use std::collections::HashSet;
// A happy number is a number that eventually reaches 1
// when repeatedly replaced by the sum of the squares of its digits.
//
// 19 = 1^2 + 9^2 = 82
// 19 -> 82 -> 68 ->100 -> 1 eventually reaches 1
// 19 = happy numbe
// Compute the sum of squares of digits of n
fn sum_of_digit_squares(mut n: i32) -> i32 {
let mut sum = 0;
while n > 0 {
let d = n % 10;
sum += d * d;
n /= 10;
}
sum
}
// Determine whether n is a happy number
fn is_happy(mut n: i32) -> bool {
let mut seen: HashSet<i32> = HashSet::new();
while n != 1 && !seen.contains(&n) {
seen.insert(n);
n = sum_of_digit_squares(n);
}
n == 1
}
fn main() {
let mut a = 1;
let mut b = 100;
if a > b {
std::mem::swap(&mut a, &mut b);
}
let mut happy_numbers: Vec<i32> = Vec::new();
for i in a..=b {
if is_happy(i) {
happy_numbers.push(i);
}
}
println!("Happy numbers in range [{}, {}]:", a, b);
for n in happy_numbers {
print!("{} ", n);
}
}
/*
run:
Happy numbers in range [1, 100]:
1 7 10 13 19 23 28 31 32 44 49 68 70 79 82 86 91 94 97 100
*/
