pub fn imperative(upper: u32) -> u32 {
   
   // Imperative approach
   // Declare accumulator variable
   let mut acc = 0;
   // Iterate: 0, 1, 2, ... to infinity
   for n in 0.. { // wow, rust can iterate to infinity
       // Square the number
       let n_squared = n * n;
       
       if n_squared >= upper {
           // Break loop if exceeded the upper limit
           break;
       } else if is_odd(n_squared) {
           // Accumulate value, if it's odd
           acc += n_squared;
       }
   }
   acc
}

pub fn declarative(upper: u32) -> u32 {
   
       (0..).map(|n| n * n)                             // All natural numbers squared
           .take_while(|&n_squared| n_squared < upper) // Below upper limit
           .filter(|n_squared| is_odd(*n_squared))     // That are odd
           .fold(0, |acc, n_squared| acc + n_squared) // Sum them
}

fn is_odd(n: u32) -> bool {
   n % 2 == 1
}