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module Day09
open System.IO
type Vec2D =
| Vec2D of int * int
static member zero = Vec2D(0, 0)
static member (+)(Vec2D (x1, y1), Vec2D (x2, y2)) = Vec2D(x1 + x2, y1 + y2)
static member (-)(Vec2D (x1, y1), Vec2D (x2, y2)) = Vec2D(x1 - x2, y1 - y2)
static member chebyshevDist (Vec2D (x1, y1)) (Vec2D (x2, y2)) = max (abs <| x2 - x1) (abs <| y2 - y1)
let directionToVec =
Vec2D
<< function
| 'U' -> (0, 1)
| 'R' -> (1, 0)
| 'D' -> (0, -1)
| 'L' -> (-1, 0)
| char -> failwithf "Invalid direction: %c" char
let parseHeadMoves =
Seq.collect (fun (line: string) -> List.replicate (int line[2..]) (directionToVec line[0]))
let constrainTail head tail =
if Vec2D.chebyshevDist head tail <= 1 then
tail
else
let (Vec2D (dx, dy)) = head - tail
tail + Vec2D(sign dx, sign dy)
let createRope length = List.replicate length Vec2D.zero
let solution ropeLength =
parseHeadMoves
>> Seq.scan
(fun rope move ->
match rope with
| h :: t -> List.scan constrainTail (h + move) t
| [] -> [])
(createRope ropeLength)
>> Seq.map Seq.last
>> Set
>> Set.count
let test = File.ReadLines("test.txt")
assert (solution 2 test = 13)
assert (solution 10 test = 1)
let input = File.ReadLines("input.txt")
printfn "%d" <| solution 2 input
printfn "%d" <| solution 10 input
|
