import type { List } from ".";
import type { Args, GenericFn, GenericResolver } from "../HKT";

/**
 * Flatten a {@link List} of {@link List}s into a single {@link List}.
 *
 * Sig: `<T>(xs: List<List<T>>) => List<T>`
 */
export type Flatten<TS extends List<List>> =
  TS extends unknown[] ? _FlattenWritable<TS> : _FlattenReadonly<TS>;
type _FlattenReadonly<TS extends List<List>> =
  TS extends readonly [infer Head extends List, ...infer Tail extends List<List>] ?
    readonly [...Head, ..._FlattenReadonly<Tail>]
  : readonly [];
type _FlattenWritable<TS extends List<List>> =
  TS extends readonly [infer Head extends List, ...infer Tail extends List<List>] ?
    [...Head, ..._FlattenWritable<Tail>]
  : [];

interface Resolver extends GenericResolver<[List<List>], List> {
  on1: ([xs]: Args<this>) => [[], List<(typeof xs)[number][number]>];
}

/**
 * [Fn] Flatten a {@link List} of {@link List}s into a single {@link List}.
 *
 * Sig: `<T>(xs: List<List<T>>) => List<T>`
 */
export default interface FlattenFn extends GenericFn<Resolver> {
  def: ([xs]: Args<this>) => Flatten<typeof xs>;
}