Biome spread

For a guide on how to halt biome spread, see Guide:Maintaining world purity.
The evil biomes and the Hallow, unlike all other biomes, can spread to other areas of a world. They slowly convert, or "infect" certain tiles which are near their own biome-exclusive tiles to their respective biome counterparts, increasing the size of the biome. The exact rules for biome spread are slightly different for each of the three infectious biomes, as described in depth below.

Collectively, these three biomes can be called the "infectious biomes". Blocks and walls which are vulnerable to infection are called "corruptible",
and those which cannot be infected under any circumstances are "non-corruptible". Blocks and walls which *could* be infected, but have *not* been, are called "pure".

Biome spread starts immediately upon world generation, is greatly enhanced and sped up after defeating the Wall of Flesh for the first time (after initiating Hardmode), and is slowed down again after defeating Plantera for the first time. Once Hardmode begins, the world's evil biome and the Hallow have few restrictions on their spread, and without player effort, will eventually take over almost the entire world between them.

This process is part of the game's "tile update cycle": Every game tick, the game chooses a handful of tiles to update, which covers several processes including biome spread. The tiles above, and below, the surface level are selected in separate passes; as an approximation, "surface" tiles will each average about one update every 140 seconds, while "underground" tiles each average about one update every 830 seconds. (That is, above ground tiles get updated about six times as often as belowground tiles, so biome spread will be that much faster above ground.) Due to the random selection method, even the averages here are approximate, and the actual length of time between a given tile's updates can vary wildly. When an infected tile is updated, it is considered as a potential "source tile", which can infect multiple other tiles in a single update.

## Spreading tiles

Only the following blocks and plants of a biome are capable of converting corruptable tiles in their vicinity. Blocks and plants can convert blocks, plants, and walls, while walls can only convert other walls (see below for the latter case). None of the three biomes can convert each other's blocks, with the exception of grass (see below for details).

Note the inconsistency here: While Crimson plants and Vicious Mushrooms are able to act as source tiles, their Corruption equivalents, Corrupt plants and Vile Mushrooms, are not.

## Pre-Hardmode spread

In pre-Hardmode, the three biomes can only spread in a very limited way: The only spreading tiles are Corrupt, Crimson, Hallowed grass and Corruption and Crimson thorny bushes. See below for the details of grass spread.

Even though the Hallow is usually not present in a pre-Hardmode world, it does spread if artificially seeded from a Hardmode world.

## Hardmode spread

In Hardmode, the pre-Hardmode grass spreading remains unchanged. However, many more tiles become corruptible, and will spread much faster. Read the table in the following way: Any of the *spreading tiles* can convert any of the *pure tiles* that are within a 7×7 square centered on the *converted tile* (i.e. within 3 blocks of the *converted tile*).

Defeating Plantera for the first time reduces the speed of the biomes' spread to ~~50\*~~1/2 (50%) of the default Hardmode speed.

## Grass

Like regular grass, the evil and Hallowed variants spread as well. The mechanics described in this section do not change upon initiating Hardmode or defeating Plantera but instead remain the same for the entirety of the game.

For the purposes of grass spread, Corruption and Crimson thorny bushes are considered Corrupt/Crimson grass blocks, respectively.

The following general restrictions apply to all forms of grass spread, including pure grass:

* Grass does not spread below the surface whatsoever. You need to be at least at 4 feet (on the Depth Meter) on the surface for the grass under your feet being able to spread.
* Grass only spreads to adjacent tiles, i.e., tiles within the eight tiles directly surrounding the spreading tile.
* Grass does not spread to tiles that have lava in one of their adjacent tiles.
* Grass only spreads to tiles that have at least one tile of open space within their adjacent tiles.

All grass variants will spread to Dirt Blocks. Furthermore, evil and Hallowed grass blocks can convert other grass as follows:

All three grass variants spread in an equal manner, hence there is none that is "faster" or "stronger", for example. However, Hallowed grass has the ability to overtake Crimson grass.

