Alpha Map Format

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Alpha worlds consist of several files, all held together in one folder under a very rigid directory structure.


The level is a file called "level.dat" in the root of the world.

Name Type Description
Data Compound General level data
Name Type Description
RandomSeed Long The seed of the world, which is used to generate new geometry.
SpawnX Int The X coordinate of the default spawn point
SpawnY Int The Y coordinate of the default spawn point
SpawnZ Int The Z coordinate of the default spawn point


Chunks are stored in folders inside the world corresponding to their coordinates in base-36. The path is "<base-36-x-63>/<base-36-z-63>/c.<base-36-x>.<base-36-z>.dat", where <base-36-x-63> is the base-36 representation of the X coordinate of the chunk AND'd bitwise with 63.

For example, if we have the chunk coordinate (39, -13), we end up with the path 13/1f/c.13.-d.dat

The main world is stored in the root directory of the world. The Nether is stored in the "DIM-1" folder in the root and uses the same chunk folder system.

The Base-36 Algorithm

To retrieve the base-36 representation of a number, use the digits "0123456789abcdefghijklmnopqrstuvwxyz", and prefix the result with a hyphen ("-") if the number is negative.

Example code in Python, from Beta:

def base36(i):
    Return the string representation of i in base 36, using lowercase letters.
    letters = "0123456789abcdefghijklmnopqrstuvwxyz"
    if i < 0:
        i = -i
        signed = True
    elif i == 0:
        return "0"
        signed = False
    s = ""
    while i:
        i, digit = divmod(i, 36)
        s = letters[digit] + s
    if signed:
        s = "-" + s
    return s

Example code in C#:

public static string Base36Encode(long input)
	if (input == 0){ return "0"; }
	string chars = "0123456789abcdefghijklmnopqrstuvwxyz";
	bool negative = (input < 0);

	StringBuilder sb = new StringBuilder();

	if (negative) {
		input = -input;
	while (input > 0) {
		sb.Insert((negative ? 1 : 0), chars[(int)(input % 36)]);
		input /= 36;
	return sb.ToString();


Persistent data for players is stored in a folder called "players". Each player has a file called "<username>.dat".

Beta player files

Player files are NBT encoded.

The file starts with a nameless TAG_Compound which contains all other data.

Tag Name Type Description
SleepTimer TAG_Short
Motion TAG_List(3 TAG_Double)
OnGround TAG_Byte
HurtTime TAG_Short
Health TAG_Short
Dimension TAG_Int
Air TAG_Short
Inventory TAG_List or TAG_Byte(when empty) See Inventory table below
Pos TAG_List(3 TAG_Double)
AttackTime TAG_Short
Sleeping TAG_Byte
Fire TAG_Short
FallDistance TAG_Float
Rotation TAG_List(2 TAG_Float)
DeathTime TAG_Short
SpawnX TAG_Int Only present when spawn is set elsewhere
SpawnY TAG_Int Only present when spawn is set elsewhere
SpawnZ TAG_Int Only present when spawn is set elsewhere

Inventory items are in the TAG_Compound format with the following tags:

Tag Name Type Description
Slot TAG_byte
id TAG_Short
Damage TAG_Short
Count TAG_Byte

Other formats

The following format is not used in Beta. May be the Alpha format.

Name Type Description
Pos List of 3 doubles X, Y, and Z coordinates of the player position, in pixel coordinates
Rotation List of 2 doubles Yaw and pitch of the player, in degrees

Map Generation

There are several approaches to map generation.


Two-dimensional noise functions can be used to provide a terrain height at certain coordinates.