// @ts-ignore - Only used in doc comments
import type { SimpleMerge } from "./SimpleMerge";
// @ts-ignore - Only used in doc comments
import type SimpleMergeFn from "./SimpleMerge";
import type { Args, Fn } from "../HKT";
import type { AssertObj } from "../helpers";

type _OptionalKeyOf<O> = {
  [K in keyof O]-?: NonNullable<unknown> extends { [P in K]: O[K] } ? K : never;
}[keyof O];

type _SpreadProperties<L, R, K extends keyof L & keyof R> = {
  [P in K]: L[P] | Exclude<R[P], undefined>;
};

type _Id<T> = T extends infer U ? { [K in keyof U]: U[K] } : never;

/**
 * Merge two objects together. Optional keys are considered.
 *
 * However, this can cause some performance issues and may break TS in rare cases,
 * so it is recommended to use {@link SimpleMerge} when you're sure that
 * the two objects have no optional properties.
 *
 * Sig: `(l: object, r: object) => object`
 *
 * @example
 * ```typescript
 * type A = { a: number; b?: string };
 * type B = { a: boolean; c: string };
 * type R = Merge<A, B>;
 * //   ^?: { a: boolean; b?: string; c: string }
 * ```
 *
 * @see {@link SimpleMerge}
 */
export type Merge<L extends object, R extends object> = AssertObj<
  _Id<
    Pick<L, Exclude<keyof L, keyof R>> &
      Pick<R, Exclude<keyof R, _OptionalKeyOf<R>>> &
      Pick<R, Exclude<_OptionalKeyOf<R>, keyof L>> &
      _SpreadProperties<L, R, _OptionalKeyOf<R> & keyof L>
  >
>;

/**
 * [Fn] Merge two objects together. Optional keys are considered.
 *
 * However, this can cause some performance issues and may break TS in rare cases,
 * so it is recommended to use {@link SimpleMerge} when you're sure that
 * the two objects have no optional properties.
 *
 * Sig: `(l: object, r: object) => object`
 *
 * @see {@link SimpleMerge}
 */
export default interface MergeFn extends Fn<[object, object], object> {
  def: ([l, r]: Args<this>) => Merge<typeof l, typeof r>;
}