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;
+}

2021/puzzle-03-02.cc

@@ -0,0 +1,81 @@
+//
+// Created by Matthew Gretton-Dann on 03/12/2021.
+//
+
+#include <cassert>
+#include <iostream>
+#include <numeric>
+#include <string>
+#include <vector>
+
+auto to_num(std::string const& s) -> std::uint64_t
+{
+  std::uint64_t result{0};
+  for (auto c : s) {
+    result <<= 1;
+    result |= (c == '1' ? 1 : 0);
+  }
+  return result;
+}
+
+auto find_oxygen_rating(std::vector<std::string> const& lines) -> std::string
+{
+  std::vector<std::string> data{lines};
+  assert(!data.empty());
+  std::size_t len{data[1].size()};
+  for (std::size_t i{0}; i < len; ++i) {
+    auto one_bits{std::accumulate(
+      data.begin(), data.end(), std::uint64_t{0},
+      [i](std::uint64_t a, std::string const& s) { return a + (s[i] == '1' ? 1 : 0); })};
+    char keep{(one_bits * 2 >= data.size()) ? '1' : '0'};
+    std::cout << "Size of data = " << data.size() << " Number of one bits = " << one_bits
+              << " keeping " << keep << '\n';
+    auto it{std::remove_if(data.begin(), data.end(),
+                           [keep, i](std::string const& s) { return s[i] != keep; })};
+    data.erase(it, data.end());
+    if (data.size() <= 1) {
+      break;
+    }
+  }
+  assert(data.size() == 1);
+  return data[0];
+}
+
+auto find_co2_rating(std::vector<std::string> const& lines) -> std::string
+{
+  std::vector<std::string> data{lines};
+  assert(!data.empty());
+  std::size_t len{data[1].size()};
+  for (std::size_t i{0}; i < len; ++i) {
+    auto one_bits{std::accumulate(
+      data.begin(), data.end(), std::uint64_t{0},
+      [i](std::uint64_t a, std::string const& s) { return a + (s[i] == '1' ? 1 : 0); })};
+    char keep{(one_bits * 2 >= data.size()) ? '0' : '1'};
+    std::cout << "Size of data = " << data.size() << " Number of one bits = " << one_bits
+              << " keeping " << keep << '\n';
+    auto it{std::remove_if(data.begin(), data.end(),
+                           [keep, i](std::string const& s) { return s[i] != keep; })};
+    data.erase(it, data.end());
+    if (data.size() <= 1) {
+      break;
+    }
+  }
+  assert(data.size() == 1);
+  return data[0];
+}
+
+auto main() -> int
+{
+  std::vector<std::string> lines;
+  std::string line;
+  while (std::getline(std::cin, line)) {
+    lines.push_back(line);
+  }
+
+  auto oxygen_rating{to_num(find_oxygen_rating(lines))};
+  auto co2_rating{to_num(find_co2_rating(lines))};
+
+  std::cout << "Oxygen generator rating * CO2 scrubber rating = " << oxygen_rating << " * "
+            << co2_rating << " = " << oxygen_rating * co2_rating << '\n';
+  return 0;
+}