bool isKaprekar(in long n) pure nothrow
in {
assert(n > 0, "isKaprekar(n): n must be > 0");
assert(n <= uint.max, "isKaprekar(n): n must be <= uint.max");
} body {
ulong powr = n ^^ 2UL;
ulong tens = 10, r, l;
while (r < n) {
r = powr % tens;
l = powr / tens;
if (r && l + r == n)
return true;
tens *= 10;
}
return false;
}
--
A positive integer is a Kaprekar number if:
-It is 1
-The decimal representation of its square may be split once into two
parts consisting of positive integers which sum to the original number.
Note that a split resulting in a part consisting purely of 0s is not
valid, as 0 is not considered positive.
Example: 2223 is a Kaprekar number, as 2223 * 2223 = 4941729, 4941729
may be split to 494 and 1729, and 494 + 1729 = 2223.
See also http://rosettacode.org/wiki/Kaprekar_numbers
