## On-rail Minecart Movement

Minecart movement is handled in two distinct regimes: on a rail and off. The
off-rail movement is handled with minetest's builtin entity movement handling.
The on-rail movement is handled with a custom algorithm. This section details
the latter.

The data for on-rail minecart movement is stored entirely inside mod storage
and indexed by a hex-encoded 128-bit universally-unique identifier (uuid). Minecart
entities store this uuid and a sequence identifier. The code for handling this
storage is in [storage.lua](./storage.lua). This was done so that minecarts can
still move while no players are connected or when out of range of players. Inspiration
for this was the [Advanced Trains mod](http://advtrains.de/). This is a behavior difference
when compared to minecraft, as carts there will stop movement when out of range of
players.

Processing for minecart movement is as follows:
1. In a globalstep handler in [carts.lua](./carts.lua), determine which carts are
   moving.
2. Call `do_movement` in [movement.lua](./movement.lua) to update
   each cart's location and handle interactions with the environment.
   1. Each movement is broken up into one or more steps that are completely
      contained inside a block. This prevents carts from ever jumping from
      one rail to another over a gap or thru solid blocks because of server
      lag. Each step is processed with `do_movement_step`
   2. Each step uses physically accurate, timestep-independent physics
      to move the cart. Calculating the acceleration to apply to a cart
      is broken out into its own function (`calculate_acceperation`).
   3. As the cart enters and leaves blocks, handlers in nearby blocks are called
      to allow the cart to efficiently interact with the environment. Handled by
      the functions `handle_cart_enter` and `handle_cart_leave`
   4. The cart checks for nearby carts and collides elastically with these. The
      calculations for these collisions are in the function `handle_cart_collision`
   5. If the cart enters a new block, determine the new direction the cart will
      move with `mcl_minecarts:get_rail_direction` in [functions.lua](./functions.lua].
      The rail nodes provide a hook `_mcl_minecarts.get_next_direction` that
      provides this information based on the previous movement direction.
3. If an entity exists for a given cart, the entity will update its position
   while loaded in.

Cart movement when on a rail occurs regarless of whether an entity for that
cart exists or is loaded into memory. As a consequence of this movement, it
is possible for carts with unloaded entities to enter range of a player.
To handle this, periodic checks are performed around players and carts that
are within range but don't have a cart have a new entity spawned.

Every time a cart has a new entity spawned, it increases a sequence number in
the cart data to allow removing old entities from the minetest engine. Any cart
entity that does not have the current sequence number for a minecart gets removed
once processing for that entity resumes.