FizzBuzz-1 in Gleam

fizzbuzz.gleam

import gleam/io
import gleam/int
import gleam/float
import gleam/list
import gleam/set
import gleam/result

pub fn main() {
  let list = generate_prime_list(100000)
  // io.debug(list)
  io.debug(list.length(list))
}

fn cond_val(cond: Bool, true_value: a, false_value: a) -> a {
  case cond { True -> true_value False -> false_value }
}

/// range_step(3, 20, 4) -> [3, 7, 11, 15, 19]
fn range_step(first: Int, last: Int, step: Int) -> List(Int) {
  let main = fn () {
    let last_diff = last - last / step * step
    let over_count = cond_val(first > last_diff, 1, 0) + int.max({ first - last_diff - 1 } / step, 0)
    list.range(0, last / step - over_count)
    |> list.map(fn (i) { i * step + first })
  }
  
  case first <= last {
    True -> main()
    False -> []
  }
}

fn generate_prime_list(n: Int) -> List(Int) {
  let main = fn () {
    // [3, 5, 7, 9, 11, 13, 15, 17, 19, 21, ...] (odd numbers >= 3)
    let root = int.square_root(n) |> result.unwrap(0.0) |> float.truncate
    let ints = range_step(3, root, 2)
    // [3, 5, 7, 9, 11, 13, 15, 17, 19, 21, ...] (prime number candidate)
    let primes = range_step(3, n, 2) |> set.from_list

    let primes = list.fold(ints, primes, fn (primes, i) {
      let sub = fn () {
        // if i == 5 -> [10, 15, 20, 25, 30, ...] (multiple of i) > i
        let removed_ints = range_step(i * 2, n, i)
        // sieve of eratosthenes
        list.fold(removed_ints, primes, fn (primes, k) {
          // remove the removed_ints from primes
          set.delete(primes, k)
        })
      }
      
      case set.contains(primes, i) {
        True -> sub()
        False -> primes
      }
    })
    // [3, 5, 7, 11, 13, 17, 19, 23, 29, 31, ...] actual primes >= 3
    let primes = set.to_list(primes) |> list.sort(int.compare)
    // add 2 as primes
    [2, ..primes]
  }

  case n {
    n, if n < 2 -> []
    _ -> main()
  }
}