import { curryN } from "@unboxing/function";

type KeyFunc<T> = (a: T) => string;

interface UniqBy {
    <T>(fn: KeyFunc<T>, list: ArrayLike<T>): T[];
    <T>(fn: KeyFunc<T>): (list: ArrayLike<T>) => T[];
}

/**
 * Returns unique items in array. Uniqueness is defined by `fn`.
 */
export const uniqueBy = curryN(2, <T>(fn: KeyFunc<T>, arr: ArrayLike<T> = []) => {
  const result = [];
  const unq = Object.create(null);

  for (let i = 0; i < arr.length; i++) {
      const key = fn(arr[i]);

      if (!unq[key]) {
          result.push(arr[i]);
          unq[key] = true;
      }
  }

  return result;
}) as UniqBy