import { randomInt } from 'crypto';

export type Snowflake = `${bigint}`;

export interface SnowflakeConfig {
	/** Epoch time, e.g. `1577836800000` for 1 Jan 2020 00:00:00 */
	epoch: number;

	/** 6-bit instance ID (0-63) */
	instance: number;
}

export function validate_snowflake_conf(conf: unknown) : asserts conf is SnowflakeConfig {
	// todo: validate config
}

/**
 * Generates a unique 64-bit integer ID.
 * 
 * Format based on Snowflake IDs (https://en.wikipedia.org/wiki/Snowflake_ID),
 * with the following modifications / details:
 * 
 * - Uses a configurable epoch time
 * - Uses a 45-bit timestamp rather than 41-bit, shortening the "instance" (since this project
 *   is never expected to operate at large scale) to 6-bits (which for now is always 0).
 */
export function create_snowflake_provider(conf: SnowflakeConfig) {
	const instance = BigInt(conf.instance) << 12n;

	return {
		uid() {
			const sequence = next_sequence();
			const timestamp = BigInt(Date.now() - conf.epoch) & timestamp_mask;
			return BigInt.asUintN(64, (timestamp << 18n) | instance | sequence);
		},
		uid_str() {
			return this.uid().toString(10) as Snowflake;
		}
	};
}

export function is_snowflake(value: string) : value is Snowflake {
	return /^\d+$/.test(value) && value.length <= 20;
}

const iterator_mask = 0xfffn;
const timestamp_mask = 0x1fffffffffffn;

let iterator = BigInt(randomInt(0xfff));

function next_sequence() {
	const value = iterator++;
	iterator &= iterator_mask;
	return value;
}