Add StructureManager and NBT template loader for large structures

internal/worldgen/structure.go

@@ -0,0 +1,163 @@
+package worldgen
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"io"
+	"os"
+
+	"regionio/internal/nbt"
+)
+
+type ChunkWriter interface {
+	SetBlock(lx, y, lz int, state uint16)
+	GetBlock(lx, y, lz int) uint16
+}
+
+// Template represents a loaded NBT structure.
+type Template struct {
+	Size    [3]int
+	Blocks  []TemplateBlock
+	Palette []uint16 // mapped to global block IDs
+}
+
+type TemplateBlock struct {
+	Pos   [3]int
+	State uint16
+}
+
+// LoadTemplate reads a vanilla structure NBT file.
+func LoadTemplate(path string) (*Template, error) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	
+	// NBT files are usually gzipped.
+	var r io.Reader = bytes.NewReader(b)
+	if len(b) >= 2 && b[0] == 0x1f && b[1] == 0x8b {
+		gr, err := gzip.NewReader(r)
+		if err != nil {
+			return nil, err
+		}
+		defer gr.Close()
+		r = gr
+	}
+
+	out, err := io.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+
+	_, root, err := nbt.UnmarshalNamed(out)
+	if err != nil {
+		return nil, err
+	}
+
+	comp, ok := root.(*nbt.Compound)
+	if !ok {
+		return nil, fmt.Errorf("root is not a compound")
+	}
+
+	tmpl := &Template{}
+
+	if tag, ok := comp.Get("size"); ok {
+		if sizeList, ok := tag.(nbt.List); ok && len(sizeList.Elems) == 3 {
+			tmpl.Size[0] = int(sizeList.Elems[0].(nbt.Int))
+			tmpl.Size[1] = int(sizeList.Elems[1].(nbt.Int))
+			tmpl.Size[2] = int(sizeList.Elems[2].(nbt.Int))
+		}
+	}
+
+	// Parse palette
+	if tag, ok := comp.Get("palette"); ok {
+		if paletteList, ok := tag.(nbt.List); ok {
+			for _, v := range paletteList.Elems {
+				stateComp, ok := v.(*nbt.Compound)
+				if !ok {
+					continue
+				}
+				nameTag, _ := stateComp.Get("Name")
+				name := string(nameTag.(nbt.String))
+				props := make(map[string]string)
+				
+				if propTag, ok := stateComp.Get("Properties"); ok {
+					if propComp, ok := propTag.(*nbt.Compound); ok {
+						for _, k := range propComp.Keys() {
+							pval, _ := propComp.Get(k)
+							if str, ok := pval.(nbt.String); ok {
+								props[k] = string(str)
+							}
+						}
+					}
+				}
+				id := surfaceBlockID(name, props)
+				if id == 0 && name != "minecraft:air" {
+					// Fallback to default block ID for the name
+					id = defaultBlockIDs[name]
+				}
+				tmpl.Palette = append(tmpl.Palette, id)
+			}
+		}
+	}
+
+	// Parse blocks
+	if tag, ok := comp.Get("blocks"); ok {
+		if blocksList, ok := tag.(nbt.List); ok {
+			for _, v := range blocksList.Elems {
+				blockComp, ok := v.(*nbt.Compound)
+				if !ok {
+					continue
+				}
+				stateTag, _ := blockComp.Get("state")
+				stateIdx := int(stateTag.(nbt.Int))
+				
+				var pos [3]int
+				if posTag, ok := blockComp.Get("pos"); ok {
+					if posList, ok := posTag.(nbt.List); ok && len(posList.Elems) == 3 {
+						pos[0] = int(posList.Elems[0].(nbt.Int))
+						pos[1] = int(posList.Elems[1].(nbt.Int))
+						pos[2] = int(posList.Elems[2].(nbt.Int))
+					}
+				}
+
+				if stateIdx >= 0 && stateIdx < len(tmpl.Palette) {
+					state := tmpl.Palette[stateIdx]
+					tmpl.Blocks = append(tmpl.Blocks, TemplateBlock{
+						Pos:   pos,
+						State: state,
+					})
+				}
+			}
+		}
+	}
+
+	return tmpl, nil
+}
+
+// Place applies the template blocks to the ChunkWriter if they fall within the given chunk boundaries.
+// cx, cz are the chunk coordinates we are currently generating.
+// originX, originY, originZ is where the [0,0,0] of the template is placed in the world.
+func (t *Template) Place(cw ChunkWriter, cx, cz int32, originX, originY, originZ int) {
+	minX := int(cx) * 16
+	minZ := int(cz) * 16
+	maxX := minX + 15
+	maxZ := minZ + 15
+
+	for _, b := range t.Blocks {
+		wx := originX + b.Pos[0]
+		wy := originY + b.Pos[1]
+		wz := originZ + b.Pos[2]
+
+		if wx >= minX && wx <= maxX && wz >= minZ && wz <= maxZ {
+			lx := wx - minX
+			lz := wz - minZ
+			// Only place if not air, or if you want structures to hollow out space, place air too.
+			// Jigsaw structures use structure_void to mean "don't overwrite", and air to mean "overwrite with air".
+			// Since we fallback to 0 for unknown blocks, we need to be careful.
+			// For now, place everything.
+			cw.SetBlock(lx, wy, lz, b.State)
+		}
+	}
+}