From cassido's May 05, 2023 Newsletter

oddSquareSum.ml

let int_sqrt x = x |> float_of_int |> sqrt |> int_of_float

(**
  * [oddSquareNumber n]
  * returns the sum of all odd square numbers less than [n]
  * 
  * it does this by going through all the odd numbers from [1] to [sqrt(n)]
  * and adding the square of each number to the sum if the square is less
  * than [n].
  *)
let oddSquareSum n =
  let rec inner max curr sum =
    if curr > max then sum
    else
      let curr_square = curr * curr in
      let sum = sum + if curr_square < n then curr_square else 0 in
      inner max (curr + 2) sum
  in
  inner (int_sqrt n) 1 0