initiative/packages/domain/src/remove-combatant.ts

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

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

/**
 * Pure function that removes a combatant from an encounter by ID.
 *
 * Adjusts activeIndex to preserve turn integrity:
 * - Removed after active → unchanged
 * - Removed before active → decrement
 * - Removed is active, mid-list → same index (next slides in)
 * - Removed is active, at end → wrap to 0
 * - Only combatant removed → 0
 *
 * roundNumber is never changed.
 */
export function removeCombatant(
	encounter: Encounter,
	id: CombatantId,
): RemoveCombatantSuccess | DomainError {
	const found = findCombatant(encounter, id);
	if (isDomainError(found)) return found;

	const { index: removedIdx, combatant: removed } = found;
	const newCombatants = encounter.combatants.filter((_, i) => i !== removedIdx);

	let newActiveIndex: number;
	if (newCombatants.length === 0) {
		newActiveIndex = 0;
	} else if (removedIdx < encounter.activeIndex) {
		newActiveIndex = encounter.activeIndex - 1;
	} else if (removedIdx > encounter.activeIndex) {
		newActiveIndex = encounter.activeIndex;
	} else {
		// removedIdx === activeIndex
		newActiveIndex =
			removedIdx >= newCombatants.length ? 0 : encounter.activeIndex;
	}

	return {
		encounter: {
			combatants: newCombatants,
			activeIndex: newActiveIndex,
			roundNumber: encounter.roundNumber,
		},
		events: [
			{
				type: "CombatantRemoved",
				combatantId: removed.id,
				name: removed.name,
			},
		],
	};
}