SPECIFICATION.md

Named Binary Tag specification

[Original specification used to be here but has been removed. The version below also covers TAG_IntArray which the original didn't cover as it was added subsequently.]

NBT (Named Binary Tag) is a tag based binary format designed to carry large amounts of binary data with smaller amounts of additional data.

A NBT file consists of a single GZIPped Named Tag of type TAG_Compound.

A Named Tag has the following format:

byte tagType
TAG_String name
[payload]

The tagType is a single byte defining the contents of the payload of the tag.

The name is a descriptive name, and can be anything (eg "cat", "banana", "Hello World!"). It has nothing to do with the tagType.

The purpose for this name is to name tags so parsing is easier and can be made to only look for certain recognized tag names.

Exception: If tagType is TAG_End, the name is skipped and assumed to be "".

The [payload] varies by tagType.

Note that ONLY Named Tags carry the name and tagType data. Explicitly identified Tags (such as TAG_String above) only contains the payload.

The tag types and respective payloads are:

Type	Name	Payload	Notes
0	`TAG_End`	*None*	This tag is used to mark the end of a list. Cannot be named! If type 0 appears where a Named Tag is expected, the name is assumed to be "". (In other words, this Tag is always just a single 0 byte when named, and nothing in all other cases)
1	TAG_Byte	A single signed byte (8 bits)
2	TAG_Short	A signed short (16 bits, big endian)
3	TAG_Int	A signed short (32 bits, big endian)
4	TAG_Long	A signed long (64 bits, big endian)
5	TAG_Float	A floating point value (32 bits, big endian, IEEE 754-2008, binary32)
6	TAG_Double	A floating point value (64 bits, big endian, IEEE 754-2008, binary64)
7	TAG_Byte_Array	TAG_Int length An array of bytes of unspecified format. The length of this array is length bytes
8	TAG_String	TAG_Short length An array of bytes defining a string in UTF-8 format. The length of this array is length bytes
9	TAG_List	TAG_Byte tagId TAG_Int length A sequential list of Tags (not Named Tags), of type typeId. The length of this array is length Tags	All tags share the same type.
10	TAG_Compound	A sequential list of Named Tags. This array keeps going until a TAG_End is found. TAG_End end	If there's a nested TAG_Compound within this tag, that one will also have a TAG_End, so simply reading until the next TAG_End will not work. The names of the named tags have to be unique within each TAG_Compound The order of the tags is not guaranteed.
11	TAG_Int_Array	TAG_Int length An array of ints of unspecified format. The length of this array is length bytes

Decoding example

(Use http://www.minecraft.net/docs/test.nbt to test your implementation)

First we start by reading a Named Tag. After unzipping the stream, the first byte is a 10. That means the tag is a TAG_Compound (as expected by the specification).

The next two bytes are 0 and 11, meaning the name string consists of 11 UTF-8 characters. In this case, they happen to be "hello world". That means our root tag is named "hello world". We can now move on to the payload.

From the specification, we see that TAG_Compound consists of a series of Named Tags, so we read another byte to find the tagType. It happens to be an 8. The name is 4 letters long, and happens to be "name". Type 8 is TAG_String, meaning we read another two bytes to get the length, then read that many bytes to get the contents. In this case, it's "Bananrama".

So now we know the TAG_Compound contains a TAG_String named "name" with the content "Bananrama".

We move on to reading the next Named Tag, and get a 0. This is TAG_End, which always has an implied name of "". That means that the list of entries in the TAG_Compound is over, and indeed all of the NBT file.

So we ended up with this:

TAG_Compound("hello world"): 1 entries
{
   TAG_String("name"): Bananrama
}

For a slightly longer test, download http://www.minecraft.net/docs/bigtest.nbt You should end up with this:

TAG_Compound("Level"): 11 entries
{
   TAG_Short("shortTest"): 32767
   TAG_Long("longTest"): 9223372036854775807
   TAG_Float("floatTest"): 0.49823147
   TAG_String("stringTest"): HELLO WORLD THIS IS A TEST STRING ÅÄÖ!
   TAG_Int("intTest"): 2147483647
   TAG_Compound("nested compound test"): 2 entries
   {
      TAG_Compound("ham"): 2 entries
      {
         TAG_String("name"): Hampus
         TAG_Float("value"): 0.75
      }
      TAG_Compound("egg"): 2 entries
      {
         TAG_String("name"): Eggbert
         TAG_Float("value"): 0.5
      }
   }
   TAG_List("listTest (long)"): 5 entries of type TAG_Long
   {
      TAG_Long: 11
      TAG_Long: 12
      TAG_Long: 13
      TAG_Long: 14
      TAG_Long: 15
   }
   TAG_Byte("byteTest"): 127
   TAG_List("listTest (compound)"): 2 entries of type TAG_Compound
   {
      TAG_Compound: 2 entries
      {
         TAG_String("name"): Compound tag #0
         TAG_Long("created-on"): 1264099775885
      }
      TAG_Compound: 2 entries
      {
         TAG_String("name"): Compound tag #1
         TAG_Long("created-on"): 1264099775885
      }
   }
   TAG_Byte_Array("byteArrayTest (the first 1000 values of (n*n*255+n*7)%100, starting with n=0 (0, 62, 34, 16, 8, ...))"): [1000 bytes]
   TAG_Double("doubleTest"): 0.4931287132182315
}