advent-of-code-2020/day2/day2.cpp

#include <algorithm>
#include <exception>
#include <fstream>
#include <iostream>
#include <regex>
#include <string>
#include <vector>

const std::string POLICY_PATTERN = "([0-9]+)-([0-9]+) (.)";
const std::string DELIM = ": ";

/**
 * Policy represents a password policy
 */
class Policy {
	public:
	Policy(int min, int max, char letter) : min(min), max(max), letter(letter) {
	}

	/**
	 * Parse a policy from an input string
	 * @param input The input to parse
	 * @return Policy The policy from the input
	 */
	static Policy parse(const std::string &input) {
		std::regex expression(POLICY_PATTERN);
		std::smatch matches;

		if (!std::regex_search(input, matches, expression)) {
			throw new std::invalid_argument("Invalid string");
		}

		return Policy(std::stoi(matches[1]), std::stoi(matches[2]), matches.str(3).at(0));
	}

	int getMin() const {
		return this->min;
	}

	int getMax() const {
		return this->max;
	}

	char getLetter() const {
		return this->letter;
	}

	private:
	int min;
	int max;
	char letter;
};

std::vector<std::string> read_input(const std::string &filename) {
	std::vector<std::string> input;
	std::string line;
	std::ifstream file(filename);
	while (std::getline(file, line)) {
		input.push_back(line);
	}

	return input;
}

class Entry {
	public:
	Entry(Policy policy, std::string password) : policy(policy), password(password) {
	}

	/**
	 * Parse an input entry from an input string
	 * @param input The input to parse
	 * @return Entry the entry from the input
	 */
	static Entry parse(const std::string &input) {
		int delimIndex = input.find(DELIM);
		std::string rawPolicy = input.substr(0, delimIndex);
		std::string password = input.substr(delimIndex + DELIM.length());
		Policy policy = Policy::parse(rawPolicy);

		return Entry(policy, password);
	}

	Policy const &getPolicy() const {
		return this->policy;
	}

	std::string const &getPassword() const {
		return this->password;
	}

	private:
	Policy policy;
	std::string password;
};

int part1(const std::vector<std::string> &input) {
	int numCorrect = 0;
	for (std::string rawEntry : input) {
		Entry entry = Entry::parse(rawEntry);
		const Policy &policy = entry.getPolicy();
		const std::string &password = entry.getPassword();
		char letter = entry.getPolicy().getLetter();
		int count = std::count_if(password.begin(), password.end(), [=](char c) { return c == letter; });
		numCorrect += (count >= policy.getMin() && count <= policy.getMax());
	}

	return numCorrect;
}

int part2(const std::vector<std::string> &input) {
	int numCorrect = 0;
	for (std::string rawEntry : input) {
		Entry entry = Entry::parse(rawEntry);
		const Policy &policy = entry.getPolicy();
		const std::string &password = entry.getPassword();
		char letter = entry.getPolicy().getLetter();
		numCorrect += ((password.at(policy.getMin() - 1) == letter) ^ (password.at(policy.getMax() - 1) == letter));
	}

	return numCorrect;
}

int main(int argc, char *argv[]) {
	if (argc != 2) {
		std::cerr << "./day1 <input_file>" << std::endl;
		return 1;
	}

	auto input = read_input(argv[1]);
	std::cout << part1(input) << std::endl;
	std::cout << part2(input) << std::endl;
}