initiative/packages/domain/src/set-side.ts

import type { DomainEvent } from "./events.js";
import {
	type CombatantId,
	type DomainError,
	type Encounter,
	findCombatant,
	isDomainError,
} from "./types.js";

export interface SetSideSuccess {
	readonly encounter: Encounter;
	readonly events: DomainEvent[];
}

const VALID_SIDES = new Set(["party", "enemy"]);

export function setSide(
	encounter: Encounter,
	combatantId: CombatantId,
	value: "party" | "enemy",
): SetSideSuccess | DomainError {
	const found = findCombatant(encounter, combatantId);
	if (isDomainError(found)) return found;

	if (!VALID_SIDES.has(value)) {
		return {
			kind: "domain-error",
			code: "invalid-side",
			message: `Side must be "party" or "enemy", got "${value}"`,
		};
	}

	const previousSide = found.combatant.side;

	const updatedCombatants = encounter.combatants.map((c) =>
		c.id === combatantId ? { ...c, side: value } : c,
	);

	return {
		encounter: {
			combatants: updatedCombatants,
			activeIndex: encounter.activeIndex,
			roundNumber: encounter.roundNumber,
		},
		events: [
			{
				type: "SideSet",
				combatantId,
				previousSide,
				newSide: value,
			},
		],
	};
}