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

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

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

/**
 * Pure function that sets, changes, or clears a combatant's initiative value.
 *
 * After updating the value, combatants are stable-sorted:
 * 1. Combatants with initiative — descending by value
 * 2. Combatants without initiative — preserve relative order
 *
 * The active combatant's turn is preserved through the reorder
 * by tracking identity (CombatantId) rather than position.
 *
 * roundNumber is never changed.
 */
export function setInitiative(
	encounter: Encounter,
	combatantId: CombatantId,
	value: number | undefined,
): SetInitiativeSuccess | DomainError {
	const targetIdx = encounter.combatants.findIndex((c) => c.id === combatantId);

	if (targetIdx === -1) {
		return {
			kind: "domain-error",
			code: "combatant-not-found",
			message: `No combatant found with ID "${combatantId}"`,
		};
	}

	if (value !== undefined && !Number.isInteger(value)) {
		return {
			kind: "domain-error",
			code: "invalid-initiative",
			message: `Initiative must be an integer, got ${value}`,
		};
	}

	const target = encounter.combatants[targetIdx];
	const previousValue = target.initiative;

	// Record active combatant's id before reorder
	const activeCombatantId =
		encounter.combatants.length > 0
			? encounter.combatants[encounter.activeIndex].id
			: undefined;

	// Create new combatants array with updated initiative
	const updated = encounter.combatants.map((c) =>
		c.id === combatantId ? { ...c, initiative: value } : c,
	);

	// Stable sort: initiative descending, undefined last
	const indexed = updated.map((c, i) => ({ c, i }));
	indexed.sort((a, b) => {
		const aHas = a.c.initiative !== undefined;
		const bHas = b.c.initiative !== undefined;

		if (aHas && bHas) {
			const aInit = a.c.initiative as number;
			const bInit = b.c.initiative as number;
			const diff = bInit - aInit;
			return diff === 0 ? a.i - b.i : diff;
		}
		if (aHas && !bHas) return -1;
		if (!aHas && bHas) return 1;
		// Both undefined — preserve relative order
		return a.i - b.i;
	});

	const sorted = indexed.map(({ c }) => c);

	// Find active combatant's new index
	let newActiveIndex = encounter.activeIndex;
	if (activeCombatantId !== undefined) {
		const idx = sorted.findIndex((c) => c.id === activeCombatantId);
		if (idx !== -1) {
			newActiveIndex = idx;
		}
	}

	return {
		encounter: {
			combatants: sorted,
			activeIndex: newActiveIndex,
			roundNumber: encounter.roundNumber,
		},
		events: [
			{
				type: "InitiativeSet",
				combatantId,
				previousValue,
				newValue: value,
			},
		],
	};
}