import type { DomainError } from "./types.js";

/**
 * Pure function that computes initiative from a resolved dice roll and modifier.
 * The dice roll must be an integer in [1, 20].
 * Returns the sum (diceRoll + modifier) or a DomainError for invalid inputs.
 */
export function rollInitiative(
	diceRoll: number,
	modifier: number,
): number | DomainError {
	if (!Number.isInteger(diceRoll) || diceRoll < 1 || diceRoll > 20) {
		return {
			kind: "domain-error",
			code: "invalid-dice-roll",
			message: `Dice roll must be an integer between 1 and 20, got ${diceRoll}`,
		};
	}

	return diceRoll + modifier;
}