[#5192, #5489] Allow configuration of initiative grouping

[Aioros]

This change adds two settings to the Initiative section of the combat configuration:

1. Roll Once per Creature: actors of the same type (roll formula - disposition - base actor) share a single initiative roll;
2. Group Combatants: actors of the same type with the same rolled initiative are grouped in the tracker (the current default functionality, but it can now be disabled).

Closes #5192
Closes #5489

[Fyorl]

Implementation looks correct. One additional requirement that I would propose is that when you roll initiative separately for a creature, with the 'roll once' setting enabled, that we check the existing combatants for a getGroupingKey match, and use that initiative so it gets added to the existing group automatically.

I suppose we would only do this if combat hasn't started or if it's the first round of combat.

[Fyorl]

Still want to _recoverUses I think

Suggested change

	if ( !updates.length ) return this;
	await this.updateEmbeddedDocuments("Combatant", updates, { turnEvents: false });
	if ( updates.length ) await this.updateEmbeddedDocuments("Combatant", updates, { turnEvents: false });

Can even remove the guard entirely since updateEmbeddedDocuments will no-op on an empty array.

[Fyorl]

I guess the desire is to match getGroupingKey's use of this.initiative, but that is solely for keeping combatants grouped in the tracker so long as they share initiative.

For this method we just want to detect 'is this creature identical to some other creature that has already rolled initiative', where 'identical' is from the wording in the PHB:

For a group of identical creatures, the DM makes a single roll, so each member of the group has the same Initiative.

Let me know if there's some other reason we are using this, otherwise I think it's safe to remove.

[arbron]

Could we use something like this to share some more of the logic:

  /**
   * Key for the group to which this combatant should belong, or `null` if it can't be grouped.
   * @returns {string|null}
   */
  getGroupingKey() {
    if ( this.group ) return this.group.id;
    const uniqueKey = this.getUniqueKey();
    return (uniqueKey === null) || (this.initiative === null) ? null : `${Math.floor(this.initiative)}:${uniqueKey}`;
  }

  /* -------------------------------------------- */

  /**
   * Key used to group unique sets of actors in the combat, or `null` if they shouldn't be grouped.
   * @returns {string|null}
   */
  getUniqueKey() {
    if ( this.token?.actorLink || !this.token?.baseActor ) return null;
    return `${this.token.disposition}:${this.token.baseActor.id}`;
  }

[Aioros]

I opted to use the initiative roll formula in the grouping key because it seemed like a reasonable interpretation of the rule. I might have a group of identical enemies where half of them are surprised at the start of combat, so I would expect two groups to roll (one normal, one with disadvantage); or I dragged four unlinked Mage tokens, changed Dex to -2 for one of them and called it Clumsy Mage, and similarly expect it to roll separately.

Let me know if that seems reasonable. To me, it seems that "having the same kind of initiative" has to be a prerequisite part of "identical creatures" in this context.

[Fyorl]

It's a reasonable interpretation. The rules are very much underspecified in this regard. I don't feel strongly, so I have approved the PR. If there's anything to be done with regards to sharing code like Jeff's suggestion, feel free to make those changes. Otherwise I will merge in due course.

[arbron]

Since it is a method, I suppose we can just pass in an initiative parameter that falls back to this.initiative if not present.

[Aioros]

Yeah, I think it can go back to being a single method (except the parameter will need to tell if it's for rolling or just for final grouping, since we need to check the appropriate setting). I'll do that and the other final fixes (probably on Monday though)

[Fyorl]

I added a canonical getUniqueKey, but I think we still need separate methods since they check different settings. If I've done something silly, feel free to just amend it.