Quest 1: Whispers in the Shell

Here's the right challenge. This was a lot simpler than the tracking of cycles.

type Direction = Left | Right  
type Instruction = { Direction: Direction; Distance: int }  
let parseInstruction (segment:string) =  
    let direction = if segment[0] = 'L' then Left else Right  
    let distance = Int32.Parse(segment.AsSpan().Slice(1)) 
    { Direction = direction; Distance = distance }  
    
let inline normalize (array: 'a array) idx =  
    if idx >= 0 then idx % array.Length else array.Length - (-idx % array.Length)  
    
let inline nextIndex instruction idx =  
    match instruction.Direction with  
    | Left -> idx - instruction.Distance  
    | Right -> idx + instruction.Distance  
    
let readFile file =  
    File.ReadAllLines(file)  
    |> fun lines ->  
        let names = lines.[0] |> _.Split(",", StringSplitOptions.RemoveEmptyEntries ||| StringSplitOptions.TrimEntries)  
        let instructions =  
            lines[2]  
            |> _.Split(",", StringSplitOptions.RemoveEmptyEntries ||| StringSplitOptions.TrimEntries)  
            |> Array.map parseInstruction  
        (names, instructions)  

let part1File file : string =  
        readFile file  
        |> fun (names, instructions) ->  
                instructions  
                |> Seq.fold (fun (idx, _) instruction  ->  
                        let next = nextIndex instruction idx |> fun v -> Math.Clamp(v, 0, names.Length - 1)  

                        match Array.tryItem next names with  
                        | Some n -> next, n  
                        | None -> failwith "off array"  
                    ) (0, "")  
        |> snd  
        
let part1 () = part1File "Inputs/Quest01/Q01_P01.txt"  

let part2File file =  
    readFile file  
    |> fun (names, instructions) ->  
        instructions  
        |> Seq.fold (fun (idx, _) instruction ->  
             let next =  nextIndex instruction idx |> normalize names  
             match Array.tryItem next names with  
                        | Some n -> next, n  
                        | None -> failwith "off array"  
                    ) (0, "")  
        |> snd  

let part2() = part2File "Inputs/Quest01/Q01_P02.txt"  

let part3File file =  
    readFile file  
    |> fun (names, instructions) ->  
        // need to preserve a reference to an array that can be mutated  
        let mutable nameArray = names  
        instructions  
        |> Seq.fold (fun _ instruction ->  
            // for this challenge the index doesn't matter  
            let next = match instruction.Direction with  
                        | Left -> names.Length - instruction.Distance  
                        | Right -> instruction.Distance  
                        |> normalize names  
            match Array.tryItem next nameArray with  
                        | Some n ->  
                            nameArray <- arraySwap 0 next names  
                            0, n  
                        | None -> failwith "off array"  
            ) (0, "")  
        |> snd  

let part3() = part3File  "Inputs/Quest01/Q01_P03.txt"