pub fn first_missing_positive(mut numbers: Vec<i32>) -> i32 {
  let numbers_length = numbers.len();

  // Mark all numbers below 1 and above the maximum range as i32::MAX.
  for index in 0..numbers_length {
    if numbers[index] < 1 || numbers[index] > numbers_length as i32 {
      numbers[index] = i32::MAX;
    }
  }

  for index in 0..numbers_length {
    // For each index get its number and subtract 1.
    let index = numbers[index].abs() as usize - 1;

    // If that index then isn't i32::MAX - 1 (ie. negative or above range).
    if index as i32 != i32::MAX - 1 {
      // Then mark the number for that index as negative.
      numbers[index] = -numbers[index].abs();
    }
  }

  // Then the first positive number we find, the index of that number will be
  // the first missing positive. And if there are only negative numbers, then
  // the next positive number is the total amount of numbers + 1.
  numbers
    .into_iter()
    .enumerate()
    .find_map(|(index, number)| {
      if number.is_positive() {
        Some(index + 1)
      } else {
        None
      }
    })
    .unwrap_or_else(|| numbers_length + 1) as i32
}