Fundamental regionising logic
A region is simply a set of owned chunk positions and implementation defined unique data object tied to that region. It is important to note that for any non-dead region x, that for each chunk position y it owns that there is no other non-dead region z such that the region z owns the chunk position y.
Each world has its own regioniser. The regioniser is a term used to describe the logic that the class "ThreadedRegioniser" executes to create, maintain, and destroy regions. Maintenance of regions is done by merging nearby regions together, marking which regions are eligible to be ticked, and finally by splitting any regions into smaller independent regions. Effectively, it is the logic performed to ensure that groups of nearby chunks are considered a single independent region.
Guarantees the regioniser provides
The regioniser provides a set of important invariants that allows regions to tick in parallel without race condtions:
The first invariant is simply that any chunk holder that exists has one, and only one, corresponding region.
The second invariant is that for every existing chunk holder x that is contained in a region that every each chunk position within the "merge radius" of x is owned by the region. Effectively, this invariant guarantees that the region is not close to another region, which allows the region to assume while ticking it can create data for chunk holders "close" to it.
The third invariant is that a ticking region cannot expand the chunk positions it owns as it ticks. The third invariant is important as it prevents ticking regions from "fighting" over non-owned nearby chunks, to ensure that they truly tick in parallel, no matter what chunk loads they may issue while ticking.
To comply with the first invariant, the regioniser will create "transient" regions around ticking regions. Specifically, around in this context means close enough that would require a merge, but not far enough to be considered independent. The transient regions created in these cases will be merged into the ticking region when the ticking region finishes ticking.
Both of the second invariant and third invariant combined allow the regioniser to guarantee that a ticking region may create and then access chunk holders around it (i.e sync loading) without the possibility that it steps on another region's toes.
The fourth invariant is that a region is only in one of four states: "transient", "ready", "ticking", or "dead."
The "ready" state allows a state to transition to the "ticking" state, while the "transient" state is used as a state for a region that may not tick. The "dead" state is used to mark regions which should not be use.
The states transistions are explained later, as it ties in with the regioniser's merge and split logic.
The regioniser implementation is a description of how the class "ThreadedRegioniser" adheres to the four invariants described previously.
Splitting the world into sections
The regioniser does not operate on chunk coordinates, but rather on "region section coordinates." Region section coordinates simply represent a grouping of NxN chunks on a grid, where N is some power of two. The actual number is left ambiguous, as region section coordinates are only an internal detail of how chunks are grouped. For example, with N=16 the region section (0,0) encompasses all chunks x in [0,15] and z in [0,15]. This concept is similar to how the chunk coordinate (0,0) encompasses all blocks x in [0, 15] and z in [0, 15]. Another example with N=16, the chunk (17, -5) is contained within region section (1, -1).
Region section coordinates are used only as a performance tradeoff in the regioniser, as by approximating chunks to their region coordinate allows it to treat NxN chunks as a single unit for regionising. This means that regions do not own chunks positions, but rather own region section positions. The grouping of NxN chunks allows the regionising logic to be performed only on the creation/destruction of region sections. For example with N=16 this means up to NxN-1=255 possible less operations in areas such as addChunk/region recalculation assuming region sections are always full.
The implemnetation variables control how aggressively the regioniser will maintain regions and merge regions.
The recalculation count is the minimum number of region sections that a region must own to allow it to re-calculate. Note that a recalculation operation simply calculates the set of independent regions that exist within a region to check if a split can be performed. This is a simple performance knob that allows split logic to be turned off for small regions, as it is unlikely that small regions can be split in the first place.
Max dead section percent
The max dead section percent is the minimum percent of dead sections in a region that must exist before a region can run re-calculation logic.
Empty section creation radius
The empty section creation radius variable is used to determine how many empty region sections are to exist around any region section with at least one chunk.
Internally, the regioniser enforces the third invariant by preventing ticking regions from owning new region sections. The creation of empty sections around any non-empty section will then enforce the second invariant.
Region section merge radius
The merge radius variable is used to ensure that for any existing region section x that for any other region section y within the merge radius are either owned by region that owns x or are pending a merge into the region that owns x or that the region that owns x is pending a merge into the region that owns y.
