module advent_of_code.tasks.day_4.Star2

open System
open System.IO

let stream = new StreamReader("tasks/day-4/input.txt")

let mutable removed = Set.empty

let getPos (grid: string list) (pos: int * int) =
    let y, x = pos.Deconstruct()
    if (y < 0 || y >= grid.Length || x < 0 || x >= grid[y].Length || removed.Contains((y, x))) then
        0
    else
        if grid[y][x] = '@' then 1 else 0

let isMovable (grid: string list) (y: int) (x: int) =
    if (grid[y][x] = '.' || removed.Contains((y, x))) then
        false
    else
        let posList = [
            (y - 1, x - 1); (y - 1, x); (y - 1, x + 1)
            (y, x - 1); (y, x + 1)
            (y + 1, x - 1); (y + 1, x); (y + 1, x + 1)
        ]
        let adjacentList = posList |> List.map (getPos grid)
        if (List.sum adjacentList < 4) then
            removed <- removed.Add((y, x))
            true
        else false

let rec countRow (grid: string list) (y: int) (x: int) =
    if x = grid[y].Length then 0 else
    (if (isMovable grid y x) then 1 else 0) + (countRow grid y (x + 1))

let rec countGrid (grid: string list) (y: int) =
    if y = grid.Length then 0 else
    (countRow grid y 0) + (countGrid grid (y + 1))

let rec loop (grid: string list) =
    let removables = countGrid grid 0
    if removables = 0 then 0 else removables + loop grid

let main =
    let grid = stream.ReadToEnd().Split('\n') |> Array.toList
    loop grid