Add 2021 day 6 puzzles

2021/puzzle-06-01.cc

@@ -0,0 +1,46 @@
+//
+// Created by Matthew Gretton-Dann on 06/12/2021.
+//
+
+#include <iostream>
+#include <numeric>
+#include <string>
+#include <vector>
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read input.\n";
+    return 1;
+  }
+
+  std::vector<std::uint64_t> fish(9, 0);
+  for (auto c : line) {
+    if (c == ',') {
+      continue;
+    }
+    if (c >= '0' && c <= '8') {
+      ++fish.at(c - '0');
+    }
+    else {
+      std::cerr << "Can't interpret " << c << '\n';
+      return 1;
+    }
+  }
+
+  constexpr unsigned days_wanted{80};
+  for (unsigned day{0}; day < days_wanted; ++day) {
+    std::vector<std::uint64_t> new_fish;
+    std::copy(fish.begin() + 1, fish.end(), std::back_inserter(new_fish));
+    new_fish.at(6) += fish.at(0);
+    new_fish.push_back(fish.at(0));
+    std::swap(fish, new_fish);
+  }
+
+  std::cout << "Fish count after " << days_wanted << " days: "
+            << std::accumulate(fish.begin(), fish.end(), std::uint64_t{0},
+                               [](auto a, auto b) { return a + b; })
+            << '\n';
+  return 0;
+}

2021/puzzle-06-02.cc

@@ -0,0 +1,46 @@
+//
+// Created by Matthew Gretton-Dann on 06/12/2021.
+//
+
+#include <iostream>
+#include <numeric>
+#include <string>
+#include <vector>
+
+auto main() -> int
+{
+  std::string line;
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read input.\n";
+    return 1;
+  }
+
+  std::vector<std::uint64_t> fish(9, 0);
+  for (auto c : line) {
+    if (c == ',') {
+      continue;
+    }
+    if (c >= '0' && c <= '8') {
+      ++fish.at(c - '0');
+    }
+    else {
+      std::cerr << "Can't interpret " << c << '\n';
+      return 1;
+    }
+  }
+
+  constexpr unsigned days_wanted{256};
+  for (unsigned day{0}; day < days_wanted; ++day) {
+    std::vector<std::uint64_t> new_fish;
+    std::copy(fish.begin() + 1, fish.end(), std::back_inserter(new_fish));
+    new_fish.at(6) += fish.at(0);
+    new_fish.push_back(fish.at(0));
+    std::swap(fish, new_fish);
+  }
+
+  std::cout << "Fish count after " << days_wanted << " days: "
+            << std::accumulate(fish.begin(), fish.end(), std::uint64_t{0},
+                               [](auto a, auto b) { return a + b; })
+            << '\n';
+  return 0;
+}