Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

internal/nbt/mutf8.go

@@ -0,0 +1,59 @@
+package nbt
+
+import (
+	"errors"
+	"unicode/utf16"
+)
+
+// errBadMUTF8 is returned when a modified-UTF-8 byte sequence is malformed.
+var errBadMUTF8 = errors.New("nbt: invalid modified UTF-8")
+
+// encodeModifiedUTF8 encodes s as Java modified UTF-8: ASCII stays one byte,
+// NUL and code points up to U+07FF take two bytes, U+0800..U+FFFF take three,
+// and supplementary code points are split into a surrogate pair (six bytes).
+func encodeModifiedUTF8(dst []byte, s string) []byte {
+	for _, u := range utf16.Encode([]rune(s)) {
+		switch {
+		case u >= 0x0001 && u <= 0x007F:
+			dst = append(dst, byte(u))
+		case u == 0 || u <= 0x07FF:
+			dst = append(dst,
+				0xC0|byte(u>>6),
+				0x80|byte(u&0x3F))
+		default:
+			dst = append(dst,
+				0xE0|byte(u>>12),
+				0x80|byte((u>>6)&0x3F),
+				0x80|byte(u&0x3F))
+		}
+	}
+	return dst
+}
+
+// decodeModifiedUTF8 decodes b (length-delimited modified UTF-8) into a string.
+func decodeModifiedUTF8(b []byte) (string, error) {
+	units := make([]uint16, 0, len(b))
+	for i := 0; i < len(b); {
+		c := b[i]
+		switch {
+		case c&0x80 == 0: // 0xxxxxxx
+			units = append(units, uint16(c))
+			i++
+		case c&0xE0 == 0xC0: // 110xxxxx 10xxxxxx
+			if i+1 >= len(b) || b[i+1]&0xC0 != 0x80 {
+				return "", errBadMUTF8
+			}
+			units = append(units, uint16(c&0x1F)<<6|uint16(b[i+1]&0x3F))
+			i += 2
+		case c&0xF0 == 0xE0: // 1110xxxx 10xxxxxx 10xxxxxx
+			if i+2 >= len(b) || b[i+1]&0xC0 != 0x80 || b[i+2]&0xC0 != 0x80 {
+				return "", errBadMUTF8
+			}
+			units = append(units, uint16(c&0x0F)<<12|uint16(b[i+1]&0x3F)<<6|uint16(b[i+2]&0x3F))
+			i += 3
+		default:
+			return "", errBadMUTF8
+		}
+	}
+	return string(utf16.Decode(units)), nil
+}