aocpp/2017/17.cpp

#include <cstdint>
#include <iostream>
#include <vector>

#include <doctest.h>

#include <aocpp/Startup.hpp>

namespace {

auto Step(std::vector<std::uint32_t> & spin, int steps) -> void {
  std::uint32_t cursor = spin.size() - 1;
  for (int i = 0; i < steps; i++) {
      cursor = spin[cursor];
    }
    spin.push_back(spin[cursor]);
    spin[cursor] = spin.size() - 1;
    cursor = spin.size() - 1;
}

auto Part1(std::uint64_t jump, std::uint64_t iterations) -> std::uint64_t {
  std::vector<std::uint32_t> spin {0};
  for (int j = 0; j < iterations; j++) {
    Step(spin, jump);
  }
  return spin.back();
}

auto Part2(std::uint64_t jump, std::uint64_t iterations) -> std::uint64_t {
  std::uint64_t cursor = 0;
  std::uint64_t after_zero = -1;
  for (std::uint64_t i = 1; i <= iterations; ++i) {
    cursor += jump;
    cursor %= i;
    if (cursor == 0) {
      after_zero = i;
    }
    cursor++;
  }

  return after_zero;
}

} // namespace

TEST_SUITE("2017-17 examples") {
  TEST_CASE("example") {
    std::vector<std::uint32_t> spin {0};
    Step(spin, 3);
    REQUIRE(spin == std::vector<std::uint32_t>{1,0});
    Step(spin, 3);
    REQUIRE(spin == std::vector<std::uint32_t>{2,0,1});
    Step(spin, 3);
    REQUIRE(spin == std::vector<std::uint32_t>{2,0,3,1});

    CHECK(Part1(3, 2017) == 638);
  }
}

auto Main(std::istream & in, std::ostream & out) -> void
{
  int steps;
  in >> steps;
  out << "Part 1: " << Part1(steps, 2017) << std::endl;
  out << "Part 2: " << Part2(steps, 50'000'000) << std::endl;
}