import type { Args, Call0, Call1W, Fn0, Fn1 } from "./HKT";
import type { List } from "./List";
import type { None, Option, Some } from "./Option";

export interface When<Pred extends Fn1<never, boolean>, Then extends Fn0 | Fn1>
  extends Fn1<unknown, Option<unknown>> {
  _tag: "When";
  def: ([value]: Args<this>) => Call1W<Pred, typeof value> extends true ?
    Some<Then extends Fn0 ? Call0<Then> : Call1W<Then, typeof value>>
  : None;
}

export interface Otherwise<Else extends Fn0 | Fn1> extends Fn1<unknown, Option<unknown>> {
  _tag: "Otherwise";
  def: ([value]: Args<this>) => Else extends Fn0 ? Call0<Else> : Call1W<Else, typeof value>;
}

type _IsAnyOtherwise<TS extends List> =
  TS extends readonly [infer Head, ...infer Tail] ?
    [Head] extends [never] ? _IsAnyOtherwise<Tail>
    : Head extends Otherwise<any> ? true
    : _IsAnyOtherwise<Tail>
  : false;

type _ExcludeNevers<TS extends List> =
  TS extends readonly [infer Head, ...infer Tail] ?
    [Head] extends [never] ?
      _ExcludeNevers<Tail>
    : readonly [Head, ..._ExcludeNevers<Tail>]
  : TS;

type _Decide<T, FS extends List> =
  FS extends readonly [infer F] ?
    F extends Otherwise<any> ? Call1W<F, T>
    : F extends When<any, any> ? Call1W<F, T>
    : never
  : FS extends readonly [infer F, ...infer Tail] ?
    F extends Otherwise<any> ? Call1W<F, T>
    : F extends When<any, any> ?
      Call1W<F, T> extends infer R ?
        R extends None ? _Decide<T, Tail>
        : _IsAnyOtherwise<Tail> extends true ?
          R extends Some<infer R> ?
            R
          : never
        : R
      : never
    : never
  : never;

type _ExtractFnReturnType<F> =
  F extends F ?
    F extends Fn0<infer R> ? R
    : F extends Fn1<any, infer R> ? R
    : never
  : never;
type _ExtractReturnType<T> =
  T extends T ?
    T extends When<any, infer F> ? _ExtractFnReturnType<F>
    : T extends Otherwise<infer F> ? _ExtractFnReturnType<F>
    : never
  : never;

export interface Decide<
  A extends When<any, any>,
  B extends When<any, any> | Otherwise<any>,
  // prettier-ignore
  C extends (_IsAnyOtherwise<[A, B]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  D extends (_IsAnyOtherwise<[A, B, C]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  E extends (_IsAnyOtherwise<[A, B, C, D]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  F extends (_IsAnyOtherwise<[A, B, C, D, E]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  G extends (_IsAnyOtherwise<[A, B, C, D, E, F]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  H extends (_IsAnyOtherwise<[A, B, C, D, E, F, G]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  I extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  J extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H, I]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  K extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H, I, J]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  L extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H, I, J, K]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  M extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H, I, J, K, L]> extends true ? never : When<any, any> | Otherwise<any>) = never,
  // prettier-ignore
  N extends (_IsAnyOtherwise<[A, B, C, D, E, F, G, H, I, J, K, L, M]> extends true ? never : When<any, any> | Otherwise<any>) = never,
> extends Fn1<
    unknown,
    _IsAnyOtherwise<[A, B, C, D, E, F, G, H]> extends true ?
      _ExtractReturnType<_ExcludeNevers<[A, B, C, D, E, F, G, H, I, J, K, L, M, N]>[number]>
    : Option<_ExtractReturnType<_ExcludeNevers<[A, B, C, D, E, F, G, H, I, J, K, L, M, N]>[number]>>
  > {
  def: ([value]: Args<this>) => _Decide<
    typeof value,
    _ExcludeNevers<[A, B, C, D, E, F, G, H, I, J, K, L, M, N]>
  >;
}