Advent of Code 2025 - December 2nd

[MyrinNew]

In this series, I'll share my progress with the 2025 version of Advent of Code.


Check the first post for a short intro to this series.


You can also follow my progress on GitHub.


December 2nd

The puzzle of day 2 was not that hard. I feared my simplistic solution for part one would not cut it for part two, but it was actually pretty easy to create a new isInvalidId function for part two, and retrofit that to part one 😅


My pitfall for this puzzle: Had to look a couple of C++ tips & tricks, but nothing major.



Solution here, do not click if you want to solve the puzzle first yourself






#include
#include
#include
#include


void extractRange(const std::string &range, std::vector<std::string> &result) {
const auto from = std::stol(range.substr(0, range.find('-')));
const auto to = std::stol(range.substr(range.find('-') + 1));
for (auto i = from; i <= to; ++i) {
result.push_back(std::to_string(i));
}
}

std::vector<std::string> loadInput(const std::string &filename) {
std::vector<std::string> result;
std::ifstream file(filename);
std::string line;
std::getline(file, line);
size_t pos = 0;
while ((pos = line.find(',')) != std::string::npos) {
auto range = line.substr(0, pos);
extractRange(range, result);
line.erase(0, pos + 1);
}
extractRange(line, result);
return result;
}


bool isInvalidId(const std::string &id, const size_t maxParts = 2) {
for (int i = 2; i <= maxParts; ++i) {
if (id.length() % i == 0) {
auto s = std::string();
auto part = id.substr(0, id.length() / i);
std:stringstream os;
for (int j = 0; j < i; j++) {
os << part;
}
if (id == os.str()) {
return true;
}
}
}
return false;
}

void partOne() {
const auto ids = loadInput("/Users/rob/projects/robvanderleek/adventofcode/2025/02/input.txt");
long result = 0;
for (const auto &id : ids) {
if (isInvalidId(id)) {
result += std::stol(id);
}
}
std::cout << result << std::endl;
assert(result == 5398419778);
}

void partTwo() {
const auto ids = loadInput("/Users/rob/projects/robvanderleek/adventofcode/2025/02/input.txt");
long long result = 0;
for (const auto &id : ids) {
if (isInvalidId(id, id.length())) {
result += std::stol(id);
}
}
std::cout << result << std::endl;
assert(result == 15704845910);
}

int main() {
partOne();
partTwo();
return 0;
}










That's it! See you again tomorrow!




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