aoc2020/day_2/main.cpp

#include <iostream>
#include <fstream>

void part1(std::ifstream& input, std::string& line) {
    int valid_count = 0;

    while (std::getline(input, line)) {
        size_t fp_i = line.find(' '); // Index of the first space in line
        size_t dash_i = line.find('-'); // Index of the only dash in line
        std::string password = line.substr(line.find(": ") + 2);

        char letter = line[fp_i + 1];
        int low = std::stoi(line.substr(0, dash_i));
        int high = std::stoi(line.substr(dash_i + 1, fp_i));

        int letter_count = std::count(password.cbegin(), password.cend(), letter);
        if (letter_count <= high && letter_count >= low) valid_count++;
    }

    std::cout << valid_count << std::endl;
}

void part2(std::ifstream& input, std::string& line) {
    int valid_count = 0;

    while(std::getline(input, line)) {
        size_t fp_i = line.find(' '); // Index of the first space in line
        size_t dash_i = line.find('-'); // Index of the only dash in line
        std::string password = line.substr(line.find(": ") + 2);

        char letter = line[fp_i + 1];
        size_t i1 = std::stoi(line.substr(0, dash_i)) - 1;
        size_t i2 = std::stoi(line.substr(dash_i + 1, fp_i)) - 1;


        if (password[i1] == password[i2]) continue; // If they are the same we can skip this line
        if (password[i1] == letter || password[i2] == letter) valid_count++;
    }

    std::cout << valid_count << std::endl;
}

int main() {
    std::ifstream input("../input.txt");
    std::string line;

    part1(input, line);

    input.clear(); // clear EOF flag
    input.seekg(0);

    part2(input, line);

    return 0;
}