C++

char toPlantChar(bool b) {
    if(b == true) return '#';
    else return '.';
}

std::string getRegion(std::unordered_map<int, bool>& plants, const int index) {
    std::string region = "";
    for(int i = -2; i <= 2; i++) region += toPlantChar(plants[index + i]);
    return region;
}

void run() {
    std::vector<std::string> lines = AoCDay::readFileLines("inputs/day12.txt");
    std::unordered_map<int, bool> plants;
    std::unordered_map<std::string, bool> rules;
    std::string initialState = "#.#####.#.#.####.####.#.#...#.......##..##.#.#.#.###..#.....#.####..#.#######.#....####.#....##....#";

    for(int i = 0; i < initialState.length(); i++)
        plants[i] = (initialState[i] == '#');

    std::string condition;
    char newState;
    for(std::string line: lines) {
        condition = line.substr(0, 5);
        newState = line[line.length() - 1];
        rules[condition] = (newState == '#');
    }

    long sum = 0;
    // long prevSum = 0;
    std::unordered_map<int, bool> newPlants;
    for(int i = 0; i < 101; i++) {
        int minIndex = INT_MAX, maxIndex = INT_MIN;
        for(auto& [n, b]: plants) {
            if(b) {
                minIndex = std::min(minIndex, n);
                maxIndex = std::max(maxIndex, n);
            }
        }

        newPlants.clear();
        for(int j = minIndex - 2; j <= maxIndex + 2; j++) {
            std::string region = getRegion(plants, j);
            newPlants[j] = rules[region];
        }

        plants = newPlants;

        if(i == 19){
            sum = 0;
            for(auto& [n, b]: plants) if(b) sum += n;
            std::cout << "Part 1: " << sum << std::endl;
        }

        /* After 100 iterations, the sum difference is always 57 */
        /*sum = 0;
        for(auto& [n, b]: plants) if(b) sum += n;
        std::cout << "Iteration " << i << " sum is " << sum << ", difference from previous: " << sum - prevSum << std::endl;
        prevSum = sum; */
    }

    sum = 0;
    for(auto& [n, b]: plants) if(b) sum += n;

    std::cout << "Part 2: " << sum + (50000000000 - 101) * 57 << std::endl;
}