ts-type-forge/src/type-level-integer/seq.d.mts

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/**
 * Creates a readonly tuple containing a sequence of number literals from 0 up to (but not including) `N`.
 * Requires `N` to be a non-negative integer literal for which `MakeTuple<unknown, N>` can be computed.
 *
 * @template N - The upper bound (exclusive) of the sequence. Must be a non-negative integer literal.
 * @returns A readonly tuple type `readonly [0, 1, ..., N-1]`.
 * @example
 * type S3 = Seq<3>; // readonly [0, 1, 2]
 * type S0 = Seq<0>; // readonly []
 * type S1 = Seq<1>; // readonly [0]
 */
type Seq<N extends number> = TSTypeForgeInternals.SeqImpl<
  MakeTuple<unknown, N>
>;

declare namespace TSTypeForgeInternals {
  /**
   * @internal
   * Implementation detail for `Seq`. Takes a tuple `T` and extracts its numeric indices as number literals into a new tuple.
   * @template T - A readonly tuple type (typically generated by `MakeTuple`).
   * @returns A readonly tuple containing the numeric indices of `T`.
   */
  type SeqImpl<T extends readonly unknown[]> = {
    // Map over each key 'i' in the tuple T.
    readonly [i in keyof T]: i extends `${number}` // Check if the key is a numeric string (e.g., "0", "1").
      ? ToNumber<i> // If yes, convert the string key to a number literal.
      : never; // If no (e.g., 'length', array methods), map to never (effectively filtered out).
  };
}