aoc2025: Implement day4 solution

userspace/aoc2025/CMakeLists.txt

@@ -2,6 +2,7 @@ set(AOC2025_PROJECTS
 	day1
 	day2
 	day3
+	day4
 	full
 )
 

userspace/aoc2025/day4/CMakeLists.txt

@@ -0,0 +1,9 @@
+set(SOURCES
+	main.cpp
+)
+
+add_executable(aoc2025_day4 ${SOURCES})
+banan_include_headers(aoc2025_day4 ban)
+banan_link_library(aoc2025_day4 libc)
+
+install(TARGETS aoc2025_day4 OPTIONAL)

userspace/aoc2025/day4/main.cpp

@@ -0,0 +1,125 @@
+#include <BAN/Vector.h>
+
+#include <inttypes.h>
+#include <stdio.h>
+
+using i8  = int8_t;
+using i16 = int16_t;
+using i32 = int32_t;
+using i64 = int64_t;
+
+using u8  = uint8_t;
+using u16 = uint16_t;
+using u32 = uint32_t;
+using u64 = uint64_t;
+
+using Grid = BAN::Vector<BAN::Vector<bool>>;
+
+static Grid parse_grid(FILE* fp)
+{
+	Grid grid;
+	MUST(grid.emplace_back());
+
+	char ch;
+	while ((ch = fgetc(fp)) != EOF)
+	{
+		switch (ch)
+		{
+			case '.':
+			case '@':
+				MUST(grid.back().push_back(ch == '@'));
+				break;
+			case '\n':
+				MUST(grid.emplace_back());
+				break;
+		}
+	}
+
+	while (grid.back().empty())
+		grid.pop_back();
+
+	return grid;
+}
+
+static i32 count_neighborgs(const Grid& grid, size_t x, size_t y)
+{
+	i32 count = 0;
+	for (i32 yoff = -1; yoff <= 1; yoff++)
+	{
+		for (i32 xoff = -1; xoff <= 1; xoff++)
+		{
+			if (yoff == 0 && xoff == 0)
+				continue;
+			if (y + yoff >= grid.size())
+				continue;
+			if (x + xoff >= grid[y + yoff].size())
+				continue;
+			count += grid[y + yoff][x + xoff];
+		}
+	}
+	return count;
+}
+
+i64 part1(FILE* fp)
+{
+	i32 result = 0;
+
+	auto grid = parse_grid(fp);
+	for (size_t y = 0; y < grid.size(); y++)
+		for (size_t x = 0; x < grid[y].size(); x++)
+			if (grid[y][x] && count_neighborgs(grid, x, y) < 4)
+				result++;
+
+	return result;
+}
+
+i64 part2(FILE* fp)
+{
+	i32 result = 0;
+
+	auto grid = parse_grid(fp);
+
+	for (;;)
+	{
+		const auto old_result = result;
+
+		for (size_t y = 0; y < grid.size(); y++)
+		{
+			for (size_t x = 0; x < grid[y].size(); x++)
+			{
+				if (!grid[y][x] || count_neighborgs(grid, x, y) >= 4)
+					continue;
+				grid[y][x] = false;
+				result++;
+			}
+		}
+
+		if (result == old_result)
+			break;
+	}
+
+	return result;
+}
+
+int main(int argc, char** argv)
+{
+	const char* file_path = "/usr/share/aoc2025/day4_input.txt";
+
+	if (argc >= 2)
+		file_path = argv[1];
+
+	FILE* fp = fopen(file_path, "r");
+	if (fp == nullptr)
+	{
+		perror("fopen");
+		return 1;
+	}
+
+	printf("part1: %" PRId64 "\n", part1(fp));
+
+	fseek(fp, 0, SEEK_SET);
+
+	printf("part2: %" PRId64 "\n", part2(fp));
+
+	fclose(fp);
+}