105 lines
2.5 KiB
C++
105 lines
2.5 KiB
C++
#include <algorithm>
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#include <array>
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#include <cinttypes>
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#include <cstdint>
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#include <cstdio>
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#include <iostream>
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#include <numeric>
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#include <string>
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#include <vector>
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#include <aocpp/Startup.hpp>
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namespace {
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struct Particle {
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std::int64_t pos;
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std::int64_t vel;
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auto operator==(Particle const& x) const -> bool = default;
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};
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auto Step(std::vector<Particle> & ps) {
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// apply gravity to update velocities
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auto n = ps.size();
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for (std::size_t i = 0; i+1 < n; i++) {
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for (std::size_t j = i+1; j < n; j++) {
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auto v = (ps[i].pos < ps[j].pos) - (ps[i].pos > ps[j].pos);
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ps[i].vel += v;
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ps[j].vel -= v;
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}
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}
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// apply velocities to update positions
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for (auto & p : ps) {
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p.pos += p.vel;
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}
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}
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auto CycleLength(std::vector<Particle> const& particles) -> std::int64_t {
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auto ps = particles;
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std::int64_t n = 0;
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do {
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Step(ps);
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n++;
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} while (!std::equal(ps.begin(), ps.end(), particles.begin()));
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return n;
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}
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/// Parse input as a vector of particle systems by axis.
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auto Parse(std::istream & in) -> std::array<std::vector<Particle>,3> {
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std::array<std::vector<Particle>,3> result;
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std::string line;
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while (std::getline(in, line)) {
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std::int64_t x, y, z;
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auto res =
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std::sscanf(
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line.c_str(),
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"<x=%" SCNi64 ", y=%" SCNi64 ", z=%" SCNi64 ">",
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&x, &y, &z);
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if (3 != res) { throw std::runtime_error{"bad input"}; }
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result[0].push_back({x,0});
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result[1].push_back({y,0});
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result[2].push_back({z,0});
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}
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return result;
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}
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/// Compute part 1 by iterating the simulation step and compute the resulting
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/// system energy.
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auto Part1(std::array<std::vector<Particle>, 3> system, std::int64_t const n) {
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for (auto && ps : system) {
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for (std::int64_t i = 0; i < n; i++) { Step(ps); }
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}
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std::int64_t result = 0;
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for (std::size_t i = 0; i < system[0].size(); i++) {
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std::int64_t xsum = 0;
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std::int64_t vsum = 0;
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for (auto && ps : system) {
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xsum += std::abs(ps[i].pos);
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vsum += std::abs(ps[i].vel);
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}
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result += xsum * vsum;
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}
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return result;
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}
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auto Part2(std::array<std::vector<Particle>, 3> const& ps) {
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std::int64_t n = 1;
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for (auto && p : ps) {
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n = std::lcm(n, CycleLength(p));
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}
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return n;
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}
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} // namespace
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auto Main(std::istream & in) -> void
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{
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auto ps = Parse(in);
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auto const part2 = Part2(ps);
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auto const part1 = Part1(std::move(ps), 1000);
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std::cout << "Part 1: " << part1 << std::endl;
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std::cout << "Part 2: " << part2 << std::endl;
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}
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