[Typescript] 45. Medium - MinusOne (Solution to solve max number of iteration by tail call)

Just for fun...

Given a number (always positive) as a type. Your type should return the number decreased by one.

For example:

type Zero = MinusOne<1> // 0
type FiftyFour = MinusOne<55> // 54

/* _____________ Your Code Here _____________ */
type Tuple<L extends number, T extends unknown[] = []> = T["length"] extends L
  ? T
  : Tuple<L, [...T, unknown]>;
type MinusOne<T extends number> = Tuple<T> extends [...infer L, unknown]
  ? L["length"]
  : never;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<MinusOne<1>, 0>>,
  Expect<Equal<MinusOne<55>, 54>>,
  Expect<Equal<MinusOne<3>, 2>>,
  Expect<Equal<MinusOne<100>, 99>>,
  Expect<Equal<MinusOne<1101>, 1100>>, // won't work
]

For `Tuple<L>`, we need adding `unknown` into tuple, until the length of tuple is the same as `L`.

For `MinusOne<L>`, using `Tuple` to accumate the tuple length, then sub one get result.

But the solution has one problem, the number of iterlation might crash the compiler.

In function programming, there is a technique called tail call. See this post: https://www.cnblogs.com/Answer1215/p/16598616.html

Trampolines is the solution idea to solve the problem. basicly wrap function into another function, inside wrapper, keep calling original function. But since everytime calling the orignial function return a new function, the callstack will be free (size will be 1 or 0), never increase. That solve the max number iteration problem.

In the Type, we can do the magic by add to the begining:

0 extends 1 ? never: <...rest of the code>

/* _____________ Your Code Here _____________ */
type Tuple<L extends number, T extends unknown[] = []> = 0 extends 1 ? never: T["length"] extends L
  ? T
  : Tuple<L, [...T, unknown]>;
type MinusOne<T extends number> = Tuple<T> extends [...infer L, unknown]
  ? L["length"]
  : never;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<MinusOne<1>, 0>>,
  Expect<Equal<MinusOne<55>, 54>>,
  Expect<Equal<MinusOne<3>, 2>>,
  Expect<Equal<MinusOne<100>, 99>>,
  Expect<Equal<MinusOne<1101>, 1100>>, // works
]

Solution 2:

type MinusOne<T extends number, ARR extends unknown[] = []> = any extends never ? never: [...ARR, 1]['length'] extends T ? ARR['length'] : MinusOne<T, [...ARR, 1]>

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<MinusOne<1>, 0>>,
  Expect<Equal<MinusOne<55>, 54>>,
  Expect<Equal<MinusOne<3>, 2>>,
  Expect<Equal<MinusOne<100>, 99>>,
  Expect<Equal<MinusOne<1101>, 1100>>,
]