Advent of Code 2020 day 25 in F#

aoc2020day25.fsx

#time "on"

let cpk, dpk = System.IO.File.ReadLines( __SOURCE_DIRECTORY__ + "/input.txt") |> Seq.toList |> fun [a;b] -> int64 a, int64 b

let magic = 20201227L

let rec transform no sn n = seq {
    yield no, n
    let n' = (n * sn) % magic
    yield! transform (no + 1) sn n'
}

let ndpk = transform 1 7L 7L |> Seq.find (snd >> (=) dpk)
let ncpk = transform 1 7L 7L |> Seq.find (snd >> (=) cpk)

transform 1 (snd ncpk) (snd ncpk) 
    |> Seq.find (fst >> (=) (fst ndpk)) 
    |> snd 
    |> printfn "part 1: %d"