FactoringANumber.fsx

let getFactors (value : bigint) = 
    let isFactor i pf = 
        i % pf = 0I
    let rec potentialFactors start i = seq {
        match start with
        | Some v when v > 2I -> for a in v..2I..i do yield a 
        | Some _ | None -> 
                    yield 2I
                    yield! potentialFactors (Some 3I) i
                  }  
    let rec loop acc lf i = 
        let findFunc = isFactor i
        let seq = potentialFactors lf i
        match Seq.tryFind findFunc seq with
            | Some (f) -> loop (f::acc) (Some (f)) (i / f)
            | None -> acc

    if (value < 0I) then loop [-1I] None (value * -1I)
    elif (value > 0I) then loop [] None value
    else []