Region section chunk shift
The region section chunk shift is simply log2(grid size N). Thus, N = 1 << region section chunk shift. The conversion from chunk position to region section is additionally defined as region coordinate = chunk coordinate >> region section chunk shift.
The regioniser is operated by invoking ThreadedRegioniser#addChunk(x, z) or ThreadedRegioniser#removeChunk(x, z) when a chunk holder is created or destroyed.
Additionally, ThreadedRegion#tryMarkTicking can be used by a caller that attempts to move a region from the "ready" state to the "ticking" state. It is vital to note that this function will return false if the region is not in the "ready" state, as it is possible that even a region considered to be "ready" in the past (i.e scheduled to tick) may be unexpectedly marked as "transient." Thus, the caller needs to handle such cases. The caller that successfully marks a region as ticking must mark it as non-ticking by using ThreadedRegion#markNotTicking.
The function ThreadedRegion#markNotTicking returns true if the region was migrated from "ticking" state to "ready" state, and false in all other cases. Effectively, it returns whether the current region may be later ticked again.
Region section state
A region section state is one of "dead" or "alive." A region section may additionally be considered "non-empty" if it contains at least one chunk position, and "empty" otherwise.
A region section is considered "dead" if and only if the region section is also "empty" and that there exist no other "empty" sections within the empty section creation radius.
The existence of the dead section state is purely for performance, as it allows the recalculation logic of a region to be delayed until the region contains enough dead sections. However, dead sections are still considered to belong to the region that owns them just as alive sections.
Addition of chunks (addChunk)
The addition of chunks to the regioniser boils down to two cases:
Target region section already exists and is not empty
In this case, it simply adds the chunk to the section and returns.
Target region section does not exist or is empty
In this case, the region section will be created if it does not exist. Additionally, the region sections in the "create empty radius" will be created as well.
Then, any region in the create empty radius + merge radius are collected into a set X. This set represents the regions that need to be merged later to adhere to the second invariant.
If the set X contains no elements, then a region is created in the ready state to own all of the created sections.
If the set X contains just 1 region, then no regions need to be merged and no region state is modified, and the sections are added to this 1 region.
Merge logic needs to occur when there are more than 1 region in the set X. From the set X, a region x is selected that is not ticking. If no such x exists, then a region x is created. Every region section created is added to the set x, as it is the section that is known to not be ticking - this is done to adhere to invariant third invariant.
Every region y in the set X that is not x is merged into x if y is not in the ticking state, otherwise x runs the merge later logic into y.
Merge later logic
A merge later operation may only take place from a non-ticking, non-dead region x into a ticking region y. The merge later logic relies on maintaining a set of regions to merge into later per region, and another set of regions that are expected to merge into this region. Effectively, a merge into later operation from x into y will add y into x's merge into later set, and add x into y's expecting merge from set.
When the ticking region finishes ticking, the ticking region will perform the merge logic for all expecting merges.
A merge operation may only take place between a dead region x and another region y which may be either "transient" or "ready." The region x is effectively absorbed into the region y, as the sections in x are moved to the region y.
The merge into later is also forwarded to the region y, such so that the regions x was to merge into later, y will now merge into later.
Additionally, if there is implementation specific data on region x, the region callback to merge the data into the region y is invoked.
The state of the region y may be updated after a merge operation completes. For example, if the region x was "transient", then the region y should be downgraded to transient as well. Specifically, the region y should be marked as transient if region x contained merge later targets that were not y. The downgrading to transient is required to adhere to the second invariant.
Removal of chunks (removeChunk)
Removal of chunks from region sections simple updates the region sections state to "dead" or "alive", as well as the region sections in the empty creation radius. It will not update any region state, and nor will it purge region sections.
Region tick start (tryMarkTicking)
The tick start simply migrates the state to ticking, so that invariants #2 and #3 can be met.
Region tick end (markNotTicking)
At the end of a tick, the region's new state is not immediately known.
First, tt first must process its pending merges.
After it processes its pending merges, it must then check if the region is now pending merge into any other region. If it is, then it transitions to the transient state.
Otherwise, it will process the removal of dead sections and attempt to split into smaller regions. Note that it is guaranteed that if a region can be possibly split, it must remove dead sections, otherwise, this would contradict the rules used to build the region in the first place.