# Aquifers

Minecraft uses *Aquifers* to determine where to place liquids and air. Without aquifers all empty space below the sea-level
would be filled with water. Aquifers thus allow caves to be dry.

## Global Fluid Picker
The **global fluid picker** is used when aquifers are disabled and in some cases with aquifers enabled.

The global fluid picker is quite simple: 
- **air** above the sea-level
- the default liquid between the sea-level and `y=-54`
- **lava** below `y=-54`

If the sea-level is below `y=-54`:
- **air** above the sea-level
- **lava** below the sea-level

## Preliminary Surface Height
The **preliminary surface height** the easiest way for the game to calculate the height of the terrain. It is calculated based on the `inital_density_without_jaggedness` density function.
Checking from top to bottom, the first height where `inital_density_without_jaggedness > 0.390625` is the preliminary surface height.

In vanilla, this height is usually about 8 blocks below the real surface, thus most calculations add 8 to the preliminary surface height.

Thus, **for brevity, the following will use *surface* to refer to "8 blocks above the *preliminary surface height*"**.

## Floodedness

### States
For each position in the world, there is one of 4 aquifer *floodedness* states:
- **Disabled**: In this case the *global fluid picker* is used. (meaning flooded caves and oceans, no water above sea-level)
- **Flooded**: In this case, the fluid is placed up to the sea-level and air above. (similar to disabled, but with fluid selection)
- **Empty**: In this case always air is placed.
- **Randomized**: In this case, the fluid level is determined randomly. See below. (This case is what most people would usually associate with aquifers)

### Determining Floodedness
The following will describe how the floodedness is determined:

- **Disabled**, if the global fluid picker picks lava.
- **Disabled**, if the position is at least 12 blocks above the surface.
- **Disabled**, if the position is at most 12 blocks below the surface (or above) **and** there is a position nearby\* where the surface is below sea-level.
- **Empty** if `erosion < -0.22499999403953552` **and** `depth > 0.8999999761581421` (i.e. deep dark region, but hardcoded)
- Otherwise, the floodedness is determined based on the value of the `fluid_level_floodedness` density function:
  - **Flooded** if `fluid_level_floodedness > min_disabled`(usally 0.8)
  - **Empty** if `fluid_level_floodedness <= max_empty`(usually 0.4)
  - **Randomized** otherwise.

\* for nearby positions the check happens offset in chunks: `{0, 0}, {-2, -1}, {-1, -1}, {0, -1}, {1, -1}, {-3, 0}, {-2, 0}, {-1, 0}, {1, 0}, {-2, 1}, {-1, 1}, {0, 1}, {1, 1}`
(yes, this is not symmetric, don't ask me why)

### Cut of values
`min_disabled` and `max_empty` can differ from the usual values **if the surface is below the sea-level**:

- `min_disabled` varies linearly from `-0.3` at the surface to `0.8` 64 blocks below the surface.
- `max_empty` varies linearly from `-0.8` at the surface to `0.4` 64  blocks below the surface.

## Randomized fluid level
If the floodedness is determined to be *Randomized*, then the fluid level is determined using the `fluid_level_spread` density function.

For this, the world is split into 16x40x16 block cells. Each cell gets a uniform fluid level.
The fluid level is calculated as `center_height + fluid_level_spread * 10` where `center_height` is the height 20 blocks above the floor of the cell.
The fluid level is then rounded down to the next multiple of 3.

**Caution: the `fluid_level_spread` density function is sampled at 1/16, 1/40, 1/16 scale.**

## Fluid selection
If the *fluid level* at the current position is above `-10`, the default fluid is always used.

Otherwise, the fluid selection is based on the `lava` density function. For this, the world is split into 64x40x64 cells (i.e. 4x1x4 fluid level cells) with each cell given a single fluid.

The fluid is the default fluid if `abs(lava) <= 0.3` and **lava** otherwise.

**Caution: the `lava` density function is sampled at 1/64, 1/40, 1/64 scale.**

## Barriers
TODO \[Somehow based on the `barrier` density function\]