path: root/solutions/day5.fs

module Solutions.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 ""