AoC21/d03.cc

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;

auto parse_input(char *file)
{
    ifstream input(file);
    if (!input.is_open()) exit(1);

    vector<string> parsed;
    for (string line; getline(input, line);) {
        parsed.push_back(line);
    }
    return parsed;
}

// Count the occurences of each bit position
vector<int> count_occurences(vector<string> input)
{
    vector<int> counts(input[0].length(), 0);
    for (string line : input) {
        for (size_t i = 0; i < line.length(); i++) {
            if (line[i] == '1') counts[i]++;
            if (line[i] == '0') counts[i]--;
        }
    }
    return counts;
}

unsigned part1(auto input)
{
    vector<int> counts = count_occurences(input);

    // Convert the counts array into a binary value
    unsigned out = 0;
    for (int i : counts) {
        out <<= 1;
        if (i > 0) out += 1;
    }

    return out * (out ^ ((1 << counts.size()) - 1));
}

unsigned part2(auto input)
{
    size_t pos;

    vector<string> numbers_ox = input;
    pos = 0;
    while (numbers_ox.size() > 1 && pos < input[0].length()) {
        int count = count_occurences(numbers_ox)[pos];
        char bit = (count >= 0) + '0';
        for (size_t i = 0; i < numbers_ox.size(); i++) {
            if (numbers_ox[i][pos] != bit) {
                numbers_ox.erase(numbers_ox.begin()+i);
                i--;
            }
        }
        pos++;
    }
    unsigned ox = stol(numbers_ox[0], NULL, 2);

    vector<string> numbers_co = input;
    pos = 0;
    while (numbers_co.size() > 1 && pos < input[0].length()) {
        int count = count_occurences(numbers_co)[pos];
        char bit = (count < 0) + '0';
        for (size_t i = 0; i < numbers_co.size(); i++) {
            if (numbers_co[i][pos] != bit) {
                numbers_co.erase(numbers_co.begin()+i);
                i--;
            }
        }
        pos++;
    }
    unsigned co = stol(numbers_co[0], NULL, 2);

    return ox * co;
}

int main(int argc, char *argv[])
{
    if (argc <= 1) return 1;
    auto input = parse_input(argv[1]);

    cout << part1(input) << endl;
    cout << part2(input) << endl;
}