Original: Advent of Code 2020 - Day 2: validating passwords
Problem statement
I decided to translate my Rust solution to Advent of Code 2020 into C++ to increase my familiarity with C++. Here's the task for Day 2:
Day 2: Password Philosophy
[...]
To try to debug the problem, they have created a list (your puzzle input) of passwords (according to the corrupted database) and the corporate policy when that password was set.
For example, suppose you have the following list:
1-3 a: abcde 1-3 b: cdefg 2-9 c: ccccccccc
Each line gives the password policy and then the password. The password policy indicates the lowest and highest number of times a given letter must appear for the password to be valid. For example,
1-3 a
means that the password must containa
at least1
time and at most3
times.In the above example,
2
passwords are valid. The middle password,cdefg
, is not; it contains no instances ofb
, but needs at least1
. The first and third passwords are valid: they contain onea
or ninec
, both within the limits of their respective policies.How many passwords are valid according to their policies?
[...]
Part Two
While it appears you validated the passwords correctly, they don't seem to be what the Official Toboggan Corporate Authentication System is expecting.
The shopkeeper suddenly realizes that he just accidentally explained the password policy rules from his old job at the sled rental place down the street! The Official Toboggan Corporate Policy actually works a little differently.
Each policy actually describes two positions in the password, where
1
means the first character,2
means the second character, and so on. (Be careful; Toboggan Corporate Policies have no concept of "index zero"!) Exactly one of these positions must contain the given letter. Other occurrences of the letter are irrelevant for the purposes of policy enforcement.Given the same example list from above:
1-3 a: abcde
is valid: position1
containsa
and position3
does not.1-3 b: cdefg
is invalid: neither position1
nor position3
containsb
.2-9 c: ccccccccc
is invalid: both position2
and position9
containc
.How many passwords are valid according to the new interpretation of the policies?
The full story can be found on the website.
My solution
lib.hpp
#ifndef INC_LIB_HPP_KwANAGBG9S
#define INC_LIB_HPP_KwANAGBG9S
#include <algorithm>
#include <cstddef>
#include <iostream>
#include <string>
#include <utility>
// must be NTBS
constexpr const char* PATH = "./data/day_2/input";
enum class Policy { Old, New };
struct Entry {
std::pair<std::size_t, std::size_t> numbers;
std::string password;
char key;
bool is_valid(Policy policy) const {
switch (policy) {
case Policy::Old: {
auto frequency = static_cast<std::size_t>(
std::count(password.begin(), password.end(), key)
);
return frequency >= numbers.first && frequency <= numbers.second;
}
case Policy::New: {
auto pos_a = numbers.first - 1;
auto pos_b = numbers.second - 1;
if (pos_a >= password.size() || pos_b >= password.size()) {
return false;
}
// logical xor
return (password[pos_a] == key) != (password[pos_b] == key);
}
}
}
};
// error checking omitted
inline std::istream& operator>>(std::istream& is, Entry& entry) {
is >> entry.numbers.first;
is.get(); // consume '-'
is >> entry.numbers.second >> entry.key;
is.get(); // consume ':'
is >> entry.password;
return is;
}
#endif
main.cpp
#include "lib.hpp"
#include <fstream>
int main() {
std::ifstream file{PATH};
std::size_t valid_count_old = 0;
std::size_t valid_count_new = 0;
for (Entry entry; file >> entry;) {
if (entry.is_valid(Policy::Old)) {
++valid_count_old;
}
if (entry.is_valid(Policy::New)) {
++valid_count_new;
}
}
std::cout << "Part One: " << valid_count_old << '\n'
<< "Part Two: " << valid_count_new << '\n';
}