Initial commit

2015/puzzle-20-01.cc

@@ -0,0 +1,37 @@
+#include <string>
+#include <iostream>
+#include <cstdlib>
+
+int main() {
+    std::string line;
+    std::getline(std::cin, line);
+
+    auto target{std::stoul(line)};
+
+    /* This is a really stupid way to do this in terms of effort, but it produces the right answer and doesn't involve
+     * thinking too much:
+     *
+     * n is the current house we're at.
+     */
+    for (auto n{2UL}; true; ++n) {
+        auto amt{0UL};
+        /* Find the number of presents delivered to house `n`.  We do this by walking through all numbers <= sqrt(n) and
+         * seeing if they are a factor.  If so we add presents for that number (i) and also (n/i), being careful not to
+         * double count for square roots.  This reduces the amount of work we have to do significantly.
+         */
+        for (auto i{1UL}; i <= (n / i); ++i) {
+            if (n % i == 0) {
+                amt += i * 10;
+                if (i != n / i) {
+                    amt += (n / i) * 10;
+                }
+            }
+        }
+        if (amt >= target) {
+            std::cout << "Target: " << target << " met at: " << n << " with amount: " << amt << '\n';
+            return EXIT_SUCCESS;
+        }
+    }
+
+    return EXIT_FAILURE;
+}