finish day 4

day4.cpp

@@ -0,0 +1,152 @@
+//
+// Created by Grant Horner on 12/4/25.
+//
+
+#include <ranges>
+#include <string>
+#include <vector>
+
+#include "doctest.h"
+#include "utils.h"
+
+namespace day4 {
+const std::string sample =
+        "..@@.@@@@.\n"
+        "@@@.@.@.@@\n"
+        "@@@@@.@.@@\n"
+        "@.@@@@..@.\n"
+        "@@.@@@@.@@\n"
+        ".@@@@@@@.@\n"
+        ".@.@.@.@@@\n"
+        "@.@@@.@@@@\n"
+        ".@@@@@@@@.\n"
+        "@.@.@@@.@.\n";
+
+enum class Point : char {
+    Paper = '@',
+    Empty = '.',
+};
+
+template<typename T>
+std::array<char, 8> get_surrounding(const std::vector<T> &lines, size_t x, size_t y) noexcept {
+    std::array<char, 8> points = {};
+    size_t i = 0;
+    for (int dy = -1; dy < 2; ++dy) {
+        if (y + dy < 0 || y + dy >= lines.size()) { continue; }
+        for (int dx = -1; dx < 2; ++dx) {
+            if (x + dx < 0 || x + dx >= lines[0].size()) { continue; }
+            if (dx == 0 && dy == 0) continue;
+
+            points[i] = lines[y + dy][x + dx];
+            i++;
+        }
+    }
+
+    return points;
+}
+
+uint32_t solve_first(const std::string_view input) noexcept {
+    const auto lines = std::string_view{input}
+                       | std::views::split('\n')
+                       | std::views::transform(to_string_view)
+                       | std::views::filter([](const auto s) { return !s.empty(); })
+                       | std::ranges::to<std::vector>();
+
+    uint32_t accessible = 0;
+
+    for (size_t y = 0; y < lines.size(); ++y) {
+        for (size_t x = 0; x < lines[0].size(); ++x) {
+            const auto c = lines[y][x];
+            if (c != '@') continue;
+            size_t num_paper = 0;
+            for (const char p: get_surrounding(lines, x, y)) {
+                if (p == '@') {
+                    num_paper++;
+                }
+                if (num_paper == 4) {
+                    break;
+                }
+            }
+
+            if (num_paper < 4) {
+                accessible++;
+            }
+        }
+    }
+
+    return accessible;
+}
+
+template<typename T>
+std::vector<std::tuple<size_t, size_t> > remove_paper(const std::vector<T> &lines) {
+    std::vector<std::tuple<size_t, size_t> > removable = {};
+
+    for (size_t y = 0; y < lines.size(); ++y) {
+        for (size_t x = 0; x < lines[0].size(); ++x) {
+            const auto c = lines[y][x];
+            if (c != '@') continue;
+            size_t num_paper = 0;
+            for (const char p: get_surrounding(lines, x, y)) {
+                if (p == '@') {
+                    num_paper++;
+                }
+                if (num_paper == 4) {
+                    break;
+                }
+            }
+
+            if (num_paper < 4) {
+                removable.emplace_back(x, y);
+            }
+        }
+    }
+
+    return removable;
+}
+
+uint32_t solve_second(const std::string_view input) {
+    std::vector<std::string> lines = std::string_view{input}
+                                     | std::views::split('\n')
+                                     | std::views::transform([](const auto r) {
+                                         return std::string(r.data(), r.size());
+                                     })
+                                     | std::views::filter([](const auto &s) { return !s.empty(); })
+                                     | std::ranges::to<std::vector>();
+    bool removed = false;
+    uint32_t total_removed = 0;
+    do {
+        removed = false;
+        const auto removable = remove_paper(lines);
+        if (!removable.empty()) {
+            removed = true;
+        }
+        for (auto [x, y]: removable) {
+            lines[y][x] = '.';
+            total_removed++;
+        }
+    } while (removed);
+
+    return total_removed;
+}
+
+TEST_SUITE_BEGIN("Day 4");
+
+TEST_CASE("solves first sample") {
+    CHECK(solve_first(sample) == 13);
+}
+
+TEST_CASE("solves first puzzle") {
+    CHECK(solve_first(read_file("day4input.txt")) == 1626);
+}
+
+
+TEST_CASE("solves second sample") {
+    CHECK(solve_second(sample) == 43);
+}
+
+TEST_CASE("solves second puzzle") {
+    CHECK(solve_second(read_file("day4input.txt")) == 9173);
+}
+
+TEST_SUITE_END();
+}