module _21

let amicalbleNumbers len =
    // get divisor sums
    let dSums = common.divisors len |> Array.map (List.fold (+) 0)
    // find amicable ones
    [2..len] |> List.filter (fun i -> 
        dSums.[i] > 0 &&
        dSums.[i] <= len && 
        not (i = dSums.[i]) &&
        i < dSums.[i] &&
        i = dSums.[dSums.[i]]
        )
        |> List.map (fun i -> (i, dSums.[i]))

let amicalbleSum =
    let a = amicalbleNumbers 10000
    let all = List.append (a |> List.map fst) (a |> List.map snd)
    all |> List.sum