Euler14.fsx

let GetChainLength value =
    let next x = match x % 2L with
                 | 0L -> x / 2L
                 | _  -> x * 3L + 1L
    let rec chain n acc = match n with
                          | x when x <= 1L -> acc
                          | x -> chain (next x) (acc+1)
    value, (chain value 1)

seq { 1L..999999L }
|> Seq.map GetChainLength
|> Seq.maxBy snd