add 13 more robust solution

13/index.ts

@@ -1,12 +1,30 @@
 import { Expect, Equal } from "type-testing";
 
-type DayCounter<
-  Start extends number,
-  End extends number,
-  Accumulator extends number[] = [Start],
-> = Accumulator["length"] extends End
-  ? [...Accumulator, End][number]
-  : DayCounter<Start, End, [...Accumulator, Accumulator["length"]]>;
+// Only work if day start at 1
+// type DayCounter<
+//   Start extends number,
+//   End extends number,
+//   Accumulator extends number[] = [Start],
+// > = Accumulator["length"] extends End
+//   ? [...Accumulator, End][number]
+//   : DayCounter<Start, End, [...Accumulator, Accumulator["length"]]>;
+
+// Thanks to https://itnext.io/implementing-arithmetic-within-typescripts-type-system-a1ef140a6f6f
+// Copy paste BuildTuple and Add tuple
+type BuildTuple<
+  Length extends number,
+  Tuple extends number[] = [],
+> = Tuple["length"] extends Length ? Tuple : BuildTuple<Length, [...Tuple, 0]>;
+
+type Add<A extends number, B extends number> = [
+  ...BuildTuple<A>,
+  ...BuildTuple<B>,
+]["length"] &
+  number;
+
+type DayCounter<Start extends number, End extends number> = Start extends End
+  ? End
+  : Start | DayCounter<Add<Start, 1>, End>;
 
 // ------------------- Test section ---------------------
 
@@ -46,3 +64,10 @@ type test_1_actual = DayCounter<1, 25>;
 //   ^?
 type test_1_expected = DaysUntilChristmas;
 type test_1 = Expect<Equal<test_1_expected, test_1_actual>>;
+
+// ------------------- Extra test section ---------------------
+
+type StartAt5 = 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12;
+type test_2_actual = DayCounter<5, 12>;
+//   ^?
+type test_2 = Expect<Equal<StartAt5, test_2_actual>>;