module _13

open System
open System.IO

let getSum =

    let numbers = File.ReadAllLines(@"13.txt")

    let add i carry =
        let sum = carry + 
                    (Seq.sum 
                        (seq 
                            { for string in numbers 
                                do yield System.Int32.Parse(string.Substring(string.Length - i, 1)) }
                         )
                     )
        (sum % 10, int(sum / 10))

    let unfold (i, carry) =
        if i = 51 then
            None
        else
            let (digit, nextCarry) = add i carry
            if i <> 50 then 
                Some(digit.ToString(), (i+1, nextCarry))
            else 
                Some(nextCarry.ToString() + digit.ToString(), (i+1, nextCarry))

    let number_string = 
        Seq.unfold unfold (1, 0)
        |> Seq.reduce (fun current next -> next + current)

    let hard = number_string.Substring(0, 10)

    hard