AoC22/d09.cc

#include <iostream>
#include <vector>
#include <unordered_set>

using namespace std;

enum Dir {
    UP,
    DOWN,
    RIGHT,
    LEFT
};

struct Instr {
    enum Dir dir;
    unsigned steps;
};

struct Pos2d {
    int x, y;

    Pos2d operator+(const Pos2d &r) const
    {
        return {this->x + r.x, this->y + r.y};
    }
    void operator+=(const Pos2d &r) { *this = *this + r; }

    Pos2d operator-(const Pos2d &r) const
    {
        return {this->x - r.x, this->y - r.y};
    }

    bool operator==(const Pos2d &r) const
    {
        return this->x == r.x && this->y == r.y;
    }
};

Pos2d dirs[] = {
    {0, 1},
    {0, -1},
    {1, 0},
    {-1, 0}
};

typedef vector<Instr> Input;

template<>
struct std::hash<Pos2d> {
    size_t operator()(const Pos2d &r) const
    {
        return hash<int>{}(r.x) ^ (hash<int>{}(r.y) << 1);
    }
};

Input parse()
{
    Input data;
    for (string line; getline(cin, line);) {
        Instr instr;
        if (line.size() < 3 || line.at(1) != ' ') throw "parse";
        switch (line.at(0)) {
        case 'U': instr.dir = UP; break;
        case 'D': instr.dir = DOWN; break;
        case 'L': instr.dir = LEFT; break;
        case 'R': instr.dir = RIGHT; break;
        }
        instr.steps = stoi(line.substr(2));
        data.push_back(instr);
    }
    return data;
}

unsigned p1(const Input &input, unsigned count = 2)
{
    auto knot = vector<Pos2d>(count);
    unordered_set<Pos2d> visited;
    visited.insert(knot.back());
    for (auto x : input) {
        for (unsigned i = 0; i < x.steps; i++) {
            knot.front() += dirs[x.dir];
            for (auto el = knot.begin(); el < knot.end() - 1; el++) {
                Pos2d dist = el[0] - el[1];
                if (abs(dist.x) >= 2 || abs(dist.y) >= 2) {
                    if (abs(dist.x) >= 2) dist.x = dist.x > 0 ? 1 : -1;
                    if (abs(dist.y) >= 2) dist.y = dist.y > 0 ? 1 : -1;
                    el[1] += dist;
                    continue;
                }
                break;
            }
            visited.insert(knot.back());
        }
    }
    return visited.size();
}

unsigned p2(const Input &input)
{
    return p1(input, 10);
}

int main()
{
    auto input = parse();
    cout << p1(input) << endl;
    cout << p2(input) << endl;
}