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strlen-arrlen.md

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StrLength<Str> and ArrLength<Arr>

StrLength<Str>

type StrLength<
  Str extends string,
  Acc extends string = '.'
> =
  Str extends `${string}${infer T}`
    ? T extends ''
      ? $$DotsToNum<Acc>
      : StrLength<T, `.${Acc}`>
    : 0;


type SL1 = StrLength<'abc'>;
// 3

type SL2 = StrLength<'x'>;
// 1

type SL3 = StrLength<''>;
// 0

ArrLength<Arr>

type ArrLength<
  Arr extends any[],
  Acc extends string = '.'
> =
  Arr extends [...items: infer H, last: any]
    ? H extends []
      ? $$DotsToNum<Acc>
      : ArrLength<H, `.${Acc}`>
    : 0;


type AL1 = ArrLength<[1, 2, true]>;
// 3

type AL2 = ArrLength<['a']>;
// 1

type AL3 = ArrLength<[]>;
// 0

alternative form:

type ArrLength<A> = A extends any[] ? A['length'] : never;


type AL1 = ArrLength<[1, 2, true]>;
// 3

type AL2 = ArrLength<['a']>;
// 1

type AL3 = ArrLength<[]>;
// 0

Generic Length<L>

type Length<
  L extends string | any[],
  Acc extends string = '.'
> =
  L extends string
    ? L extends `${string}${infer T}`
      ? T extends ''
        ? $$DotsToNum<Acc>
        : Length<T, `.${Acc}`>
      : 0
    : L extends any[]
      ? L['length']
      : never;


type LenStr1 = Length<'ABC'>;
// 3

type LenStr2 = Length<''>;
// 0

type LenArr1 = Length<[1, 'a', true, ]>;
// 3

type LenArr2 = Length<[]>;
// 0

Dot-Based Numeral System

type $$DotsToNum<
  T extends string,
  Acc extends string[] = []
> =
  T extends ''
    ? Acc['length']
    : T extends `.${infer R}`
      ? $$DotsToNum<R, [...acc: Acc, x: string]>
      : [...acc: Acc, x: string]['length'];


type DTN1 = $$DotsToNum<''>;
// 0

type DTN2 = $$DotsToNum<'.'>;
// 1

type DTN3 = $$DotsToNum<'....'>;
// 4