Advent of Code F# - Day 15 | theburningmonk.com

Yan Cui

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Day 15

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ps. look out for all my other solutions for Advent of Code challenges here.

Day 15

See details of the challenge here.

Today’s challenge is pretty straight forward, and a brute force approach would have suffice – try every t from 0 to inifinity and return the first t that satisfies this equation for all the discs:

(t + disc.Number – (disc.Positions – disc.Time0Position)) % disc.Positions = 0

That said, there’s a simple optimization we can apply by restricting ourselves to values of t that gets you through the first slot. Give my input for the challenge:

Disc #1 has 13 positions; at time=0, it is at position 11.
Disc #2 has 5 positions; at time=0, it is at position 0.
Disc #3 has 17 positions; at time=0, it is at position 11.
Disc #4 has 3 positions; at time=0, it is at position 0.
Disc #5 has 7 positions; at time=0, it is at position 2.
Disc #6 has 19 positions; at time=0, it is at position 17.

The first t that will get us through disc #1 is t=1 where disc #1 reaches position 0 on t=2 (which is 1s away from t). From there, we only need to check every 13s, ie t=14, t=27, t=40, …

	type Disc =
	{
	Number : int
	Positions : int
	Time0Pos : int
	}

	let solve (discs : Disc list) =
	// what's the first time we can press and get through the first slot?
	let fstDisc = discs.[0]
	let timeToZero = fstDisc.Positions - fstDisc.Time0Pos
	let fstT = timeToZero - fstDisc.Number

	Seq.initInfinite (fun i -> i * fstDisc.Positions + fstT)
	\|> Seq.filter (fun t ->
	discs \|> List.forall (fun disc ->
	(t + disc.Number - (disc.Positions - disc.Time0Pos)) % disc.Positions = 0))
	\|> Seq.head

	let input1 =
	[
	{ Number = 1; Positions = 13; Time0Pos = 11 }
	{ Number = 2; Positions = 5; Time0Pos = 0 }
	{ Number = 3; Positions = 17; Time0Pos = 11 }
	{ Number = 4; Positions = 3; Time0Pos = 0 }
	{ Number = 5; Positions = 7; Time0Pos = 2 }
	{ Number = 6; Positions = 19; Time0Pos = 17 }
	]
	let part1 = solve input1

view raw Day15_Part1.fsx hosted with ❤ by GitHub

Part 2

After getting the first capsule (it contained a star! what great fortune!), the
machine detects your success and begins to rearrange itself.

When it’s done, the discs are back in their original configuration as if it were
time=0 again, but a new disc with 11 positions and starting at position 0 has
appeared exactly one second below the previously-bottom disc.

With this new disc, and counting again starting from time=0 with the
configuration in your puzzle input, what is the first time you can press the
button to get another capsule?

	let input2 = input1 @ [ { Number = 7; Positions = 11; Time0Pos = 0 } ]
	let part2 = solve input2

view raw Day15_Part2.fsx hosted with ❤ by GitHub

Advent of Code F# – Day 15

Day 15

Part 2

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