aoc2025: Implement day2 solution

userspace/aoc2025/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(AOC2025_PROJECTS
 	day1
+	day2
 	full
 )
 

userspace/aoc2025/day2/CMakeLists.txt

@@ -0,0 +1,9 @@
+set(SOURCES
+	main.cpp
+)
+
+add_executable(aoc2025_day2 ${SOURCES})
+banan_include_headers(aoc2025_day2 ban)
+banan_link_library(aoc2025_day2 libc)
+
+install(TARGETS aoc2025_day2 OPTIONAL)

userspace/aoc2025/day2/main.cpp

@@ -0,0 +1,106 @@
+#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;
+
+i64 part1(FILE* fp)
+{
+	const auto is_invalid_id =
+		[](u64 id) -> bool
+		{
+			u64 temp_id = id;
+			u64 repeat = 0;
+			u64 mult = 1;
+
+			while (repeat * mult + repeat < id)
+			{
+				repeat += (temp_id % 10) * mult;
+				temp_id /= 10;
+				mult *= 10;
+			}
+
+			if (temp_id < mult / 10)
+				return false;
+			return repeat * mult + repeat == id;
+		};
+
+	u64 result = 0;
+
+	u64 start, end;
+	while (fscanf(fp, "%" SCNu64 "-%" SCNu64 ",", &start, &end) >= 2)
+		for (u64 id = start; id <= end; id++)
+			if (is_invalid_id(id))
+				result += id;
+
+	return result;
+}
+
+i64 part2(FILE* fp)
+{
+	const auto is_invalid_id =
+		[](u64 id) -> bool
+		{
+			u64 temp_id = id;
+			u64 repeat = 0;
+			u64 mult = 1;
+
+			while (repeat * mult + repeat < id)
+			{
+				repeat += (temp_id % 10) * mult;
+				temp_id /= 10;
+				mult *= 10;
+
+				if (repeat < mult / 10)
+					continue;
+
+				u64 attempt = repeat * mult + repeat;
+				while (attempt < id)
+					attempt = attempt * mult + repeat;
+				if (attempt == id)
+					return true;
+			}
+
+			return false;
+		};
+
+	u64 result = 0;
+
+	u64 start, end;
+	while (fscanf(fp, "%" SCNu64 "-%" SCNu64 ",", &start, &end) >= 2)
+		for (u64 id = start; id <= end; id++)
+			if (is_invalid_id(id))
+				result += id;
+
+	return result;
+}
+
+int main(int argc, char** argv)
+{
+	const char* file_path = "/usr/share/aoc2025/day2_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);
+}