4518 - Fill (simplest solution)

[MajorLift]

/**
 * @typedef Fill
 * @params START_SAFE?, END_SAFE?: target range of fill operation adjusted for out-of-bounds input
 * @params COUNT?: number of consecutive elements to replace with fill operation
 * @returns START > END ? T : [...T.slice(0, START_SAFE), ...new Array(COUNT).fill(V), ...T.slice(END_SAFE)]
 */
type Fill<
  T extends unknown[],
  V extends unknown,
  START extends number = 0,
  END extends number = T["length"],
  START_SAFE extends number = LessThan<T["length"], START> extends true ? T["length"] : START,
  END_SAFE extends number = LessThan<T["length"], END> extends true ? T["length"] : END,
  COUNT extends number = Subtract<END_SAFE, START_SAFE>,
  RESULT extends unknown[] = LessThan<END, START> extends true 
    ? T
    : [...Pop<T, Subtract<T["length"], START_SAFE>>, ...Repeat<COUNT, V>, ...Shift<T, END_SAFE>],
> = RESULT;
;

/** Utilities */
type Shift<T extends any[], N extends number = 1> = 
  N extends 0 ? T 
    : T extends [infer _, ...infer Rest]
      ? Shift<Rest, Subtract<N, 1>>
      : [];

type Pop<T extends any[], N extends number = 1> = 
  N extends 0 ? T
    : T extends [...infer Rest, infer _]
      ? Pop<Rest, Subtract<N, 1>> 
      : [];

type LessThan<T extends number, U extends number> = 
  Equal<T, U> extends true ? false
    : Subtract<T, U> extends never ? true : false;

type Subtract<M extends number, S extends number> = 
  Repeat<M> extends [...Repeat<S>, ...infer Rest] ? Rest["length"] : never;

type Repeat<N extends number, T extends unknown = null, M extends T[] = []> = 
  M["length"] extends N ? M : Repeat<N, T, [...M, T]>;

See also: 216 - Slice #21502

[MajorLift]

Added new test case #21521:

Expect<Equal<Fill<[1, 2, 3], true, 10, 20>, [1, 2, 3]>>

• Previously, solutions could pass all test cases without returning T if End >= Start >= T["length"].

// Example: `Fill<[1, 2, 3], true, 10, 20>` resolves to `never`, but passes all test cases.
    : [...Pop<T, Subtract<T["length"], Start>>, ...Repeat<M, N>, ...Shift<T, End>]
// Fix: `Fill<[1, 2, 3], true, 10, 20>` correctly resolves to `[1, 2, 3]`
    : [...Pop<T, Subtract<T["length"], MStart>>, ...Repeat<M, N>, ...Shift<T, MEnd>]