2023/14/src/part2.cpp


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#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>

using Dish = std::vector<std::string>;
using History = std::vector<std::vector<bool>>;

constexpr int CYCLES = 1'000'000'000;

Dish parse(const char* file)
{
    Dish v;

    if (std::ifstream input{ file }; input.is_open())
    {
        std::string line;
        while (not std::getline(input,line).eof())
        {
            v.push_back(std::move(line));
        }
    }

    return v;
}

std::vector<bool> encode(const Dish& v)
{
    auto len = v.size();
    std::vector<bool> e(len * len, 0);
    for (int x = 0; x < len; ++x)
    {
        for (int y = 0; y < len; ++y)
        {
            e[x * len + y] = v[x][y] == 'O';
        }
    }
    return e;
}

Dish& tilt_north(Dish& v)
{
    for (int c = 0; c < v.size(); ++c)
    {
        int floor{};
        for (int r = floor; r < v.size(); ++r)
        {
            switch (v[r][c])
            {
                case '#':
                    floor = r + 1;
                    break;
                case 'O':
                    v[r][c] = '.';
                    v[floor++][c] = 'O';
            }
        }
    }
    return v;
}

Dish& tilt_south(Dish& v)
{
    for (int c = 0; c < v.size(); ++c)
    {
        int floor{ static_cast<int>(v.size() - 1) };
        for (int r = floor; r >= 0; --r)
        {
            switch (v[r][c])
            {
                case '#':
                    floor = r - 1;
                    break;
                case 'O':
                    v[r][c] = '.';
                    v[floor--][c] = 'O';
            }
        }
    }
    return v;
}

Dish& tilt_west(Dish& v)
{
    for (int r = 0; r < v.size(); ++r)
    {
        int floor{};
        for (int c = floor; c < v.size(); ++c)
        {
            switch (v[r][c])
            {
                case '#':
                    floor = c + 1;
                    break;
                case 'O':
                    v[r][c] = '.';
                    v[r][floor++] = 'O';
            }
        }
    }
    return v;
}

Dish& tilt_east(Dish& v)
{
    for (int r = 0; r < v.size(); ++r)
    {
        int floor{ static_cast<int>(v.size() - 1) };
        for (int c = floor; c >= 0; --c)
        {
            switch (v[r][c])
            {
                case '#':
                    floor = c - 1;
                    break;
                case 'O':
                    v[r][c] = '.';
                    v[r][floor--] = 'O';
            }
        }
    }
    return v;
}

Dish& cycle(Dish& v)
{
    return tilt_east(tilt_south(tilt_west(tilt_north(v))));
}

int load(const std::vector<bool>& v, int l)
{
    int load{};
    for (int x = 0; x < l; ++x)
    {
        for (int y = 0; y < l; ++y)
        {
            load += (l - x) * v[x * l + y];
        }
    }
    return load;
}

int dup(History& hist, Dish dish)
{
    auto h = encode(dish);
    if (auto found = std::find(hist.cbegin(), hist.cend(), h); found != hist.cend())
    {
        return hist.cend() - found;
    }
    hist.push_back(std::move(h));
    return 0;
}

int main(int argc, char* argv[])
{
    auto v = parse(argv[1]);

    History history;
    history.push_back(encode(v));

    int p, np;
    for (p = 0; p == 0; p = dup(history, cycle(v)));
    np = history.size() - p;

    std::cout << load(history[np + (CYCLES - np) % p], v.size()) << std::endl;
    return 0;
}