07.cpp

#include <chrono>
#include <iostream>

using std::string;
using size_type = std::string::size_type;

bool find_bab_inside(const string& line, char aba[2]) {
  bool inside_square_brackets = false;
  for (size_type j = 0; j < line.length() - 2; j++) {
    if (inside_square_brackets) {
      if (line[j] == aba[1] && line[j + 2] == aba[1] && line[j + 1] == aba[0]) {
        return true;
      }
    }

    if (line[j] == '[' || line[j] == ']') {
      inside_square_brackets = !inside_square_brackets;
    }
  }
  return false;
}

int main() {
  auto tstart = std::chrono::high_resolution_clock::now();
  int pt1 = 0;
  int pt2 = 0;

  string input;
  while (std::getline(std::cin, input)) {
    bool inside_square_brackets = false;
    bool has_abba_outside = false;
    bool has_abba_inside = false;
    bool has_bab = false;
    char aba[2] = {0, 0};

    for (size_type i = 0; i < input.length(); i++) {

      // detect ABBA
      if ((i < input.length() - 3) && input[i] != input[i + 1] &&
          input[i] == input[i + 3] && input[i + 1] == input[i + 2]) {
        if (inside_square_brackets) {
          has_abba_inside = true;
        } else {
          has_abba_outside = true;
        }
      }

      // detect ABA
      if (!inside_square_brackets && i < input.length() - 2 &&
          input[i] != input[i + 1] && input[i] == input[i + 2]) {
        aba[0] = input[i];
        aba[1] = input[i + 1];

        // loop over entire line to find BAB
        if (find_bab_inside(input, aba)) {
          has_bab = true;
        }
      }

      if (input[i] == '[' || input[i] == ']') {
        inside_square_brackets = !inside_square_brackets;
      }
    }

    // part 1: ABBA outside but not inside
    if (has_abba_outside && !has_abba_inside) {
      pt1 += 1;
    }

    // part 2: ABA
    if (has_bab) {
      pt2 += 1;
    }
  }

  std::cout << "--- Day 7: Internet Protocol Version 7 ---\n";
  std::cout << "Part 1: " << pt1 << "\n";
  std::cout << "Part 2: " << pt2 << "\n";

  auto tstop = std::chrono::high_resolution_clock::now();
  auto duration =
      std::chrono::duration_cast<std::chrono::microseconds>(tstop - tstart);
  std::cout << "Time: " << duration.count() << " μs"
            << "\n";
  return EXIT_SUCCESS;
}