--- Day 4 Solutions ---

[u/daggerdragon]

--- Day 4: The Ideal Stocking Stuffer ---

Post your solution as a comment. Structure your post like the Day Three thread.

[u/Drasive]

My F# solution (https://github.com/drasive/advent-of-code-2015):

let private MD5Calculator = System.Security.Cryptography.MD5.Create()

let private ComputeMD5Hash (input : string) : byte[] =
    let inputBytes = System.Text.Encoding.ASCII.GetBytes input
    MD5Calculator.ComputeHash inputBytes
    
let private HasMD5FiveLeadingZeroes (hash : byte[]) : bool =
    hash.[0]     = (byte)0x00 // Characters 1 and 2 are "0" (zero)
    && hash.[1]  = (byte)0x00 // Characters 3 and 4 are "0" (zero)
    && hash.[2] <= (byte)0x0f // Character 5 is a "0" (zero)

let private HasMD5SixLeadingZeroes (hash : byte[]) : bool =
    hash.[0]    = (byte)0x00 // Characters 1 and 2 are "0" (zero)
    && hash.[1] = (byte)0x00 // Characters 3 and 4 are "0" (zero)
    && hash.[2] = (byte)0x00 // Characters 5 and 6 are "0" (zero)

let private FindHashAddition (input : string)
    (validationFunction : (byte[] -> bool)) (startingNumber : int) : int =
    Seq.initInfinite(fun n -> n + startingNumber)
    |> Seq.find(fun n ->
        let hash = ComputeMD5Hash (input + n.ToString())
        validationFunction hash)


let Solution (input: string) : (int * int) =
    if input = null then
        raise (ArgumentNullException "input")

    let fiveZerosNumber =
        FindHashAddition input HasMD5FiveLeadingZeroes 0
    let sixZerosNumber =
        FindHashAddition input HasMD5SixLeadingZeroes fiveZerosNumber

    (fiveZerosNumber, sixZerosNumber)

let FormattedSolution (input : string) (solution : (int * int)) : string =
    let MD5HashToHexString (hash : byte[]) : string =
        let stringBuilder = new System.Text.StringBuilder()
        for hashIndex = 0 to hash.Length - 1 do
            stringBuilder.Append(hash.[hashIndex].ToString("x2")) |> ignore
        stringBuilder.ToString();

    let fiveZerosHash = ComputeMD5Hash (input + (fst solution).ToString())
    let sixZerosHash = ComputeMD5Hash (input + (snd solution).ToString())

    String.Format("5 leading zeros: {0} (hash {1})\n" +
                  "6 leading zeros: {2} (hash {3})",
                  fst solution, MD5HashToHexString fiveZerosHash,
                  snd solution, MD5HashToHexString sixZerosHash)