Advent of Code 2023, December 8

AdventOfCode2023.12.08.fsx

#time "on"

open System
open System.IO
open System.Collections.Generic

let input =
    File.ReadAllLines(__SOURCE_DIRECTORY__ + "/../data/input08.txt")

let parseLine (str: string) =
    str.Substring(0, 3), str.Substring(7, 3), str.Substring(12, 3)

let instructions = input[0] |> Array.ofSeq

let points = Dictionary<string, string * string>()

input 
|> Seq.toList 
|> List.skip 2 
|> List.iter (fun str -> 
    let (x,y,z) = parseLine str
    points.Add(x, (y,z)))

let rec play (instructions: char array) instrIndex currentPoint stopCondition count =
    let leftPoint, rightPoint = points[currentPoint]
    let direction = instructions[instrIndex]
    let nextPoint = if direction = 'L' then leftPoint else rightPoint
    if stopCondition nextPoint then
        count + 1
    else
        let instrIndex = if instrIndex = instructions.Length-1 then 0 else instrIndex + 1
        play instructions instrIndex nextPoint stopCondition (count + 1)

// Part One

play instructions 0 "AAA" (fun x -> x = "ZZZ") 0

// Part Two
        
let rec gcd x y = if y = 0L then abs x else gcd y (x % y)
let lcm x y = x * y / (gcd x y)

points.Keys 
|> Seq.filter (fun x -> x.EndsWith "A") 
|> Seq.map (fun x -> play instructions 0 x (fun (x: string) -> x.EndsWith "Z") 0)
|> Seq.fold (fun acc elt -> lcm elt acc) 1L