1 files changed, 119 insertions, 0 deletions

diff --git a/day04/solution.cpp b/day04/solution.cpp
new file mode 100644
index 0000000..6fa5ccd
--- /dev/null
+++ b/day04/solution.cpp
@@ -0,0 +1,119 @@
+#include <array>
+#include <iterator>
+#include <print>
+#include <iostream>
+#include <ranges>
+#include <string_view>
+#include <algorithm>
+#include <functional>
+#include <utility>
+#include <vector>
+#include <tuple>
+#include <map>
+#include <unordered_map>
+#include <set>
+#include <unordered_set>
+#include <numeric>
+
+using namespace std::literals;
+namespace views = std::views;
+namespace ranges = std::ranges;
+
+const auto parse_input() {
+  std::vector<std::string> grid;
+
+  for (std::string line; std::getline(std::cin, line);) {
+    grid.push_back(std::move(line));
+  }
+
+  return grid;
+}
+
+using IndexPair = std::pair<int, int>;
+
+void part1(auto input) {
+  uint64_t answer{0};
+  const int width(input[0].size());
+  const int height(input.size());
+
+  constexpr auto zeros = views::repeat(0, 4);
+  constexpr auto incr = views::iota(0, 4);
+  constexpr auto decr = incr | views::transform(std::negate<>{});
+  // Pack directions into a tuple because they have different types
+  constexpr auto directions = std::tuple{
+    views::zip(decr, zeros),    // top
+    views::zip(decr, incr),     // top-right
+    views::zip(zeros, incr),    // right
+    views::zip(incr, incr),     // bottom-right
+    views::zip(incr, zeros),    // bottom
+    views::zip(incr, decr),     // bottom-left
+    views::zip(zeros, decr),    // left
+    views::zip(decr, decr),     // top-left
+  };
+
+  for (const auto [i, j] :
+         views::cartesian_product(views::iota(0, width),
+                                  views::iota(0, height))) {
+
+    // std::apply is needed to unfold the ‘directions’ tuple.
+    std::apply([=, &answer](auto &&... dirs) {
+      auto add_offset = [i, j](IndexPair e) {
+        const auto [di, dj] = e;
+        return std::tuple{i + di, j + dj};
+      };
+
+      auto in_bounds = [width, height](IndexPair e) -> bool {
+        const auto [i, j] = e;
+        return 0 <= i and i < height and 0 <= j and j < width;
+      };
+
+      auto get_input_element = [input](IndexPair e) -> char {
+        const auto [i, j] = e;
+        return input[i][j];
+      };
+
+      (((answer += ranges::equal(dirs
+                                 | views::transform(add_offset)
+                                 | views::filter(in_bounds)
+                                 | views::transform(get_input_element), "XMAS"sv))), ...);
+    }, directions);
+  }
+
+  std::println("{}", answer);
+}
+
+void part2(auto input) {
+  uint64_t answer{0};
+  int width(input[0].size());
+  int height(input.size());
+
+  constexpr size_t window_size = 3;
+  for (auto &&v_window : input | views::slide(window_size)) {
+    for (const auto &[r1, r2, r3] : views::zip(v_window[0] | views::slide(window_size),
+                                               v_window[1] | views::slide(window_size),
+                                               v_window[2] | views::slide(window_size))) {
+      const std::array<char, 3> diagonals[] = {
+        {r1[0], r2[1], r3[2]},
+        {r3[0], r2[1], r1[2]},
+      };
+
+      answer += ranges::all_of(diagonals, [](const auto &diag) {
+        return ranges::equal(diag, "MAS"sv) or ranges::equal(diag, "SAM"sv);
+      });
+    }
+  }
+  std::println("{}", answer);
+}
+
+int main() {
+  const auto input = parse_input();
+
+#ifndef NO_PART1
+  part1(input);
+#endif
+
+#ifndef NO_PART2
+  part2(input);
+#endif
+  return 0;
+}