Add 2021 day 3 puzzles

2021/puzzle-03-01.cc

@@ -0,0 +1,44 @@
+//
+// Created by Matthew Gretton-Dann on 03/12/2021.
+//
+
+#include <cassert>
+#include <iostream>
+#include <string>
+#include <vector>
+
+auto main() -> int
+{
+  std::vector<unsigned> bit_counts;
+  std::string line;
+  unsigned line_count{0};
+  while (std::getline(std::cin, line)) {
+    ++line_count;
+    if (bit_counts.empty()) {
+      bit_counts.resize(line.size(), 0);
+    }
+    assert(bit_counts.size() == line.size());
+    for (std::size_t i{0}; i < line.size(); ++i) {
+      if (line[i] == '1') {
+        bit_counts[i]++;
+      }
+    }
+  }
+
+  std::uint64_t gamma_rate{0};
+  std::uint64_t epsilon_rate{0};
+  for (auto count : bit_counts) {
+    gamma_rate <<= 1;
+    epsilon_rate <<= 1;
+    if (count > line_count / 2) {
+      gamma_rate |= 1;
+    }
+    else {
+      epsilon_rate |= 1;
+    }
+  }
+
+  std::cout << "Gamma rate * epsilon rate = " << gamma_rate << " * " << epsilon_rate << " = "
+            << gamma_rate * epsilon_rate << '\n';
+  return 0;
+}