Editing Field map encounter mechanics (section)

== Which encounters do you get? (Formation) ==

Each field that can be hostile has up to two encounter tables, one of which can be set as the current active encounter table. The encounter table consists of two sections: Standard encounters and Special encounters. Special encounters are back, side, and pincer attacks, whereas standard encounters are anything else (including preempts, which are determined differently).

The game first generates a random number and uses it to determine if this encounter will be a Special formation and, if so, which of them it is. If a special formation has been selected, the procedure quits and that is the battle generated.

If a special battle was not selected, another random number is chosen, and this value is used to determine which of the Standard encounters will be generated. If this generated encounter is the same as the Last Encounter Formation (more on this later), the game "rerolls" your encounter by generating another random number and using this as the generated encounter formation (even if it is the same as the last encounter formation again).

After any standard encounter is selected (with one exception, see Quirks), the Last Encounter Formation is set to the newly generated encounter formation. This Last Encounter Formation mechanic is meant to reduce how often the same encounter appears twice in a row, and it makes a generated formation sometimes dependent on the formation generated by the previous encounter.

Encounter tables are structured as follows: There are six slots for Standard encounters and four slots for Special encounters. Each encounter slot consists of a formation ID and an encounter rate. Formation IDs are a number from 0 to 999, and each represents a (usually) unique battle formation, a lookup table for which can be found here. 

Encounter rates are a value from 0 to 63, and the higher the value, the more likely that encounter is to appear. Each Special slot is assigned a specific function, with the first two being assigned to Back Attack formations, the third for a Side Attack formation, and the fourth for a Pincer formation. The sum of all weights for Standard encounters always equals 64, and the sum of all weights for Special encounters is always significantly less than 64.

To select a formation, a random number from 0 to 63 is chosen. We then iterate through all of the encounter slots in either the special or standard encounter table area, whichever we are currently looking through. Throughout this, we keep track of a threshold value that starts at zero.

For each of these encounter slots, we start by adding this slot’s encounter rate to the threshold value. If the random value is less than this threshold value, then this is the encounter we use.

In effect, this means that we can imagine each encounter table as a list of 64 slots, each with an encounter formation. The randomly chosen number will be the index of one of these slots, and that will be the encounter chosen by this procedure.

For example, let’s take the first screen in the game, whose standard encounter table is as follows:
{| class="wikitable"
|Encounter Formation
|Encounter Rate
|-
|301
|24
|-
|302
|22
|-
|303
|18
|}
In order, the game checks the first, then second, then third of these slots until it finds a formation that satisfies the random value. This can be represented by our 64-slot model having the first 24 slots (from 0 to 23) represent encounter formation 301, the next 22 slots (from 24 to 45) represent encounter formation 302, and the last 18 slots (from 46 to 63) representing encounter formation 303.
{| class="wikitable"
|0 …………………………... 23
|24 ……………………….… 45
|46 ……………………….… 63
|-
|301
|302
|303
|}
As for when a special formation is attempting to be generated, this table consists of some slots filled at the beginning of the table, followed by the rest of the table containing no encounter. If a section with no encounter is chosen, then the game has failed to generate a special encounter and will then go to generate a standard encounter instead.

Note that having sufficiently many and high-leveled Preemptive Materia equipped on the third party member (this is a bug) causes you to enter a Mastered Preempt state. In this state, the encounter rate for the Back Attack and Pincer slots in the special encounter slots are halved. The side attack slot is unaffected. This is the only difference between the different special encounter slots, as the data for whether each formation is a back, side, or pincer attack are stored in the formation data elsewhere.
(TODO: find exact thresholds and conditions for Mastered Preempt state)

=== Manipulating Formations ===

As you might have been able to guess, a similar method is used to generate the random numbers used for formations as the method used for danger thresholds, however it’s even simpler this time. A different random index is used for formation generation, making it independent from danger threshold generation. The index that determines what encounters you get is a 1-byte value called the Formation Accumulator, or just formation for short. When this generator calls for a random number, formation is incremented by one, then the RNG table value at the index of the new formation value is returned and is used as the random value.

This generator returns a value between 0 and 255, so to get a value between 0 and 63, it is divided by 4 and truncated to an integer.

Formation RNG is called once to determine if this encounter is a special formation. If it isn’t, it’s called a second time to determine what this encounter’s formation is. If a reroll is needed, it’s called a third time. As a result, the formation value can advance by 1, 2, or 3 depending on the current state of the formation value and the last encounter formation, which is what determines when a reroll occurs.

Formations can be manipulated by skipping or not skipping certain encounters to cause formation to increase or not increase at specific points. It can also be affected by getting encounters on certain fields, depending on if they have a special encounter table or not, or if their encounter table results in a reroll. This reroll is also known as a “bump” or “getting bumped”, since you’re bumped past an encounter to the next, or bumped past the usual formation increase of 2 and up to 3.

For example, you can intentionally waste movement and force an encounter at the end of a field instead of going to the next field, even if it was possible to make it to the next field, just because getting an encounter on the first field would result in a different and possibly more favorable enemy formation or formation increase than getting it on the second field. You can also intentionally avoid walking under an encounter that could be skipped because getting that encounter at that point would result in a more favorable formation increase.

In general, formation manipulation is needed to ensure getting specific encounters, such as those that give Enemy Skills such as Death Claws for Laser or a Boundfat for Death Sentence, or to avoid getting particularly bad encounters, such as pincers. However, formations are in general more difficult to manipulate since you don’t have much flexibility or control over their increase, but just knowing what formations you will get is often enough to prepare for a particularly bad encounter formation if one were to occur. Formation manipulation and routing is especially important in 100% because the category requires many drops, steals, morphs, enemy skills, and even kills from specific enemies all over the game.