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ABMs: Improve Introduction

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_en/map/abms.md
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@ -9,22 +9,29 @@ redirect_from: /en/chapters/abms.html
## Introduction
An **A**ctive **B**lock **M**odifier (**ABM**) allows you to run code on
certain nodes at specific intervals.
An Active Block Modifier (ABM) is a method of periodically running a
function on nodes matching a criteria.
As the name implies, this only works on loaded MapBlocks.
Please be warned, ABMs which are too frequent or act on many nodes
cause massive amounts of lag. Use them sparingly.
ABMs are best suited for nodes which are frequently found in the world,
such as grass.
ABMs have a high CPU overhead, as Minetest needs to scan all Active Blocks
to find matching nodes, but they have a low memory and storage overhead.
* [Example: Growing Alien Grass](#example-growing-alien-grass)
For nodes which are uncommon or already use metadata, such as furnaces
and machines, node timers should be used instead.
Node timers do not require scanning a MapBlock, but they do require slightly
more memory and storage to keep track of running timers.
* [Registering an ABM](#registering-an-abm)
* [Your Turn](#your-turn)
## Example: Growing Alien Grass
## Registering an ABM
Alien grass, for the purposes of this chapter, is a type of grass which
has a chance to appear near water.
To create it, you first need to register the grass, then
write an ABM.
```lua
minetest.register_node("aliens:grass", {
@ -48,21 +55,23 @@ minetest.register_abm({
})
```
This ABM runs every ten seconds. There is a 1 in 5 chance of the ABM running on each
node that has the correct name and the correct neighbours. If the ABM runs on a
node, an alien grass node is placed above it. Please be warned, this will delete any
node previously located in that position. To prevent this you should include a check
using minetest.get_node to make sure there is space for the grass.
This ABM runs every ten seconds.
There is a 1 in 50 chance of the ABM running on each node that has the
correct name and the correct neighbours.
If the ABM runs on a node, an alien grass node is placed above it.
Please be warned, this will delete any node previously located in that position.
To prevent this you should include a check using minetest.get_node to make sure there is space for the grass.
Specifying a neighbour is optional. If you specify multiple neighbours, only one of them
needs to be present to meet the requirements.
Specifying a neighbour is optional.
If you specify multiple neighbours, only one of them needs to be
present to meet the requirements.
Specifying chance is also optional. If you don't specify the chance, the ABM will
always run when the other conditions are met.
Specifying chance is also optional.
If you don't specify the chance, the ABM will always run when the other conditions are met.
## Your Turn
* **Midas touch**: Make water turn to gold blocks with a 1 in 100 chance, every 5 seconds.
* **Decay**: Make wood turn into dirt when water is a neighbour.
* **Burnin'**: Make every air node catch on fire. (Tip: "air" and "fire:basic_flame").
* Midas touch: Make water turn to gold blocks with a 1 in 100 chance, every 5 seconds.
* Decay: Make wood turn into dirt when water is a neighbour.
* Burnin': Make every air node catch on fire. (Tip: "air" and "fire:basic_flame").
Warning: expect the game to crash.