13.ts

// TS playground link: https://tsplay.dev/advent-of-typescript-2023-13

// Count the Days
// The elves are SPENT. They need some motivation. They are (literally) counting down the days until Christmas.
//     side note on performance bonuses.. Santa promised that this year they'd get a bonus on the 26th (as well as an extra 2 PAID days off over the course of the next year!). Santa actually promised this last year (and the year before) but no one got a bonus because (according to Santa) "global warming has caused rising sea levels which in turn has eaten coastline, causing a need for many repairs at some of the high-density apartment complexes Santa owns in Florida, resulting in lower cashflow for the parent organization". That's what he said, anyway.
// So, as a small token of our appreciation, let's help the elves by implementing a type, DayCounter, that they can use to keep track of how many days are remaining before Christmas.
// The first argument is the beginning of the count (inclusive), and the second argument is the last number to count to (also inclusive). It should return a union of numbers representing the remaining days.

type OrderedAdventDates = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25];
type PossibleAdventDate = OrderedAdventDates[number]

type DayCounter2<OrderedAdventDates2, From extends PossibleAdventDate, To extends PossibleAdventDate> =
    OrderedAdventDates2 extends [infer Start, ...infer Middle, infer End]
        ? End extends To
            ? Start extends From ?
                OrderedAdventDates2[number]
                : DayCounter2<[...Middle, End], From, To>
            : DayCounter2<[Start, ...Middle], From, To>
        : never

type DayCounter<From extends PossibleAdventDate, To extends PossibleAdventDate> =
    From extends To
		? From
		: OrderedAdventDates extends [From, ...infer Middle, To]
			? (From | Middle[number] | To)
			: DayCounter2<OrderedAdventDates, From, To>