04518-medium-fill.ts

// ============= Test Cases =============
import type { Equal, Expect } from "./test-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]>>
];

// ============= Your Code Here =============
// ref: https://github.com/type-challenges/type-challenges/issues/14102

type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T["length"],
  Count extends any[] = [],
  NeedReplace extends boolean = Count["length"] extends Start ? true : false
// > = Count["length"] extends End
> = End extends Count["length"]
  ? T
  : T extends [infer L, ...infer R]
  ? NeedReplace extends true
    ? [N, ...Fill<R, N, Start, End, [...Count, 0], true>]
    : [L, ...Fill<R, N, Start, End, [...Count, 0]>]
  : T;