aocpp/2022/11.cpp

#include <cstdint>
#include <iostream>
#include <numeric>
#include <sstream>
#include <stdexcept>
#include <string>
#include <vector>

#include <doctest.h>

#include <aocpp/Startup.hpp>

namespace {

struct Monkey {
  std::vector<std::int64_t> items;
  std::int64_t divisor;
  std::size_t true_ix, false_ix;
  std::optional<std::int64_t> rhs;
  char op;
};

auto Parse(std::istream & in) -> std::vector<Monkey>
{
  std::string word;
  std::vector<Monkey> monkies;

  while (in >> word) {
    monkies.emplace_back();
    Monkey & m = monkies.back();

    in >> word >> word >> word; // 0: Starting items:

    for (;;) {
      std::int64_t elt;
      in >> elt;
      m.items.push_back(elt);

      char c;
      in >> c;
      if (c != ',') break;
    }

    in >> word >> word >> word >> word; // Operation: new = old

    in >> m.op >> word;
    if (word != "old") m.rhs = std::stoll(word);

    in >> word >> word >> word >> m.divisor;
    in >> word >> word >> word >> word >> word >> m.true_ix
       >> word >> word >> word >> word >> word >> m.false_ix;
  }

  return monkies;
}

auto Solve(std::vector<Monkey> const& input, std::size_t const rounds, std::optional<std::int64_t> const modulus) -> std::int64_t
{
  std::vector<std::int64_t> throws(input.size());
  for (std::size_t i = 0; i < input.size(); ++i) {
    for (auto item : input[i].items) {
      auto cursor = i;
      for (auto n = rounds; n > 0;) {
        throws[cursor]++;
        auto const& m = input[cursor];

        auto const y = m.rhs.value_or(item);
        switch (m.op) {
          case '+': item += y; break;
          case '*': item *= y; break;
          default: throw std::runtime_error{"bad input"};
        }
        if (modulus.has_value()) {
          item %= *modulus;
        } else {
          item /= 3;
        }

        auto const next = (item % m.divisor == 0) ? m.true_ix : m.false_ix;
        if (next < cursor) n--;
        cursor = next;
      }
    }
  }

  // Compute top 2 elements of the vector
  std::partial_sort(throws.begin(), throws.begin()+2, throws.end(), std::greater());

  return throws[0] * throws[1];
}

auto Part1(std::vector<Monkey> const& input) -> std::int64_t
{
  return Solve(input, 20, {});
}

auto Part2(std::vector<Monkey> const& input) -> std::int64_t
{
  auto const modulus =
    std::transform_reduce(
      input.cbegin(), input.cend(),
      std::int64_t{1}, std::lcm<std::int64_t, std::int64_t>,
      [](Monkey const& m) { return m.divisor; });
  return Solve(input, 10'000, modulus);
}

} // namespace

TEST_SUITE("2022-11 examples") {
  TEST_CASE("example") {
    std::istringstream in {
R"(Monkey 0:
  Starting items: 79, 98
  Operation: new = old * 19
  Test: divisible by 23
    If true: throw to monkey 2
    If false: throw to monkey 3

Monkey 1:
  Starting items: 54, 65, 75, 74
  Operation: new = old + 6
  Test: divisible by 19
    If true: throw to monkey 2
    If false: throw to monkey 0

Monkey 2:
  Starting items: 79, 60, 97
  Operation: new = old * old
  Test: divisible by 13
    If true: throw to monkey 1
    If false: throw to monkey 3

Monkey 3:
  Starting items: 74
  Operation: new = old + 3
  Test: divisible by 17
    If true: throw to monkey 0
    If false: throw to monkey 1
)"
    };
    auto const input = Parse(in);
    CHECK(Part1(input) == 10605);
    CHECK(Part2(input) == 2713310158);
  }
}

auto main(int argc, char** argv) -> int
{
  auto const input = Parse(*aocpp::Startup(argc, argv));
  std::cout << "Part 1: " << Part1(input) << std::endl;
  std::cout << "Part 2: " << Part2(input) << std::endl;
}