216 - Slice

[alexfung888]

// remove the tail starting with End (non-negative)
// Head is the substring starting from Arr[0]
// Tail is the rest of it
// therefore cut when Head['length'] is End
// notice the Head/Tail here are not the same as the Head/Tail in my caller (Slice3<>).
type Cut<
  Arr extends number[], End extends number, 
  Head extends number[], Tail extends number[], R extends number[] = []
> = End extends Head['length']
  ? R
  : Tail extends [infer A1 extends number, ...infer T extends number[]]
    ? Cut<Arr, End, [...Head, A1], T, [...R, A1]>
    : []
;

// Start and End are non-negative
// find the substring starting at Start and pass to Cut<> to remove tail
// Head is a substring starting from Arr[0], growing from []
// Tail is the rest of Arr[], starting with the whole array
// The final result is a left$() of Tail
type Slice3<
  Arr extends number[], Start extends number = 0, End extends number = Arr['length'], 
  Head extends number[] = [], Tail extends number[] = Arr
> = Start extends Head['length']
  ? Cut<Arr, End, Head, Tail>
  : Tail extends [infer A1 extends number, ...infer T extends number[]]
    ? Slice3<Arr, Start, End, [...Head, A1], T>
    : [];

// convert negative length to positive equivalent
// Input: Length > 0 (negative sign already removed)
// Output: Arr['length'] - Length
type N2P<Arr extends number[], Length extends number, R extends number[] = [], L extends unknown[] = []> = Arr extends [...infer T extends number[], infer A9 extends number]
  ? Length extends R['length']
    ? Arr['length']
    : N2P<T, Length, [A9, ...R]>
  : L['length'];

// handle negative indexes to always return a non-negative index
// leave positive indexes untouched
type UnNeg<Arr extends number[], Length extends number> = `${Length}` extends `-${infer S extends number}`
  ? N2P<Arr, S>
  : Length;

type Slice<
  Arr extends number[], Start extends number = 0, End extends number = Arr['length'], 
  Head extends number[] = [], Tail extends number[] = Arr
> = Slice3<Arr, UnNeg<Arr, Start>, UnNeg<Arr, End>, Head, Tail>// your answers