5kyu(cpp): add „(Ready for) Prime Time“

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

5kyu/prime_time/solution.cpp

@@ -0,0 +1,63 @@
+#include <cassert>
+#include <vector>
+
+namespace {
+
+inline std::size_t index(unsigned p) { return p - 2; }
+inline bool is_prime(const std::vector<bool> &primes, unsigned p) {
+  return primes[index(p)];
+}
+
+void discard_multiples(std::vector<bool> &primes, unsigned p) {
+  for (auto k = 2; index(p * k) < primes.size(); k++) {
+    primes[index(p * k)] = false;
+  }
+}
+
+void discard_even(std::vector<bool> &primes) { discard_multiples(primes, 2); }
+
+std::vector<unsigned> construct_primes(const std::vector<bool> &primes) {
+  std::vector<unsigned> result;
+
+  for (auto p = 0u; p < primes.size(); p++) {
+    if (primes[p]) {
+      result.push_back(p + 2);
+    }
+  }
+
+  return result;
+}
+
+} // namespace
+
+// Generate an array containing every prime number between 0 and the num
+// specified (inclusive)
+std::vector<unsigned> prime(unsigned n) {
+  if (n < 2) {
+    return {};
+  }
+
+  std::vector<bool> primes(n - 1, true);
+  discard_even(primes);
+
+  for (auto p = 3u; p <= n; p += 2) {
+    if (!is_prime(primes, p)) {
+      continue;
+    }
+
+    discard_multiples(primes, p);
+  }
+
+  return construct_primes(primes);
+}
+
+int main() {
+  assert(prime(0) == std::vector<unsigned>{});
+  assert(prime(1) == std::vector<unsigned>{});
+  assert(prime(2) == std::vector<unsigned>{2});
+  assert((prime(23) == std::vector<unsigned>{2, 3, 5, 7, 11, 13, 17, 19, 23}));
+  assert(
+      (prime(30) == std::vector<unsigned>{2, 3, 5, 7, 11, 13, 17, 19, 23, 29}));
+
+  return 0;
+}

README.md

@@ -98,6 +98,7 @@
 - [Greed is Good](https://www.codewars.com/kata/5270d0d18625160ada0000e4) - [solution](5kyu/greed_is_good)
 - [Snakes and Ladders](https://www.codewars.com/kata/587136ba2eefcb92a9000027) - [solution](5kyu/snakes_and_ladders)
 - [The Clockwise Spiral](https://www.codewars.com/kata/536a155256eb459b8700077e) - [solution](5kyu/the_clockwise_spiral)
+- [(Ready for) Prime Time](https://www.codewars.com/kata/521ef596c106a935c0000519) - [solution](5kyu/prime_time)
 
 ### Python