/**
 * Checks if a variable is defined and not null.
 * After this check, typescript sees the variable as defined.
 *
 * @param arg - The input to be verified
 *
 * @returns true if the input variable is defined.
 *
 * @beta This API may change without a BREAKING CHANGE notice.
 */
export function isDefined<T>(arg: T): arg is Exclude<T, null | undefined> {
  return arg !== null && typeof arg !== 'undefined'
}

/**
 * Ensures that a given input is an array. If the input is a single item, it is converted to an array with a single
 * entry.
 *
 * @param arg - a variable that needs to be converted to array
 *
 * @beta This API may change without a BREAKING CHANGE notice.
 */
export function asArray<T>(arg?: T | T[] | any): (T | any)[] {
  return isDefined(arg) ? (Array.isArray(arg) ? arg : [arg]) : []
}

/**
 * Checks if an object is iterable (can be used for `for..of`)
 * @param obj - the variable to be checked
 *
 * @beta This API may change without a BREAKING CHANGE notice.
 */
export function isIterable<T>(obj: any): obj is Iterable<T> {
  return obj != null && typeof obj[Symbol.iterator] === 'function'
}

/**
 * Compute the intersection of two arrays
 * Elements are compared by reference so object types will appear as unique even if they contain the same data.
 *
 * @param a - first array
 * @param b - second array
 * @returns The intersection of the two arrays.
 *
 * @public
 */
export function intersect<T>(a: T[] | any, b: any[] | any): T[] {
  const setB = new Set<T>(asArray(b));
  return [...new Set(asArray(a))].filter(x => setB.has(x));
}