advent-of-code-2019-day2.fsx

open System.IO

type Opcode = int
type InstructionPointer = int
type Noun = int
type Verb = int
type Program = int []
type Output = int

let input =
    File.ReadAllText("./2019/data/day2.txt")
    |> (fun (line: string) -> line.Split [| ',' |])
    |> Seq.map int
    |> Seq.toArray

let getValue (n: int) (arr: int []) =
  Array.get arr (Array.get arr n)

let applyOp (arr: int []) (n: int) : int [] =
  let f = match Array.get arr n with
          | 1 -> (+)
          | 2 -> (*)
          | x -> failwithf "Unsupported operation: %d" x
  let val1 = getValue (n + 1) arr
  let val2 = getValue (n + 2) arr
  let target = arr.[n + 3]
  arr.[target] <- f val1 val2
  arr

let compute (input: int []) (noun: Noun) (verb: Verb) =
  let rec inner (currentPos: int) (arr: int []) =
    match arr.[currentPos] with
    | 99 -> arr
    | n -> inner (currentPos + 4) (applyOp arr currentPos)
  input.[1] <- noun
  input.[2] <- verb
  inner 0 (Array.copy input)

let findInputsForOutput (program: Program) (out: Output): (Noun * Verb) =
  let result =
    seq { 
      for noun in [0..99] do
        for verb in [0..99] do
          let result = compute program noun verb
          if result.[0] = out 
          then yield noun, verb
    }
  Seq.head result

// part 2
let answer = 
  let (noun, verb) = findInputsForOutput input 19690720
  100 * noun + verb