add fill

fill.ts

@@ -0,0 +1,41 @@
+import type { Equal, Expect } from '@type-challenges/utils'
+
+type cases = [
+  Expect<Equal<Fill<[], 0>, []>>,
+  Expect<Equal<Fill<[], 0, 0, 3>, []>>,
+  Expect<Equal<Fill<[1, 2, 3], 0, 0, 0>, [1, 2, 3]>>,
+  Expect<Equal<Fill<[1, 2, 3], 0, 2, 2>, [1, 2, 3]>>,
+  Expect<Equal<Fill<[1, 2, 3], 0>, [0, 0, 0]>>,
+  Expect<Equal<Fill<[1, 2, 3], true>, [true, true, true]>>,
+  Expect<Equal<Fill<[1, 2, 3], true, 0, 1>, [true, 2, 3]>>,
+  Expect<Equal<Fill<[1, 2, 3], true, 1, 3>, [1, true, true]>>,
+  Expect<Equal<Fill<[1, 2, 3], true, 10, 0>, [1, 2, 3]>>,
+  Expect<Equal<Fill<[1, 2, 3], true, 10, 20>, [1, 2, 3]>>,
+  Expect<Equal<Fill<[1, 2, 3], true, 0, 10>, [true, true, true]>>,
+]
+
+type FillInternal<
+  T extends unknown[],
+  N,
+  Start extends number = 0,
+  End extends number = T['length'],
+  Left extends unknown[] = [],
+  Mid extends unknown[] = [],
+  Right extends unknown[] = T,
+> =
+Right extends [infer Head, ...infer Tail]
+  ? Left['length'] extends Start
+    ? Mid['length'] extends End
+      ? [...Mid, Head, ...Tail]
+      : FillInternal<T, N, Start, End, Left, [...Mid, N], Tail>
+    : FillInternal<T, N, Start, End, [...Left, Head], [...Left, Head], Tail>
+  : [...Mid, ...Right]
+;
+
+type Fill<
+  T extends unknown[],
+  N,
+  Start extends number = 0,
+  End extends number = T['length'],
+> = FillInternal<T, N, Start, End>
+;