// 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
function sumOfDigitSquares(int $n): int {
$sum = 0;
while ($n > 0) {
$d = $n % 10;
$sum += $d * $d;
$n = intdiv($n, 10);
}
return $sum;
}
// Determine whether n is a happy number
function isHappy(int $n): bool {
$seen = [];
while ($n !== 1 && !isset($seen[$n])) {
$seen[$n] = true;
$n = sumOfDigitSquares($n);
}
return $n === 1;
}
function main() {
$a = 1;
$b = 100;
if ($a > $b) {
$temp = $a;
$a = $b;
$b = $temp;
}
$happyNumbers = [];
for ($i = $a; $i <= $b; $i++) {
if (isHappy($i)) {
$happyNumbers[] = $i;
}
}
echo "Happy numbers in range [$a, $b]:\n";
foreach ($happyNumbers as $n) {
echo $n . " ";
}
echo "\n";
}
main();
/*
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
*/
