even fibs

Leonardo.fs

module Tests

open System
open Xunit
open FsUnit.Xunit

let fib (a,b) =
    let sum = a+b
    Some (sum,(b,sum))

let fibseq = Seq.unfold fib (0,1)

let sumEvensNotExceeding notToExceed numbers =
    numbers |>
    Seq.takeWhile (fun n -> n <= notToExceed) |>
    Seq.filter (fun n -> n % 2 = 0) |>
    Seq.map (fun n -> bigint n) |>
    Seq.reduce (+)


let result = fibseq |> sumEvensNotExceeding 4_000_000
// 4_613_732


let [<Fact>] ``fib 0,1`` () = fib (0,1) |> should equal (Some (1,(1,1)))
let [<Fact>] ``fibseq take 1`` () = fibseq |> Seq.take 1 |> Seq.toList |> should equal [1]
   
let [<Fact>]``fibseq take 5`` () = fibseq |> Seq.take 5 |> Seq.toList |> should equal [1;2;3;5;8]
   
let [<Fact>]``sum evens not exceeding 10`` () = fibseq |> sumEvensNotExceeding 10 |> should equal 10I

let [<Fact>]``fencepost sum evens not exceeding 8`` () = fibseq |> sumEvensNotExceeding 8 |> should equal 10I

let [<Fact>] ``drylab result for 1,000,000`` () = fibseq |> sumEvensNotExceeding 4_000_000 |> should equal 42I