aocpp/2019/14.cpp

#include <algorithm>
#include <cstdint>
#include <iostream>
#include <sstream>
#include <iterator>
#include <numeric>
#include <map>
#include <set>
#include <vector>
#include <tuple>

#include <aocpp/Startup.hpp>

namespace {

using Recipes = std::map<std::string, std::pair<std::int64_t, std::vector<std::pair<std::int64_t, std::string>>>>;

auto Parse(std::istream & in) -> Recipes {
  Recipes result;

  std::string line;

  std::int64_t n;
  std::string word;

  while (std::getline(in, line)) {
    std::vector<std::pair<std::int64_t, std::string>> components;
    std::istringstream sin {line};

    bool leftSide;
    do {
      sin >> n >> word;
      leftSide = word.back() == ',';
      if (leftSide) word.pop_back();
      components.push_back({n, word});
    } while(leftSide);
    sin >> word; // skip
    sin >> n >> word;
    result.insert({word, {n, std::move(components)}});
  }

  return result;
}

auto Topsort(Recipes const& recipes) -> std::vector<std::string>
{
  // Kahn's algorithm
  std::set<std::string> perm;
  std::set<std::string> temp;
  std::vector<std::string> result;

  auto visit_ = [&](auto& rec, std::string name) -> void {
    if (!perm.contains(name)) {
      if (!temp.insert(name).second) throw std::runtime_error{"cyclic recipes"};
      if (recipes.contains(name)) {
        for (auto && [_, c] : recipes.at(name).second) {
          rec(rec, c);
        }
      }
      temp.erase(name);
      perm.insert(name);
      result.push_back(name);
    }
  };
  auto visit = [&](auto name) { visit_(visit_, name); };

  for (auto && [k,_] : recipes) {
    visit(k);
  }

  std::reverse(result.begin(), result.end());

  return result;
}

class Machine {
  struct Entry {
    std::int64_t need;
    std::int64_t batch;
    std::vector<std::pair<std::int64_t, std::int64_t&>> components;
  };
  std::int64_t ore_;
  std::vector<Entry> process_;
public:
  Machine(Recipes const& recipes)
  {
    auto order = Topsort(recipes);
    order.pop_back();
    process_.resize(order.size(), {});

    std::map<std::string, std::size_t> indexes;
    for (auto && name : order) {
      indexes.insert({name, indexes.size()});
    }

    for (auto && [k,v] : recipes) {
      auto & entry = process_[indexes[k]];
      entry.batch = v.first;
      for (auto && [m,c] : v.second) {
        entry.components.push_back({m, c == "ORE" ? ore_ : process_[indexes[c]].need});
      }
    }
  }

  auto operator()(std::int64_t fuel) {
    for (auto && entry : process_) {
      entry.need = 0;
    }
    process_[0].need = fuel;
    ore_ = 0;

    for (auto && entry : process_) {
      auto batches = (entry.need + entry.batch - 1) / entry.batch;
      for (auto && [m,c] : entry.components) {
        c += batches*m;
      }
    }

    return ore_;
  }
};

auto ComputeFuel(Machine & machine, std::int64_t ore) {
  std::int64_t tooHi = 1;
  while (machine(tooHi) <= ore) {
    tooHi *= 2;
  }

  std::int64_t lo = 0;
  while (lo+1 != tooHi) {
    auto mid = std::midpoint(lo, tooHi);
    (machine(mid) > ore ? tooHi : lo) = mid;
  }

  return lo;
}

} // namespace

auto main(int argc, char** argv) -> int {
  auto recipes = Parse(aocpp::Startup(argc, argv));
  auto machine = Machine(recipes);
  auto part1 = machine(1);
  auto part2 = ComputeFuel(machine, 1'000'000'000'000);
  std::cout << "Part 1: " << part1 << std::endl;
  std::cout << "Part 2: " << part2 << std::endl;
}