import { IsEqual } from 'type-fest';

/**
 * Takes an array and returns the types that make up its parts. The prefix is
 * anything before the rest parameter (if any), split between it's required
 * part, and it's optional part. The suffix is anything after the rest
 * parameter (if any), and the item is the type of the rest parameter.
 *
 * NOTE: The prefix is split into 2 tuples where all items are non-optional so
 * that types that rely on the presence of a specific element can be built more
 * accurately.
 *
 * TODO: Some existing types use the `prefix` accessor which doesn't handle the optional part correctly. We need to fix each of those types before we can remove it.
 *
 * The output could be used to reconstruct the input: `[
 *   ...TupleParts<T>["required"],
 *   ...Partial<TupleParts<T>["optional"]>,
 *   ...CoercedArray<TupleParts<T>["item"]>,
 *   ...TupleParts<T>["suffix"],
 * ]`.
 */
type TupleParts<T, PrefixRequired extends Array<unknown> = [], PrefixOptionals extends Array<unknown> = [], Suffix extends Array<unknown> = []> = T extends readonly [infer Head, ...infer Tail] ? TupleParts<Tail, [...PrefixRequired, Head], PrefixOptionals, Suffix> : T extends readonly [...infer Head, infer Tail] ? TupleParts<Head, PrefixRequired, PrefixOptionals, [Tail, ...Suffix]> : IsTupleRestOnly<T> extends true ? T extends ReadonlyArray<infer Item> ? {
    prefix: [...PrefixRequired, ...Partial<PrefixOptionals>];
    required: PrefixRequired;
    optional: PrefixOptionals;
    item: Item;
    suffix: Suffix;
} : never : T extends readonly [(infer OptionalHead)?, ...infer Tail] ? TupleParts<Tail, PrefixRequired, [
    ...PrefixOptionals,
    OptionalHead
], Suffix> : never;
/**
 * Helper type for `TupleParts`. Checks if T = ReadonlyArray<U> for some U.
 */
type IsTupleRestOnly<T> = T extends readonly [] ? true : T extends readonly [unknown?, ...infer Tail] ? IsEqual<Readonly<T>, Readonly<Tail>> : false;

export type { TupleParts as T };