aocpp/2020/01.cpp

#include <algorithm>
#include <bitset>
#include <cstdint>
#include <iostream>
#include <sstream>
#include <iterator>
#include <vector>

#include <doctest.h>

#include <aocpp/Startup.hpp>

namespace {

auto Parse(std::istream & in) -> std::vector<std::int64_t>
{
  std::vector<int64_t> result;
  std::int64_t x;
  while (in >> x) {
    result.push_back(x);
  }
  std::sort(result.begin(), result.end());
  return result;
}

auto Part1(std::vector<std::int64_t> const& entries) {
  std::bitset<2020> numbers;
  for (auto const i : entries) {
    if (i >= 2020) {
      break;
    }
    numbers.set(i);
  }
  for (auto const i : entries) {
    if (numbers.test(2020 - i)) {
      return (2020 - i) * i;
    }
  }
  throw std::runtime_error{"no part 1 solution"};
}

auto Part2(std::vector<std::int64_t> const& entries) {
  std::bitset<2020> numbers;
  for (auto const i : entries) {
    if (i >= 2020) { break; }
    if (i < 2020) { numbers.set(i); }
  }

  for (auto it1 = entries.begin(); it1 != entries.end(); it1++) {
    for (auto it2 = std::next(it1); it2 != entries.end(); it2++) {
      auto target = 2020 - (*it1) - (*it2);
      if (target < 0) break;
      if (target < 2020 && numbers.test(target)) { return target * *it1 * *it2;}
    }
  }
  throw std::runtime_error{"no part 2 solution"};
}

}

TEST_SUITE("documented examples") {
  std::istringstream in {
    "1721\n"
    "979\n"
    "366\n"
    "299\n"
    "675\n"
    "1456\n"};
  auto entries = Parse(in);

TEST_CASE("part 1") {
  REQUIRE(Part1(entries) == 514579);
}

TEST_CASE("part 2") {
  REQUIRE(Part2(entries) == 241861950);
}

}

auto Main(std::istream & in) -> void
{
  auto const entries {Parse(in)};
  std::cout << "Part 1: " << Part1(entries) << std::endl;
  std::cout << "Part 2: " << Part2(entries) << std::endl;
}