aocpp/2022/25.cpp

#include <cstdint> // int64_t
#include <functional> // plus
#include <iostream> // istream
#include <iterator> // istream_iterator
#include <numeric> // transform_reduce
#include <sstream> // istringstream
#include <string> // string
#include <tuple> // pair, make_pair

#include <doctest.h>

#include <aocpp/Startup.hpp>

namespace {

using It = std::istream_iterator<std::string>;

// 
template <typename T>
auto DivMod(T dividend, T divisor) -> std::pair<T, T>
{
  auto const quotient  = dividend / divisor;
  auto const remainder = dividend % divisor;
  if (remainder == 0 || (remainder > 0) == (divisor > 0)) {
    return {quotient, remainder};
  } else {
    return {quotient - 1, remainder + divisor};
  }
}

template <typename T>
auto FromInt(T x) -> std::string
{
  std::string output;
  char const digits[] {"=-012"};
  while (x != 0) {
    auto [x_, i] = DivMod<T>(x + 2, 5); // plain % and / are insufficient for negatives
    x = x_;
    output.push_back(digits[i]);
  }
  std::reverse(output.begin(), output.end());
  return output;
}

template <typename T>
auto ToInt(std::string const& str) -> T
{
  T acc {0};
  for (char c : str) {
    acc *= 5;
    switch (c) {
      case '2': acc += 2; break;
      case '1': acc += 1; break;
      case '0': break;
      case '-': acc -= 1; break;
      case '=': acc -= 2; break;
    }
  }
  return acc;
}

auto Solve(std::istream & in) -> std::string
{
  return FromInt(std::transform_reduce(It{in}, It{}, std::int64_t{}, std::plus(), ToInt<std::int64_t>));
}

} // namespace

TEST_SUITE("2022-25 examples") {
  TEST_CASE("example") {
    std::istringstream in {
        "1=-0-2\n"
        "12111\n"
        "2=0=\n"
        "21\n"
        "2=01\n"
        "111\n"
        "20012\n"
        "112\n"
        "1=-1=\n"
        "1-12\n"
        "12\n"
        "1=\n"
        "122\n"
    };
    CHECK(Solve(in) == "2=-1=0");
  }
  // The example input doesn't test out any negative numbers
  TEST_CASE("Negative numbers") {
    CHECK(ToInt<int>("-") == -1);
    CHECK(ToInt<int>("=") == -2);
    CHECK(ToInt<int>("-=") == -7);
    CHECK(ToInt<int>("=-") == -11);
    CHECK(FromInt(-1) == "-");
    CHECK(FromInt(-2) == "=");
    CHECK(FromInt(-7) == "-=");
    CHECK(FromInt(-11) == "=-");
  }

  TEST_CASE("DivMod behavior") {
    CHECK(DivMod<int>(2,5) == std::make_pair(0,2));
    CHECK(DivMod<int>(-2,5) == std::make_pair(-1,3));
    CHECK(DivMod<int>(2,-5) == std::make_pair(-1,-3));
    CHECK(DivMod<int>(-2,-5) == std::make_pair(0,-2));
    
    CHECK(DivMod<int>(-5,-5) == std::make_pair(1,0));
    CHECK(DivMod<int>(-5,5) == std::make_pair(-1,0));
    CHECK(DivMod<int>(5,-5) == std::make_pair(-1,0));
    CHECK(DivMod<int>(-5,-5) == std::make_pair(1,0));
  }
}

auto main(int argc, char** argv) -> int
{
  std::cout << "Part 1: " << Solve(*aocpp::Startup(argc, argv)) << std::endl;
}