aocpp/2022/13.cpp

#include <array>
#include <cstdint>
#include <iostream>
#include <sstream>
#include <variant>
#include <vector>

#include <boost/phoenix.hpp>
#include <boost/range/adaptor/indexed.hpp>
#include <boost/spirit/include/qi.hpp>

#include <doctest.h>

#include <aocpp/Startup.hpp>

namespace {

namespace phx = boost::phoenix;
namespace qi = boost::spirit::qi;

struct Packet;
using Packets = std::vector<Packet>;
using PacketPair = std::array<Packet, 2>;
using PacketPairs = std::vector<PacketPair>;

// A packet is an integer or a list of packets.
struct Packet {
  std::variant<std::int64_t, Packets> rep;
};

auto operator<(Packet const& lhs, std::int64_t rhs) -> bool;
auto operator<(std::int64_t lhs, Packet const& rhs) -> bool;

auto operator<(std::int64_t const lhs, Packets const& rhs) -> bool {
  switch (rhs.size()) {
    case 0: return false;
    case 1: return lhs < rhs[0];
    default: return !(rhs[0] < lhs);
  }
}

auto operator<(Packets const& lhs, std::int64_t const rhs) -> bool {
  switch (lhs.size()) {
    case 0: return true;
    default: return lhs[0] < rhs;
  }
}

auto operator<(Packet const& lhs, std::int64_t rhs) -> bool {
  return std::visit([rhs](auto const& rep) { return rep < rhs; }, lhs.rep);
}

auto operator<(std::int64_t lhs, Packet const& rhs) -> bool {
  return std::visit([lhs](auto const& rep) { return lhs < rep; }, rhs.rep);
}

// Less-than relation on packets.
// Two integers are compared using their integer values.
// Two lists are compared using a lexicographic ordering.
// When an integer is compared to a list, that integer is promoted to a singleton list.
auto operator<(Packet const& lhs, Packet const& rhs) -> bool {
  return std::visit(std::less(), lhs.rep, rhs.rep);
}

// Input file grammar:
// Each packet should be newline-terminated.
// Each pair should be newline-separated.
// Packets can be integer literals or a list of packets.
// A list of packets is bracketed with square brackets and is comma-separated.
// No additional whitespace is tolerated in the input stream.
template <typename It>
class Grammar : public qi::grammar<It, PacketPairs()> {
  qi::rule<It, std::int64_t> integer;
  qi::rule<It, Packets()> packets;
  qi::rule<It, Packet()> packet;
  qi::rule<It, PacketPair()> packetPair;
  qi::rule<It, PacketPairs()> packetPairs;

public:
  Grammar() : Grammar::base_type{packetPairs} {
    using namespace qi::labels;

    integer  = qi::long_long;
    packets  = "[" >> -(packet % ",") >> "]";
    packet   = integer [ bind(&Packet::rep, _val) = _1 ]
             | packets [ bind(&Packet::rep, _val) = _1 ];
    packetPair = packet [_val[0] = _1] >> "\n" >>
                 packet [_val[1] = _1] >> "\n";
    packetPairs = -(packetPair % "\n");
  }
};

/// Parse the complete input stream according to the rules defined in 'Grammar'.
/// Throws: std::runtime_error on failed parse
auto Parse(std::istream & in) -> PacketPairs
{
  auto result = PacketPairs{};
  auto const content = std::string{std::istreambuf_iterator{in}, {}};
  auto b = content.begin(); // updated on successful parse
  auto const e = content.end();

  if (!qi::parse(b, e, Grammar<decltype(b)>{}, result) || b != e) {
    throw std::runtime_error{"Bad packet: " + content};
  }

  return result;
}

// Return the sum 1-based indexes of the pairs where the first element is less than the second.
auto Part1(PacketPairs const& packetPairs) -> std::size_t
{
  auto result = std::int64_t{0};
  for (auto const& [i, pair] : boost::adaptors::index(packetPairs)) {
    if (pair[0] < pair[1]) {
      result += i + 1;
    }
  }
  return result;
}

// Returns the product of the 1-based indexes of the elements [[2]] and [[6]]
// when those elements are added to all the packets in the packet pair list
// and that list is then soted.
auto Part2(PacketPairs const& packetPairs) -> std::size_t
{
  auto two = Packet{2}; // Indistinguishable from [[2]] and [[6]]
  auto six = Packet{6};
  auto ix2 = std::size_t{1};
  auto ix6 = std::size_t{2};
  for (auto const& pair : packetPairs) {
    for (auto const& Packet : pair) {
      if (Packet < six) {
        ix6++;
        if (Packet < two) {
          ix2++;
        }
      }
    }
  }
  return ix2 * ix6;
}

} // namespace

TEST_SUITE("2022-13 examples") {
  TEST_CASE("simple ordering") {
    CHECK(Packet{1} < Packet{2});
    CHECK(!(Packet{2} < Packet{1}));
    CHECK(Packet{Packets{Packet{1}}} < Packet{Packets{Packet{2}}});
    CHECK(Packet{1} < Packet{Packets{Packet{2}}});
    CHECK(!(Packet{Packets{Packet{2}}} < Packet{1}));
    CHECK(Packet{Packets{}} < Packet{Packets{Packet{2}}});
    CHECK(Packet{Packets{}} < Packet{Packets{Packet{2}}});
    CHECK(Packet{1} < Packet{Packets{Packet{2},Packet{3}}});
    CHECK(!(Packet{Packets{Packet{2},Packet{3}}} < Packet{1}));
    CHECK(Packet{2} < Packet{Packets{Packet{2},Packet{3}}});
    CHECK(!(Packet{Packets{Packet{2},Packet{3}}} < Packet{2}));
  }
  TEST_CASE("documented example") {
    auto in = std::istringstream{
      "[1,1,3,1,1]\n"
      "[1,1,5,1,1]\n"
      "\n"
      "[[1],[2,3,4]]\n"
      "[[1],4]\n"
      "\n"
      "[9]\n"
      "[[8,7,6]]\n"
      "\n"
      "[[4,4],4,4]\n"
      "[[4,4],4,4,4]\n"
      "\n"
      "[7,7,7,7]\n"
      "[7,7,7]\n"
      "\n"
      "[]\n"
      "[3]\n"
      "\n"
      "[[[]]]\n"
      "[[]]\n"
      "\n"
      "[1,[2,[3,[4,[5,6,7]]]],8,9]\n"
      "[1,[2,[3,[4,[5,6,0]]]],8,9]\n"
    };
    auto const packetPairs = Parse(in);
    CHECK(13 == Part1(packetPairs));
    CHECK(140 == Part2(packetPairs));
  }
}

auto Main(std::istream & in, std::ostream & out) -> void
{
  auto const packetPairs = Parse(in);
  out << "Part 1: " << Part1(packetPairs) << std::endl;
  out << "Part 2: " << Part2(packetPairs) << std::endl;
}