day_04a.cpp

#include <algorithm>
#include <numeric>
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include <cmath>

std::vector<int> extract_numbers(const std::string& s) {
  std::cout << s << '\n';
  std::vector<int> numbers;
  std::size_t start = 0;
  std::size_t end = s.find(' ');
  for (int i = 0; i < s.size(); i += 3) {
    if (s[i] == ' ') {  
      numbers.push_back(std::stoi(s.substr(i+1, i+2)));
    } else {
      numbers.push_back(std::stoi(s.substr(i, i+2)));
    }
  }
  return numbers;
}

int main(int argc, char * argv[]) {
  std::string input = "../input/day_04_input";
  if (argc > 1) {
    input = argv[1];
  }

  std::string line;
  std::fstream file(input);

  std::size_t total_points = 0;
   while(std::getline(file, line)) {
    std::size_t start = 0;
    std::size_t end = line.find(':');
    const int card_id = std::stoi(line.substr(5, end - 5));
    start = end + 2;
    end = line.find('|', start);
    const auto winning_numbers = extract_numbers(line.substr(start, end - start));
    const auto card_numbers = extract_numbers(line.substr(end + 2, line.size() - end));
    int count = 0;
    for (const auto& card_number : card_numbers) {
      if (std::find(std::begin(winning_numbers), std::end(winning_numbers), card_number) !=  std::end(winning_numbers)) {
        count++;
      }
    }
    if (count > 0) {
      total_points += std::pow(2, count - 1);
    }
   }
  std::cout << total_points << '\n';
   return 0;
}