chore(cpp): implement isqrt

Signed-off-by: Matej Focko <mfocko@redhat.com>

.common/cpp/skeleton.cpp

@@ -158,6 +158,28 @@ long pow(long base, long exp) {
   return half * half * base;
 }
 
+template <typename T>
+T isqrt(T x) {
+  assert(x >= 0);
+
+  auto max_shift = 8 * sizeof(T) - 1;
+  auto shift = (max_shift - std::countl_zero(x)) & ~1;
+  auto bit = 1 << shift;
+
+  T result = 0;
+  while (bit != 0) {
+    if (x >= (result + bit)) {
+      x -= result + bit;
+      result = (result >> 1) + bit;
+    } else {
+      result = (result >> 1);
+    }
+    bit = bit >> 2;
+  }
+
+  return result;
+}
+
 template <std::int64_t MODULO = 1000000007>
 struct Z {
   Z(std::int64_t x = 0) : x(x % MODULO) {}