// Euler Problem 17: Number letter counts
 
let rec numberEnglish n =
    match n with
    | _ when n < 1 -> ""
    | 1 -> "one"
    | 2 -> "two"
    | 3 -> "three"
    | 4 -> "four"
    | 5 -> "five"
    | 6 -> "six"
    | 7 -> "seven"
    | 8 -> "eight"
    | 9 -> "nine"
    | 10 -> "ten"
    | 11 -> "eleven"
    | 12 -> "twelve"
    | _ when n >= 13 && n <= 19 -> teens n
    | _ when n >= 20 && n <= 99 -> teens2 n
    | _ when n >= 100 && n <= 999 -> hundreds n
    | 1000 -> "one thousand"
    | _ -> ""
 
and teens n =
    match n with
    | 13 -> "thir"
    | 14 -> "four"
    | 15 -> "fif"
    | 16 -> "six"
    | 17 -> "seven"
    | 18 -> "eigh"
    | 19 -> "nine"
    | _ -> ""
    + "teen"
 
and teens2 n =
    let m1 =
        match n / 10 with
        | 2 -> "twenty"
        | 3 -> "thirty"
        | 4 -> "forty"
        | 5 -> "fifty"
        | 6 -> "sixty"
        | 7 -> "seventy"
        | 8 -> "eighty"
        | 9 -> "ninety"
        | _ -> ""
    let m2 =
        match n % 10 with
        | 0 -> ""
        | m -> "-" + numberEnglish m
    m1 + m2
 
and hundreds n =
    let m1 = numberEnglish (n / 100) + " hundred"
    let m2 =
        match n % 100 with
        | 0 -> ""
        | m -> " and " + numberEnglish m
    m1 + m2
 
let euler017 () =
    let sb = System.Text.StringBuilder ()
    (sb, { 1 .. 1000 })
    ||> Seq.fold (fun b n -> b.Append (numberEnglish n))
    |> string
    |> Seq.sumBy (fun c -> if Core.char.IsLetter c then 1 else 0)
 
let timer f =
    let sw = System.Diagnostics.Stopwatch ()
    sw.Start ()
    let r = f ()
    sw.Stop ()
    r, sw.Elapsed
 
let _ =
    let r, t = timer euler017
    printfn "Result: %A" r
    printfn "Time: %A" t
 

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