aocpp/2019/17.cpp

#include <algorithm>
#include <iostream>
#include <map>
#include <tuple>
#include <utility>
#include <vector>

#include <aocpp/Coord.hpp>
#include <aocpp/Grid.hpp>
#include <aocpp/Startup.hpp>
#include <intcode/intcode.hpp>

using namespace aocpp;
using namespace intcode;

namespace {

auto GatherOutput(Machine m) -> Grid
{
    std::vector<std::string> rows;
    bool eol = true; // next output starts a new line

    Run(m, []() -> ValueType {
      throw std::runtime_error{"input not supported"};
    },
    [&](ValueType c) -> void {
      if (eol) {
        rows.emplace_back();
        eol = false;
      }
      if ('\n' == c) {
        eol = true;
      } else {
        rows.back().push_back(static_cast<char>(c));}
      }
    );
    return Grid{std::move(rows)};
}

/// Find all the 4-way intersections.
/// @return Sum of product of coordinates
auto ScaffoldAlignments(Grid const& grid) -> std::size_t
{
  std::int64_t answer = 0;
  std::int64_t h = grid.rows.size();
  for (std::int64_t y = 1; y+1 < h; y++) {
    auto const& row = grid.rows[y];
    std::int64_t w = row.size();
    for (std::int64_t x = 1; x+1 < w; x++) {
      Coord c = {x,y};
      if (grid[c] == '#' &&
        grid[Up(c)] == '#' &&
        grid[Down(c)] == '#' &&
        grid[Left(c)] == '#' &&
        grid[Right(c)] == '#')
      {
        answer += x*y;
      }
    }
  }
  return answer;
}

auto ComputePath(Grid const& grid)
  -> std::vector<std::pair<char, std::int64_t>>
{
  // Determine starting location and direction
  Coord start{}, step{};
  grid.each([&](Coord c, char v) {
    switch (v) {
      case '^': start = c; step = { 0, -1}; break;
      case '>': start = c; step = { 0,  1}; break;
      case '<': start = c; step = {-1,  0}; break;
      case 'v': start = c; step = { 0,  1}; break;
    }
  });

  auto is_path = [&](Coord c) {
    return grid.contains(c) && grid[c] == '#';
  };

  std::vector<std::pair<char, std::int64_t>> result;
  for(;;) {
    char dir;
    if (is_path(start + CW(step))) {
      dir = 'R';
      step = CW(step);
    } else if (is_path(start + CCW(step))) {
      dir = 'L';
      step = CCW(step);
    } else {
      return result;
    }

    std::int64_t n = 0;
    while (is_path(start + step)) {
      start += step;
      n++;
    }
    result.emplace_back(dir, n);
  }
}

template <typename It>
auto RenderCommands(It begin, It end) -> std::string
{
  std::string result;
  while (begin != end) {
    auto const& [m,n] = *begin++;
    result += m;
    result += ',';
    result += std::to_string(n);
    result += ',';
  }
  result.pop_back(); // trailing comma
  return result;
}

template<class InputIter1, class InputIter2>
auto constexpr prefix(
  InputIter1 first1, InputIter1 last1,
  InputIter2 first2, InputIter2 last2
) -> bool
{
  for(;;) {
    if (first1 == last1) return true;
    if (first2 == last2) return false;
    if (*first1++ != *first2++) return false;
  }
}

using Path = std::vector<std::pair<char, std::int64_t>>;

auto ComputeProgramLoop(
  Path::const_iterator begin,
  Path::const_iterator end,
  std::vector<char> & solution,
  std::map<char, std::pair<Path::const_iterator, Path::const_iterator>> & subroutines
) -> bool
{
  // no path left to encode, we're done
  if (begin == end) return true;

  // Try using an existing subroutine
  for (auto const& [k,v] : subroutines) {
    if (prefix(v.first, v.second, begin, end)) {
      solution.push_back(k);
      auto begin_ = begin + (v.second - v.first);
      if (ComputeProgramLoop(begin_, end, solution, subroutines)) return true;
      solution.pop_back(); // rollback k
    }
  }

  // Try allocating a new subroutine
  if (subroutines.size() == 3) return false;
  auto next_routine = 'A' + subroutines.size();
  auto sub = subroutines.insert({next_routine, {}}).first;
  solution.push_back(next_routine);

  // Divide begin-end into begin-cursor and cursor-end
  // Assign begin-end to a new subroutine and recursively solve
  for (
    auto cursor = begin+1;
    cursor != end && RenderCommands(begin, cursor).size() <= 20;
    ++cursor
  ) {
    sub->second = {begin, cursor};
    if (ComputeProgramLoop(cursor, end, solution, subroutines)) return true;
  }

  // Deallocate the new subroutine; there was no solution
  subroutines.erase(sub);
  solution.pop_back();
  return false;
}

auto ComputeProgram(Path const& path) -> std::string {
  std::vector<char> solution;
  std::map<char, std::pair<Path::const_iterator, Path::const_iterator>> subroutines;
  
  if (!ComputeProgramLoop(path.begin(), path.end(), solution, subroutines)) {
    throw std::runtime_error{"no solution found"};
  }

  std::string result;
  for (char c : solution) {
    result += c;
    result += ',';
  }
  result.back() = '\n';

  for (auto const& [_,v] : subroutines) {
    result += RenderCommands(v.first, v.second);
    result += '\n';
  }
  result += "n\n"; // no video feed

  return result;
}

auto GatherScore(Machine m, std::string const& program) -> ValueType {
  ValueType score {};
  auto it = program.begin();
  m.At(0) = 2; // put into fancy mode
  Run(m,
    [&]() -> ValueType {
      if (it < program.end()) { return *it++; }
      throw std::runtime_error{"insufficient input"};
    },
    [&](ValueType o) { score = o; });
  return score;
}

} // namespace

auto Main(std::istream & in) -> void
{
  auto machine = Machine{ParseStream(in)};
  auto const grid = GatherOutput(machine);
  auto const part1 = ScaffoldAlignments(grid);
  auto const path = ComputePath(grid);
  auto const program = ComputeProgram(path);
  std::cout << "Part 1: " << part1 << std::endl;
  
  auto const part2 = GatherScore(std::move(machine), program);
  std::cout << "Part 2: " << part2 << std::endl;
}