FindAll

Problem Link

Problem

Given a pattern string P and a text string T, implement the type FindAll<T, P> that returns an array containing all the indices where P occurs in T.

type Matched = FindAll<'Collection | Mutation | Query', 'Collection'> // [0]
type Matched2 = FindAll<'TwoTwo', 'Two'>  // [0, 3]
type Empty    = FindAll<'', 'Two'>        // []
type NotFound = FindAll<'Collection | Mutation | Query', 'Foo'> // []

Solution

type FindAll<
  T extends string,
  P extends string,
  Acc extends number[] = [],
  Idx extends any[] = []
> =
  P extends ''
    ? []
    : T extends `${string}${infer Rest}`
      ? T extends `${P}${string}`
        ? FindAll<Rest, P, [...Acc, Idx['length']], [...Idx, unknown]>
        : FindAll<Rest, P, Acc, [...Idx, unknown]>
      : Acc

How it works:

1. Use Idx as a counter (tuple length = current index).
2. At each position, check if the remaining string T starts with P using T extends \${P}${string}``.
3. If yes, record Idx['length'] in Acc and advance one character.
4. Otherwise just advance one character.
5. Return Acc when the string is exhausted.

Key Takeaways

• Tracking position with a tuple counter (Idx['length']) is the standard way to get numeric indices in TS type recursion.
• Checking T extends \${P}${string}`` tests whether the current suffix starts with the pattern.
• Advancing by one character at a time (T extends \${string}${infer Rest}``) allows overlapping matches.