/**
 * Returns a filter lambda to be used in array.filter() to only allow the
 * first occurrence of duplicate values.
 * @param isEqual Custom equality check, defaults to `a === b`
 */
export function unique<E>(
  isEqual: (a: E, b: E) => boolean = (a, b) => a === b
) {
  return (self: E, index: number, arr: ReadonlyArray<E>) =>
    arr.findIndex((v) => isEqual(v, self)) === index;
}