Advent Of Code 2016 - Day 5

Day5.fs

module Day5

open System
open System.Security.Cryptography
open System.Text

let input = @"reyedfim"

let bytes  (s:string) =  Encoding.ASCII.GetBytes(s)

let md5 (data : byte array) : string =
    use md5 = MD5.Create()
    (StringBuilder(), md5.ComputeHash(data))
    ||> Array.fold (fun sb b -> sb.Append(b.ToString("x2")))
    |> string


let rec decode (s:string) (n:int) acc = 
    if (Seq.length acc) = 8 then acc
    else
        let candidate = s + n.ToString()
        let hash = bytes candidate |> md5
        if hash.StartsWith("00000") then decode s (n + 1) (acc @ [hash.Substring(5,1)])
        else decode s (n + 1) acc

let position c = 
    match c with
    | "0" -> Some 0
    | "1" -> Some 1
    | "2" -> Some 2
    | "3" -> Some 3
    | "4" -> Some 4
    | "5" -> Some 5
    | "6" -> Some 6
    | "7" -> Some 7
    | _ -> None

let rec decode2' (s:string) (n:int) acc = 
    if (Seq.length acc) = 8 then acc
    else
        let candidate = s + n.ToString()
        let hash = bytes candidate |> md5
        if hash.StartsWith("00000") then
            let pos = position ( hash.Substring(5 , 1) )
            match pos with 
            | Some p -> 
                    if  (Seq.exists (fun t -> (fst t) = p) acc) then decode2' s (n + 1) acc                        
                    else decode2' s (n + 1) (acc @ [(p, hash.Substring(6,1))]) 
            | _ -> decode2' s (n + 1) acc
        else decode2' s (n + 1) acc

let decode2 s = decode2' s 0 [] |> Seq.sort |> (Seq.map snd) |> String.Concat

let day5 = 
    printfn "\n\nDay 5\n"    
        
    printfn "Part 1: %A" (decode input 0 [])
    printfn "Part 2: %A"  (decode2 input)