(*
    Problem 2
    Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
    1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
    Find the sum of all the even-valued terms in the sequence which do not exceed four million.
*)
let problem_2 bound =
    let rec fsum prev preprev sum =
        let nfib = prev + preprev

        if nfib >= bound then sum
        else
            if nfib % 2 <> 0 then
                printfn "nfib %i, sum %i" nfib sum
                fsum nfib prev sum
            else
                printfn "nfib %i, sum %i" nfib (sum + nfib)
                fsum nfib prev (sum + nfib)

    //simlify the task a little, start from 31
    fsum 2 1 2

//problem_2 1000000
//Array.fold(fun acc e -> acc + e) 0 [|1; 2; 3; 5; 8; 13; 21; 34; 55; 89|];;