From d38030e86355e2357ac84349198b638bd74a495e Mon Sep 17 00:00:00 2001
From: Orfeas <38209077+0xfea5@users.noreply.github.com>
Date: Mon, 8 Dec 2025 20:14:20 +0200
Subject: day04

---
 day04/solution.cpp | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 day04/solution.cpp

(limited to 'day04/solution.cpp')

diff --git a/day04/solution.cpp b/day04/solution.cpp
new file mode 100644
index 0000000..48afb58
--- /dev/null
+++ b/day04/solution.cpp
@@ -0,0 +1,82 @@
+#include <bits/stdc++.h>
+
+namespace views = std::views;
+namespace ranges = std::ranges;
+
+void pad_grid(std::vector<std::string>& input) {
+    for (size_t i = 0; i < input.size(); ++i) {
+        input[i] = "." + input[i] + ".";
+    }
+    int const row_size = input[0].size();
+    input.emplace(input.begin(), row_size, '.');
+    input.emplace_back(row_size, '.');
+}
+
+auto parse_input(std::istream& is) {
+    const std::string input = {
+        std::istreambuf_iterator<char>(is),
+        std::istreambuf_iterator<char>()
+    };
+
+    auto lines = views::split(input, '\n')
+        | views::filter([](auto&& line) { return not line.empty(); })
+        | ranges::to<std::vector<std::string>>();
+
+    pad_grid(lines);
+    return lines;
+}
+
+constexpr std::array<char, 3> tuple_to_array(std::tuple<char, char, char>&& t) {
+    return {std::get<0>(t), std::get<1>(t), std::get<2>(t)};
+}
+
+auto next_state(std::vector<std::string> const& input) {
+    int nremoved = 0;
+    auto forklift_pass = [&nremoved](auto&& columns) -> char {
+        if (columns[1][1] == '@' and ranges::count(columns | views::join, '@') < 5) {
+            ++nremoved;
+            return '.';
+        }
+        return columns[1][1];
+    };
+
+    std::vector<std::string> next_state;
+    for (auto const& rows : input | views::slide(3)) {
+        next_state.push_back(
+                             views::zip(rows[0], rows[1], rows[2])
+                                 | views::transform(tuple_to_array)
+                                 | views::slide(3)
+                                 | views::transform(forklift_pass)
+                                 | ranges::to<std::string>());
+    }
+    pad_grid(next_state);
+    return std::make_tuple(next_state, nremoved);
+}
+
+void part1(auto input) {
+    auto const [_, answer] = next_state(input);
+    std::println("{}", answer);
+}
+
+void part2(auto input) {
+    // Write second part solution here
+    int answer = 0;
+    while (true) {
+        auto const [_next_state, increment] = next_state(input);
+        if (increment == 0) {
+            break;
+        }
+        input = _next_state;
+        answer += increment;
+    }
+    std::println("{}", answer);
+}
+
+int main() {
+    auto input = parse_input(std::cin);
+
+    part1(input);
+    part2(input);
+
+    return 0;
+}
-- 
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