Add part one of a 2021 day

adventofcode/2021/day/3/binary_diagnostic.cpp

@@ -0,0 +1,33 @@
+#include <iostream>
+#include <string>
+#include <string_view>
+#include <vector>
+
+int bin2dec(std::string_view bin) {
+    int base{1};
+    int ans{0};
+    for (ssize_t i = bin.size() - 1; i >= 0; i--) {
+        ans += (bin[i] - '0')*base;
+        base *= 2;
+    }
+    return ans;
+}
+
+int main() {
+    size_t num_numbers{0};
+    std::string line;
+    std::vector<size_t> ones_count;
+    while (std::getline(std::cin, line)) {
+        num_numbers++;
+        if (!ones_count.size())
+            ones_count = std::vector<size_t>(line.length(), 0);
+        size_t i{0};
+        for (const char c : line) ones_count[i++] += c == '1';
+    }
+    std::string gamma_rate, epsilon_rate;
+    for (const size_t count : ones_count) {
+        gamma_rate.append(  count > num_numbers / 2 ? "1" : "0");
+        epsilon_rate.append(count > num_numbers / 2 ? "0" : "1");
+    }
+    std::cout << "Part one: " << bin2dec(gamma_rate) * bin2dec(epsilon_rate) << '\n';
+}