1 files changed, 81 insertions, 0 deletions

diff --git a/day02/solution.cpp b/day02/solution.cpp
new file mode 100644
index 0000000..e4f47b8
--- /dev/null
+++ b/day02/solution.cpp
@@ -0,0 +1,81 @@
+#include <numeric>
+#include <cassert>
+#include <print>
+#include <iostream>
+#include <vector>
+#include <ranges>
+#include <algorithm>
+#include <sstream>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+const auto parse_input() {
+  std::vector<std::vector<int>> result;
+  for (std::string s; std::getline(std::cin, s);) {
+    std::stringstream ss{s};
+    std::vector<int> row;
+    for (int l; ss >> l;) {
+      row.push_back(l);
+    }
+    result.push_back(row);
+  }
+  return result;
+}
+
+bool safe_report(const auto &report) {
+  assert(report.size() > 0);
+  std::vector<int> _diffs(report.size());
+  std::adjacent_difference(report.begin(), report.end(), _diffs.begin());
+
+  // The first diff is from 0 (zero) to the first layer, so we ignore it
+  const auto diffs = ranges::subrange(_diffs.begin() + 1, _diffs.end());
+  const bool ascending = std::count_if(diffs.begin(), diffs.end(), [](int d) {
+    return d > 0;
+  }) > (diffs.size() / 2);
+
+  const bool safe = ranges::all_of(diffs, [ascending](int d) {
+    const bool bounded = (1 <= std::abs(d) and std::abs(d) <= 3);
+    const bool monotonic = ((d > 0 and ascending) or (d < 0 and not ascending));
+    return monotonic and bounded;
+  });
+  return safe;
+}
+
+void part1(const auto &input) {
+  int answer{0};
+
+  for (const auto &report : input) {
+    if (safe_report(report)) {
+      ++answer;
+    }
+  }
+
+  std::println("{}", answer);
+}
+
+void part2(const auto &input) {
+  int answer{0};
+  for (const auto &report : input) {
+    if (ranges::any_of(views::iota(size_t(0), report.size()), [&report](size_t i) {
+      return safe_report(views::concat(report | views::take(i),
+                                       report | views::drop(i + 1)));
+    })) {
+      ++answer;
+    }
+  }
+  std::println("{}", answer);
+}
+
+int main() {
+  const auto input = parse_input();
+
+#ifndef NO_PART1
+  part1(input);
+#endif
+
+#ifndef NO_PART2
+  part2(input);
+#endif
+  return 0;
+}