From d3622ff806436bcfebb1e5da68e9a9c3d8a151dd Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Tue, 7 Dec 2021 16:09:20 +0000
Subject: [PATCH] Add 2016 day 20 puzzles

---
 2016/puzzle-20-01.cc |  87 +++++++++++++++++++++++++++++++++++++
 2016/puzzle-20-02.cc | 101 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 188 insertions(+)
 create mode 100644 2016/puzzle-20-01.cc
 create mode 100644 2016/puzzle-20-02.cc

diff --git a/2016/puzzle-20-01.cc b/2016/puzzle-20-01.cc
new file mode 100644
index 0000000..0e6afc0
--- /dev/null
+++ b/2016/puzzle-20-01.cc
@@ -0,0 +1,87 @@
+#include <iostream>
+#include <regex>
+#include <string>
+
+struct Ranges
+{
+  void add(std::uint32_t low, std::uint32_t high)
+  {
+    static constexpr auto max{std::numeric_limits<std::uint32_t>::max()};
+    auto it{ranges_.begin()};
+    while (it != ranges_.end()) {
+      if (low != 0 && it->second < low - 1) {
+        ++it;
+      }
+      else if (high == max || high + 1 < it->first) {
+        ranges_.insert(it, {low, high});
+        return;
+      }
+      else {
+        it->first = std::min(low, it->first);
+        it->second = std::max(high, it->second);
+        auto it2{it + 1};
+        while (it2 != ranges_.end() && (it->second == std::numeric_limits<std::uint32_t>::max() ||
+                                        it2->first <= it->second + 1)) {
+          it->second = it2->second;
+          ++it2;
+        }
+        ranges_.erase(it + 1, it2);
+        return;
+      }
+    }
+
+    ranges_.push_back({low, high});
+  }
+
+  Ranges inverse() const
+  {
+    Ranges result;
+    auto it{ranges_.begin()};
+    if (it == ranges_.end()) {
+      result.ranges_.push_back({0, std::numeric_limits<std::uint32_t>::max()});
+      return result;
+    }
+    if (it->first != 0) {
+      result.ranges_.push_back({0, it->first - 1});
+    }
+    auto low{it->second + 1};
+    while (it != ranges_.end()) {
+      result.ranges_.push_back({low, it->first - 1});
+      low = it->second + 1;
+      ++it;
+    }
+    if (low != 0) {
+      result.ranges_.push_back({low, std::numeric_limits<std::uint32_t>::max()});
+    }
+    return result;
+  }
+
+  auto lowest() const -> std::uint32_t
+  {
+    if (ranges_.empty()) {
+      return std::numeric_limits<std::uint32_t>::max();
+    }
+    return ranges_.front().first;
+  }
+
+private:
+  std::vector<std::pair<std::uint32_t, std::uint32_t>> ranges_;
+};
+
+auto main() -> int
+{
+  static std::regex match_re{"(\\d+)-(\\d+)"};
+  std::string line;
+  std::smatch m;
+  Ranges ranges;
+  while (std::getline(std::cin, line)) {
+    if (!std::regex_search(line, m, match_re)) {
+      std::cerr << "Unable to match: " << line << '\n';
+      return 1;
+    }
+    ranges.add(std::stoul(m.str(1)), std::stoul(m.str(2)));
+  }
+
+  std::cout << "First available IP address: " << ranges.inverse().lowest() << '\n';
+  return 0;
+}
\ No newline at end of file
diff --git a/2016/puzzle-20-02.cc b/2016/puzzle-20-02.cc
new file mode 100644
index 0000000..7eb7ca5
--- /dev/null
+++ b/2016/puzzle-20-02.cc
@@ -0,0 +1,101 @@
+#include <iomanip>
+#include <iostream>
+#include <numeric>
+#include <regex>
+#include <string>
+
+struct Ranges
+{
+  auto operator==(Ranges const& rhs) const noexcept { return ranges_ == rhs.ranges_; }
+
+  void add(std::uint32_t low, std::uint32_t high)
+  {
+    static constexpr auto max{std::numeric_limits<std::uint32_t>::max()};
+    auto it{ranges_.begin()};
+    while (it != ranges_.end()) {
+      if (low != 0 && it->second < low - 1) {
+        ++it;
+      }
+      else if (high == max || high + 1 < it->first) {
+        ranges_.insert(it, {low, high});
+        return;
+      }
+      else {
+        it->first = std::min(low, it->first);
+        it->second = std::max(high, it->second);
+        auto it2{it + 1};
+        while (it2 != ranges_.end() && (it->second == std::numeric_limits<std::uint32_t>::max() ||
+                                        it2->first <= it->second + 1)) {
+          it->second = std::max(it->second, it2->second);
+          ++it2;
+        }
+        ranges_.erase(it + 1, it2);
+        return;
+      }
+    }
+
+    ranges_.emplace_back(low, high);
+  }
+
+  [[nodiscard]] auto inverse() const -> Ranges
+  {
+    Ranges result;
+    auto it{ranges_.begin()};
+    assert(it->first <= it->second);
+    if (it == ranges_.end()) {
+      result.ranges_.emplace_back(0, std::numeric_limits<std::uint32_t>::max());
+      return result;
+    }
+    if (it->first != 0) {
+      result.ranges_.emplace_back(0, it->first - 1);
+    }
+    auto low{it->second + 1};
+    for (++it; it != ranges_.end(); ++it) {
+      assert(low < it->first);
+      assert(it->first <= it->second);
+      result.ranges_.emplace_back(low, it->first - 1);
+      low = it->second + 1;
+    }
+    if (low != 0) {
+      result.ranges_.emplace_back(low, std::numeric_limits<std::uint32_t>::max());
+    }
+    return result;
+  }
+
+  [[nodiscard]] auto lowest() const -> std::uint32_t
+  {
+    if (ranges_.empty()) {
+      return std::numeric_limits<std::uint32_t>::max();
+    }
+    return ranges_.front().first;
+  }
+
+  [[nodiscard]] auto count() const -> std::uint32_t
+  {
+    return std::accumulate(ranges_.begin(), ranges_.end(), std::uint32_t{0},
+                           [](auto a, auto const& r) { return a + (r.second - r.first + 1); });
+  }
+
+private:
+  std::vector<std::pair<std::uint32_t, std::uint32_t>> ranges_;
+};
+
+auto main() -> int
+{
+  static std::regex match_re{"(\\d+)-(\\d+)"};
+  std::string line;
+  std::smatch m;
+  Ranges ranges;
+  while (std::getline(std::cin, line)) {
+    if (!std::regex_search(line, m, match_re)) {
+      std::cerr << "Unable to match: " << line << '\n';
+      return 1;
+    }
+    ranges.add(std::stoul(m.str(1)), std::stoul(m.str(2)));
+  }
+
+  auto inverse{ranges.inverse()};
+  std::cout << "First available IP address: " << inverse.lowest() << '\n';
+  std::cout << "Number of available IP addresses: " << inverse.count() << '\n';
+  return 0;
+}
\ No newline at end of file