/** Which end of a creature's Hit Dice range its max HP is taken from. */
export type HpVariant = "min" | "max";

export const VALID_HP_VARIANTS: ReadonlySet<string> = new Set(["min", "max"]);

export interface HpRange {
	/** Every Hit Die rolls a 1 (never below 1 HP). */
	readonly min: number;
	/** The statblock's printed average. */
	readonly average: number;
	/** Every Hit Die rolls its maximum. */
	readonly max: number;
}

/**
 * Matches a plain Hit Dice pool: "9d8", "3d6 + 6", "2d6 - 2".
 * Bestiary data uses several dash characters, so hyphen, minus sign, en dash
 * and em dash all count as a negative modifier.
 */
const DICE_FORMULA_REGEX =
	/^\s*(\d+)\s*[dD]\s*(\d+)\s*(?:([-+−–—])\s*(\d+))?\s*$/;

/**
 * Derives the min/average/max HP of a creature from its Hit Dice formula.
 * Returns null when the formula is not a plain dice pool — some sources carry
 * prose (`hp.special`) or an empty string instead.
 */
export function hpRange(
	hp: Readonly<{ average: number; formula: string }>,
): HpRange | null {
	const match = DICE_FORMULA_REGEX.exec(hp.formula);
	if (!match) return null;

	const count = Number(match[1]);
	const sides = Number(match[2]);
	if (count < 1 || sides < 1) return null;

	const modifier =
		match[4] === undefined
			? 0
			: (match[3] === "+" ? 1 : -1) * Number.parseInt(match[4], 10);

	return {
		min: Math.max(1, count + modifier),
		average: hp.average,
		max: Math.max(1, count * sides + modifier),
	};
}

/** The HP value a variant selects; undefined means the printed average. */
export function hpForVariant(
	range: HpRange,
	variant: HpVariant | undefined,
): number {
	if (variant === "min") return range.min;
	if (variant === "max") return range.max;
	return range.average;
}