half-earth/gdd.md

# Half-Earth Citybuilder Prototype

## TODO

* Zoning demand
* Journey Pathfinding
* Resident health
* Education
* Supply chains

## Interaction Modes

Interaction modes can be changed via keyboard shortcut.

### Selection Mode

This is the default interaction mode.
It can be accessed at any time by pressing the `esc` key.

### Build Mode

Build mode can be entered by pressing the `b` key.

## Critical Path

### Grid

Add a 10x10 grid.
Tiles are highlighted a different colour when the player hovers over them with the cursor.
Tiles can be interacted with, which turns them another colour.

### Diffusion

Tiles in the grid contain floating point values.
These values propogate to neighbouring cells a tick happens.
Ticks can be applied manually or they can be applied over time.
Add a tick counter as a UI element.

* [ ] understand different options for representing the rate of diffusion

Diffusing should use 'pull' operations; tiles should pull values from surrounding tiles.
This is an attempt to think forwards to parallelising computation of diffusiong - ideally using the GPU.

#### Diffusion Algorithm

Diffusion is modelled as two-dimensional steady-state heat conduction.
This means that the amount of a value does not change between the start and end of the computation of the diffusion step.
The value at one 2D location is calculated based on the values of itself and its neighbours in the previous step.
A diffusion coefficient controls the rate of diffusion.

[Video explanation](https://www.youtube.com/watch?v=RGbV7T_iWT8)
[Derivation](https://www.tec-science.com/thermodynamics/heat/heat-equation-diffusion-equation/)

Implementation:

```cs
private float TransferHeat(float t0, float alpha, float nx, float ny, float px, float py)
{
    float d2tdx2 = nx - 2 * t0 + px;
    float d2tdy2 = ny - 2 * t0 + py;
    return t0 + alpha * (d2tdx2 + d2tdy2);
}
```

### Tile Types

Tiles start as wild tiles.
The player can use a UI element or a keyboard shortcut to enable build mode.
In build mode, the player can click on a wile tile to turn it into a developed tile.

* Wild tiles are green
* Developed tiles are grey

### Heat

Developed tiles produce heat while wild tiles absorb it.
Every tile has a floating-point heat value which diffuses to its neighbours.
Add a heat map view which can be toggled or cycled to.
It is possible to cycle the view using a UI element.

### Tile Health

Add another value to wild tiles: health.
A wild tile can accept a certain amount of heat each tick from surrounding tiles.
If it receives more heat than threshold it takes damage based on the surplus.

A wild tile should be able to safely absorb heat from 3 surrounding developed cells, but take damage from 4.
A wild tile's health impacts its ability to absorb heat.
The amount of heat input required to damage a wild tile is known as its damage threshold.

    damage taken = max((heat received - damage threshold), 0)

Its effectiveness at absorbing heat scales with its remaining health.

    heat absorbed = health * base absorbtion rate

Diffusion still occurs to move heat away from depleted wild tiles.

### Rewilding

Add a rewilding mode option to the UI.
In rewilding mode, the player can click on a developed tile to turn in into a wild tile.
The wild tile starts with 0 health.

### Wild Recovery

Depleted wile tiles can recover health each tick.
The recovery rate is based on the difference between the damage threshold and the actual amount they received.
The recovery rate is increased if the tile has wild neighbours.

    neighbour multiplier = lerp(1, max, number of neighbours / 4)
    health restored = max((damage threshold - heat received), 0) * neighbour multiplier.

### Developed tile types

Split developed tiles into 3 types:

* Residential
* Commercial
* Industrial

When placing developed tiles, the player can select which type they want to place.
These types generate different amounts of heat:

    industrial > commercial > residential

### Journeys

Developed tiles have a chance to generate journeys between them.
A journey instances a vehicle which traverses the map at a constant speed from its origin to its destination tile.
A vehicle adds a constant amount of heat to every tile it traverses.

### Moving In

Residential tiles have two new integer resources, current residents and maximum residents.
Every tick there is a chance for residents to move in to a residential tile if the current number is less than the maximum number of residents.
The initial number of maximm residents per residential tile is 4.

### Jobs

Industrial and commercial tiles have two new integer resources, available jobs and occupied jobs.
The chance for residents to move in depends on there being being fewer occupied than available jobs.
Industrial and commercial tiles can only generate journeys if they have at least 1 occupied job.
Their chance of generating a journey is proportional to

    occupied jobs / available jobs .

### Commutes

A resident moves in to a residential tile and gets a job at an industrial or commercial 'job' tile.
The residential tile regularly generates journeys to the job tile, and the job tile regularly generates journeys in the other direction.

### Comfortable Living Temperatures

If a residential tile is too hot, people will not want to live there.
Residential tiles have a critical temperature at which the chance of someone moving into a tile becomes negative, becoming a chance that someone moves out.
This should not be implemented as a multiplication of other factors, but should rather contribute to an additive modifier to the move-in chance.
This is to avoid the situation where an extremely attractive locations becomes extremely unattractive due to a slight over-temperature multiplying a large number.

When someone moves out, one of the jobs associated with that residential tile is removed.

# Not critical

### Wealth

Journeys generate a new integer tile resource, wealth.
When a journey is completed, the destination tile accrues 1 wealth.
Wealth is lost if a tile is rewilded; only developed tiles have wealth.