In Hardmode, Corrupt, Crimson, and Hallowed grass will still grow to Dirt Blocks and infect other grass as described above. However, they additionally gain the ability to convert any corruptable tile within three tiles (just like the other spreading tiles) – including pure grass. Converting a tile that way is handled differently and thus the restrictions outlined above do not apply. This allows them, for example, to infect pure grass below the surface, something which is not possible pre-Hardmode.

Evil and Hallowed grass will never convert any tile underneath a Sunflower. Note that while in pre-Hardmode, Sunflowers thus serve as an effective barrier, but spreading grass will be able to spread to tiles up to three tiles away in Hardmode, easily bypassing a single Sunflower.

## Background Walls

For a technical explanation and the complete algorithm in detail, see the talk page.
Background walls cannot infect blocks, but they *can* be infected by blocks, and they can infect certain other walls within certain restrictions. Walls can also be converted by the initial Hardmode transition, or manually by the player.

* Natural or placed Grass Walls, Jungle Walls, and Flower Walls are all converted to infected Grass walls. Only pure walls are infected, and afterwards cannot be re-infected by a different biome.
* Natural or placed Sandstone Walls or Hardened Sand Walls corrupt to the corresponding biome wall. Even if infected, these *can* be re-infected and converted to a different biome.
* Stone walls cannot be infected this way, but Ebonstone and Crimstone walls can infect the other wall types listed above.
* In all cases, any wall undergoing biome conversion (including from Thrown Water or the Clentaminator) thereafter qualifies as a *natural* wall. This may affect enemy spawning, and perhaps the presence of the various Underground Desert biomes.

There are two overlapping patterns ("grassy" and "sandy") for infection of walls, depending on the contents of the source tile. Both patterns are considered for each tile update, and can separately infect different walls. The behavior can also vary according to whether the tile is above or below surface level, and which infection is being considered.

### Common Rules

* In all cases, wall infection range is 2 tiles (a 5×5 square centered on the source tile), compared to 3 tiles for most blocks.
	+ As with blocks, Hallow cannot infect Jungle Walls
* For a source tile to infect a wall, it must find a qualifying foreground block or plant with the infection in question (collectively, "biome blocks") either *in* the source tile, or within 20 tiles of it.
	+ The qualifying biome blocks vary with context, but the following blocks always qualify: **Grass, Ice, Stone, Sand, and Vines.**
	+ For Crimson, or to infect any grassy wall below surface level, the biome block must be in the source tile.
	+ Otherwise (infecting a grassy wall above surface level with Corruption or Hallow or any sandy wall), the biome block can be found anywhere in a 20-tile range (a 41×41 square centered on the source tile).

### Grassy Wall Infection

If the source tile includes any **natural infected Grass Wall**, **natural infected Stone Wall** (*except* Pearlstone Wall) or **infected grass block**, then it can attempt to **infect a *pure* Grass, Flower, or Jungle Wall**. Both natural and player-placed walls of these types can be infected.

* Each of the infectious biomes is checked in turn: Crimson, then Corruption, then Hallow. If the tile has a qualifying wall and a grass tile from different biomes, it can attempt to spread both biomes.
	+ For each infection attempt, a single tile within range is chosen, and if it is a pure grassy wall, it will be infected.
	+ Again, infection needs a "biome block" as above:
		- For Crimson or if below surface level, then the biome block must be in the source tile, otherwise within 20 tiles.
		- In addition to the list from above (Grass, Ice, Stone, Sand, and Vines), Demonite Ore qualifies for Corruption. It only qualifies as a biome block in this step, and Crimtane Ore does *not* qualify for Crimson.
		- Most biome-specific plants (if infected) also qualify as biome blocks here. This includes thorny bushes, tall grass/flowers (that is, growing from proper grass blocks), Vile/Vicious Mushrooms, and (again) vines.
	+ As above, Hallow cannot infect Jungle Walls, and Pearlstone Walls cannot infect grassy walls at all.

### Sandy Wall Infection

If the source tile contains **any Sandstone Wall**, **any Hardened Sand Wall**, and/or **any Sandstone Block**, then it can attempt to **infect a Hardened Sand or Sandstone Wall**.

