https://projecteuler.net/problem=12

prob12.fs

// attempt 2 - use recursion, threading the index and current triangle number to each call
//             less memory intensive, shouldn't be too terrible on stack (tail recursion)
//             but the "countFactors" function sticks out as a little nasty
//
//             run in f# jupyter notebook, or using mono (fsharpc prob12.fs && mono prob12.exe)

let minFactors = 5
// let minFactors = 500

let isFactor x y = (x % y = 0UL)

let countFactors n = 
    [ 1UL.. 1UL + (n/2UL) ]
    |> List.filter (isFactor n)
    |> List.length    
    
let rec prob20 n triangle =
    if (countFactors triangle) >= minFactors
    then triangle
    else prob20 (n+1UL) (triangle+n+1UL)

prob20 1UL 1UL
// |> printfn "%u" // for printing to stdout when running in cmdline