/**
 * The device Overview is scoped to, as URL state beside the period.
 *
 * The API takes one exact address (no list) and matches the text the logger
 * stored, so that is what the route lets through: a value that is not an IPv4
 * or IPv6 address is dropped from the URL rather than sent on to a 400, and an
 * IPv6 address is reduced to the RFC 5952 spelling the logger writes, so an
 * uppercase or expanded form in a pasted link still finds its client. The
 * server writes hex groups only, so an IPv4 tail is folded into the two hextets
 * it names and an IPv4-mapped address is reduced to its plain IPv4. An absent
 * scope is the whole household.
 */

const IPV4 = /^(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)\.){3}(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)$/;
const HEXTET = /^[0-9a-f]{1,4}$/i;

/** The two hextets an IPv4 tail names: `192.0.2.30` is `c000:21e`. */
function ipv4Hextets(dotted: string): string[] {
	const octets = dotted.split(".").map(Number);
	return [((octets[0] << 8) | octets[1]).toString(16), ((octets[2] << 8) | octets[3]).toString(16)];
}

/**
 * One side of a `::` as hextets, or null. An IPv4 tail is folded into the two
 * hextets it names — the server prints an IPv6 address as hex groups only,
 * never with a dotted tail — and it is read only where the address ends, since
 * the tail is the last 32 bits and nothing may follow it.
 */
function sideHextets(parts: string[], dottedAllowed: boolean): string[] | null {
	const last = parts[parts.length - 1];
	const hextets =
		last !== undefined && last.includes(".")
			? dottedAllowed && IPV4.test(last)
				? [...parts.slice(0, -1), ...ipv4Hextets(last)]
				: null
			: parts;
	if (hextets === null) return null;
	return hextets.every((group) => HEXTET.test(group)) ? hextets : null;
}

/**
 * RFC 4291 text — up to eight hextets, one `::` at most, an IPv4 tail allowed
 * as the last 32 bits — parsed to its eight hextets, or null.
 */
function ipv6Groups(value: string): string[] | null {
	const halves = value.split("::");
	if (halves.length > 2) return null;
	const split = halves.length === 2;
	const head = sideHextets(halves[0] === "" ? [] : (halves[0] as string).split(":"), !split);
	const tail = sideHextets(split && halves[1] !== "" ? (halves[1] as string).split(":") : [], split);
	if (head === null || tail === null) return null;
	const width = head.length + tail.length;
	if (split ? width >= 8 : width !== 8) return null;
	const zeros = Array.from({ length: 8 - width }, () => "0");
	const expanded = split ? [...head, ...zeros, ...tail] : head;
	return expanded.map((group) => group.replace(/^0+(?=.)/, "").toLowerCase());
}

/** RFC 5952: lowercase, no leading zeros, the longest run of two or more zero groups as `::` (the first on a tie). */
function compressIpv6(groups: string[]): string {
	let best = { start: -1, length: 0 };
	for (let i = 0; i < groups.length;) {
		if (groups[i] !== "0") {
			i += 1;
			continue;
		}
		let j = i;
		while (j < groups.length && groups[j] === "0") j += 1;
		if (j - i >= 2 && j - i > best.length) best = { start: i, length: j - i };
		i = j;
	}
	if (best.start < 0) return groups.join(":");
	const head = groups.slice(0, best.start).join(":");
	const tail = groups.slice(best.start + best.length).join(":");
	return `${head}::${tail}`;
}

/**
 * The dotted IPv4 an IPv4-mapped address stands for, or null. The server
 * normalizes `::ffff:a.b.c.d` to the plain `a.b.c.d`, so a link that spells the
 * mapped form has to be reduced the same way to find its client.
 */
function mappedIpv4(groups: string[]): string | null {
	if (groups.slice(0, 5).some((group) => group !== "0") || groups[5] !== "ffff") return null;
	const high = Number.parseInt(groups[6] as string, 16);
	const low = Number.parseInt(groups[7] as string, 16);
	return [high >> 8, high & 0xff, low >> 8, low & 0xff].join(".");
}

export function parseClient(value: unknown): string | undefined {
	if (typeof value !== "string") return undefined;
	if (IPV4.test(value)) return value;
	const groups = ipv6Groups(value);
	if (groups === null) return undefined;
	return mappedIpv4(groups) ?? compressIpv6(groups);
}