package main
import (
"fmt"
)
// 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
func sumOfDigitSquares(n int) int {
sum := 0
for n > 0 {
d := n % 10
sum += d * d
n /= 10
}
return sum
}
// Determine whether n is a happy number
func isHappy(n int) bool {
seen := make(map[int]bool)
for n != 1 && !seen[n] {
seen[n] = true
n = sumOfDigitSquares(n)
}
return n == 1
}
func main() {
a, b := 1, 100
if a > b {
a, b = b, a // swap
}
happyNumbers := []int{}
for i := a; i <= b; i++ {
if isHappy(i) {
happyNumbers = append(happyNumbers, i)
}
}
fmt.Printf("Happy numbers in range [%d, %d]:\n", a, b)
for _, n := range happyNumbers {
fmt.Printf("%d ", 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
*/
