type TNum = 0[];
type NumOf<N extends number, _Acc extends 0[] = []> = _Acc["length"] extends N ? _Acc : NumOf<N, [..._Acc, 0]>;
type Gte<This extends TNum, That extends TNum> = This extends [...That, ...infer R] ? true : false;
type Add1<N extends number, _T = [...NumOf<N>, 0]["length"]> = _T extends number ? _T : never;

type SplitAtIdx<Arr extends unknown[], I extends number, _Head extends unknown[] = []> =
  _Head["length"] extends I
  ? Arr extends [infer _, ...infer R] ? [_Head, R] : [_Head, []]
  : Arr extends [infer H, ...infer R] ? SplitAtIdx<R, I, [..._Head, H]>
  : [_Head, []]
type Assign<Arr extends unknown[], T, I extends number> = Gte<NumOf<I>, NumOf<Arr["length"]>> extends true ? Arr
  : [...SplitAtIdx<Arr, I>[0], T, ...SplitAtIdx<Arr, I>[1]];

type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T['length'],
> =
  Gte<NumOf<Start>, NumOf<T["length"]>> extends true ? T
  : Gte<NumOf<Start>, NumOf<End>> extends true ? T
  : Fill<Assign<T, N, Start>, N, Add1<Start>, End>;

import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<Fill<[], 0>, []>>,
  Expect<Equal<Fill<[], 0, 0, 3>, []>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 0, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 2, 2>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0>, [0, 0, 0]>>,
  Expect<Equal<Fill<[1, 2, 3], true>, [true, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 1>, [true, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 1, 3>, [1, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 10, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 10>, [true, true, true]>>,
]