Kaprekar numbers: Difference between revisions

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 <pre>[1, 9, 45, 55, 99, 297, 703, 999, 2223, 2728, 4879, 4950, 5050, 5292, 7272, 7777, 9999]
 </pre>
-===Lower level version===
-More than ten times faster (same output):
+Alternative version, right to left, produces same output
-<lang d>import std.stdio, std.algorithm, std.range;
-pure nothrow bool isKaprekar(in long n)
+<lang d>bool isKaprekar(in long n) {
+    ulong a = n * n, b = a % 10;
-in {
-    assert(n > 0, "isKaprekar(n) is defined for n > 0.");
+    for (ulong d = 1, t = 0; a > 0; d *= 10) {
+        b += t * d;
-    assert(n <= 3_162_277_660UL, "isKaprekar(n) overflow.");
+        a /= 10;
-} body {
-    immutable ulong nn = n * n;
+        if (b && a + b == n)
-    ulong tens = 1;
+            return 1;
-    while (tens < nn)
+        t = a % 10;
-        tens *= 10;
+    }
+    return 0;
-    while ((tens /= 10) > n)
-        if (nn - n == (nn / tens) * (tens - 1))
-            return true;
-    return n == 1;
-}
-void main() {
-    writeln(filter!isKaprekar(iota(1, 10_000)));
-    writeln(count!isKaprekar(iota(1, 1_000_000)));
 }</lang>