* Only Hardened Sand or Sandstone Walls, pure *or with any infection*, are converted this way.
* Unlike with grassy walls, only one biome is chosen, and then a single nearby tile is chosen for an infection attempt.
	+ To attempt infection, the source tile must have an infected wall and/or an infected biome block.
		- Qualifying walls include Grass, Stone, Sandstone, or Hardened Sand. (Note that the first two imply a Sandstone Block from the first step)
		- Qualifying biome blocks include the list from above (Grass, Ice, Stone, Sand, and Vines), or Sandstone Blocks, but not other plants.
		- If the tile has different infections for wall and block/vine, Crimson takes priority, then Hallow. Corruption applies only if there is no competing infection.

## Notes

* Spreading tiles can spread their biome even if there are non-corruptible blocks "in the way". For example, a one-block-wide wall of Wood between a Corrupt grass block and a pure grass block in Hardmode will not prevent the grass from being converted because the Corrupt grass can convert any tile within three tiles. Thus, to effectively stop the spread of a biome, there must be at least a three-block-wide obstacle of either open air or non-corruptible blocks.
* The Crimson and the Hallow grow vines from beneath their respective grass blocks. These vines can spread their biomes to nearby blocks (in Hardmode) and walls as above. There are no Corrupt vines, and vines themselves cannot be converted except by converting the block they're connected to.
* Stone Blocks with moss on them will be treated like regular Stone Blocks. The moss will be destroyed upon conversion.
* Thorny bushes grow from Corrupt and Crimson grass blocks, can extend a significant distance in any direction, and can furthermore spread the Corruption and Crimson, even over non-corruptible blocks. Therefore, it is possible for an evil biome to spread over a gap in pre-Hardmode, if its thorny bushes happen to grow across it and touch dirt or grass on the other side.
* Chlorophyte Ore and Chlorophyte Bricks limit the spread of the evil biomes:
	+ If the Corruption attempts to convert either a Mud Block or a Jungle grass block, *or* if at least one of the four tiles directly surrounding the target tile is a Mud or Jungle grass block, then it will check for the existence of nearby Chlorophyte: If there are at least 5 Chlorophyte tiles (regardless of whether Ore or Brick, both are added) within a radius of 10 tiles, the conversion will fail entirely. If there are fewer than 5, but more than 0 Chlorophyte tiles, there is a chance of *<tiles>*/5 (e.g. 3/5 if there are 3 Chlorophyte tiles) for the conversion to fail.
	+ The Crimson will always check for nearby Chlorophyte, regardless of the target or adjacent blocks.
	+ Chlorophyte Ore can also turn nearby dirt into mud, partly countering conversion damage to the Jungle.
* Because the Hallow will not spread to mud or Jungle grass, creating artificial Hallow between the Jungle and adjacent biomes can also help to preserve the Jungle. For a more controlled solution, combine this with the above note and place Hallowed blocks directly next to small Chlorophyte veins near the edges of the Underground Jungle.
	+ If the evil biome spreads near the vein by way of Jungle grass, and it is not large enough to outright cancel that spread, the Hallow will then begin spreading, and due to the slowing effect of the Chlorophyte will quickly overtake the evil biome.
	+ If the evil biome does not spread near the vein, either by player action or due to it having grown large enough to stop the spread, then the Hallow will not spread at all.

## References

1. ↑ A tick is a time unit countable by the software. Most of *Terraria*'s updating logic happens every tick. A tick has the length of 1/60th of a second, hence there are 60 ticks in a second and 3600 ticks in a minute.
2. ↑ Information taken from the PC 1.4.2.3 source code, method `HardUpdateWorld()` in `Terraria.WorldGen.cs`. There may be inaccuracies, as the current  PC version is 1.4.4.9.
3. ↑ Information taken from the PC 1.3.5.3 source code, method `nearbyChlorophyte()` in `Terraria.WorldGen.cs`. There may be inaccuracies, as the current  PC version is 1.4.4.9.