Chunk

A chunk is a 16x128x16 volume of blocks. Usually these are compressed with the deflate algorithm.

NBT

Inside of an uncompressed chunk file are only a few NBT Tags

Field	Type	Description
Entities	List	All entities in this chunk
TileEntities	List	All tile entities in this chunk, such as Mob Spawners, Chests or Signs
LastUpdate	Long	The last tick value for when the chunk was updated
TerrainPopulated	Byte	If the chunk is populated with foliage
xPos	Integer	The Chunk-Space X Coordinate of the Chunk
zPos	Integer	The Chunk-Space Z Coordinate of the Chunk
Blocks	Byte Array	The block ID values of the chunk
Data	Byte Array	The block data values of the chunk
BlockLight	Byte Array	The block light values of the chunk
SkyLight	Byte Array	The sky light values of the chunk
HeightMap	Byte Array	A top-down heightmap of the chunk

Network

Over the network, chunks are a lot simpler, as they only carry block information. The number of blocks that're sent is determined by the chunk packet size information, usually (15,127,15) which becomes a 16x128x16 area. See more on how this data is sent on the chunk packet page.

Field	Unit	Description
Blocks	Byte	The block ID values of the chunk
Data	Nibble	The block data values of the chunk
BlockLight	Nibble	The block light values of the chunk
SkyLight	Nibble	The sky light values of the chunk

Bytes VS Nibbles

All non-block data is sent and stored as nibbles, meaning that to read a blocks metadata value, for example, one must read the top or bottom 4-bits of the relevant Byte. As such, the Data, BlockLight and SkyLight sections are half as big as the blocks section.

Reading

unsigned char lower = GetBlockLight(x, y  , z);
unsigned char upper = GetBlockLight(x, y+1, z);
data[index] = (upper << 4 | lower);

Writing

unsigned char lower = data[index] & 0xF
unsigned char upper = data[index] >> 4

Block ordering

Blocks are stored as vertical columns (Y-Axis). To iterate over a stored Byte Array from a Chunk the ordering is as follows.

for (int x = 0; x < 16; x++) {
    for (int z = 0; z < 16; z++) {
        for (int y = 0; y < 128; y++) {
            // Y is in Z which is in X
            block[y + (z*128) + (x*128*16)];
        }
    }
}