let serial = 5719
let calcPower x y serial =
  let rackId = x + 10;
  let powerLevel = ((rackId * y) + serial) * rackId;
  let strPower = string powerLevel

  if strPower.Length < 3 then 0 else

  strPower
  |> Seq.item (strPower.Length - 3)
  |> fun i -> int (string i) - 5

let createGrid maxX maxY serial =
  let grid = Array2D.create maxX maxY 0

  for x in [0 .. maxX - 1] do
    for y in [0 .. maxY - 1] do
      grid.[x,y] <- calcPower x y serial

  grid

let maxX, maxY = 300,300

let findCell (grid: int[,]) cellSize =
  let mutable biggestPower = 0
  let mutable biggestPowerPos = -1,-1

  for x in [0 .. maxX - cellSize - 1] do
    for y in [0 .. maxY - cellSize - 1] do
      let power =
        grid.[x .. x + cellSize - 1, y .. y + cellSize - 1]
        |> Seq.cast
        |> Seq.reduce (fun acc i -> acc + i)

      if power > biggestPower then
        biggestPower <- power
        biggestPowerPos <- x,y

  biggestPower, biggestPowerPos

// part 1
createGrid maxX maxY serial
|> findCell <| 3
|> printfn "%A"

let findBestGridSize grid =
  let mutable lastMaxPower = (-1000, (-1, -1))
  let mutable bestSize = 0

  let mutable currentSize = 1
  let mutable powerIncreasing = true

  // we'll iterate until the power seems to drastically decrease,
  // then we'll know there is no point looking further
  while powerIncreasing do
    let power = grid |> findCell <| currentSize

    // give some stupid buffer as it seems to fluctuate
    if fst power + 20 < (fst lastMaxPower) then powerIncreasing <- false

    if fst power > fst lastMaxPower
    then
      lastMaxPower <- power
      bestSize <- currentSize

    currentSize <- currentSize + 1

  snd lastMaxPower, bestSize

// part 2
createGrid maxX maxY serial
|> findBestGridSize
|> printfn "%A"