt", flag.ContinueOnError)
+	FileFlag   = cmd.String("file", "", "The MCA file to extract")
+	IndexXFlag = cmd.Int("x", 0, "The x chunk coordinate")
+	IndexZFlag = cmd.Int("z", 0, "The z chunk coordinate")
+	ToFlag     = cmd.String("to", "mca-chunk.nbt", "The output file")
+)
+
+func Run(args []string) (err error) {
+	err = cmd.Parse(args)
+	if err != nil {
+		return
+	}
+	if *FileFlag == "" {
+		return fmt.Errorf("no file specified")
+	}
+	f, err := os.Open(*FileFlag)
+	if err != nil {
+		return
+	}
+	defer f.Close()
+	mca, err := anvil.ReadChunkHeader(f)
+	if err != nil {
+		return
+	}
+	t, err := mca.TimeAt(f, int32(*IndexXFlag), int32(*IndexZFlag))
+	if err != nil {
+		return
+	}
+	slog.Info("Chunk last written", "time", t)
+	n, err := mca.ChunkAt(f, int32(*IndexXFlag), int32(*IndexZFlag))
+	if err != nil {
+		return
+	}
+	err = nbt.WriteUnstructuredFilePath(*ToFlag, n)
+	if err != nil {
+		return
+	}
+	return
+}

cmd/strom/main.go

@@ -10,6 +10,7 @@ import (
 	"slices"
 
 	"github.com/admin-else/strom/cmd/strom/api"
+	"github.com/admin-else/strom/cmd/strom/extract_mca"
 	"github.com/admin-else/strom/cmd/strom/offline_uuid"
 	"github.com/admin-else/strom/cmd/strom/packet_inspector"
 	"github.com/admin-else/strom/cmd/strom/print_nbt"
@@ -39,10 +40,13 @@ var subcommands = map[string]func(args []string) error{
 	"packet-spy":   packet_inspector.Run,
 	"api":          api.Run,
 	"version":      Version,
+	"extract-mca":  extract_mca.Run,
 }
 
 func Help(args []string) (err error) {
-	fmt.Println("Available subcommands", slices.Collect(maps.Keys(subcommands)))
+	subcommandNames := slices.Collect(maps.Keys(subcommands))
+	slices.Sort(subcommandNames)
+	fmt.Println("Available subcommands", subcommandNames)
 	return
 }