solutions/day5.fs


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namespace Solutions

module Day5 =
    open System.IO
    open System.Text.RegularExpressions

    // forgive me lord for what i'm about to do
    // too lazy to figure out how to parse initial state so hardcoding instead
    let initState = [|
                    ['D'; 'T'; 'W'; 'N'; 'L']; 
                    ['H'; 'P'; 'C'];
                    ['J'; 'M'; 'G'; 'D'; 'N'; 'H'; 'P'; 'W'];
                    ['L'; 'Q'; 'T'; 'N'; 'S'; 'W'; 'C'];
                    ['N'; 'C'; 'H'; 'P'];
                    ['B'; 'Q'; 'W'; 'M'; 'D'; 'N'; 'H'; 'T'];
                    ['L'; 'S'; 'G'; 'J'; 'R'; 'B'; 'M'];
                    ['T'; 'R'; 'B'; 'V'; 'G'; 'W'; 'N'; 'Z'];
                    ['L'; 'P'; 'N'; 'D'; 'G'; 'W'] 
                    |]

    type instruction = {
        amount: int;
        source: int;
        target: int
    }

    let getInstruction inst =
        let matches = Regex.Matches(inst, "move (\d+) from (\d+) to (\d+)")
                        |> Seq.head
                        |> fun x -> x.Groups
                        |> List.ofSeq |> List.tail
                        |> List.map (fun x -> int x.Value)
        assert (List.length matches = 3)
        {amount = matches[0]; source = matches[1] - 1; target = matches[2] - 1}
    
    let instructions = File.ReadLines("inputs/day5.txt") 
                        |> Array.ofSeq 
                        |> fun x -> Array.splitAt (Array.findIndex ((=) "") x) x
                        |> snd |> Array.tail |> Array.map getInstruction

    let executeInstruction1 (state: list<char>[]) inst =
        for _ in 1..inst.amount do
            match state[inst.source] with
            | top::bottom ->
                state[inst.source] <- bottom
                state[inst.target] <- top::state[inst.target]
            | _ -> failwith "this shouldn't happen"

    let part1 () = 
        let mutable currentState = initState
        Array.iter (executeInstruction1 currentState) instructions
        Array.map (List.head >> string) currentState |> String.concat ""

    let executeInstruction2 (state: char [] []) inst =
        state[inst.target] <- Array.append state[inst.source].[..inst.amount-1] state[inst.target]
        state[inst.source] <- state[inst.source][inst.amount..]
        
    let part2 () = 
        let mutable currentState = initState |> Array.map Array.ofList
        Array.iter (executeInstruction2 currentState) instructions
        Array.map (Array.head >> string) currentState |> String.concat ""