aoc2017-day3part2

    struct point {
        int x;
        int y;
    };

    // this is arbitrary. just to make ordered containers work.
    bool operator<(const point & lhs, const point & rhs) {
        if(lhs.x != rhs.x) return lhs.x < rhs.x;
        return lhs.y < rhs.y;
    }

    typedef map<point, int> memory_map;

    point rotate(const point & p) {
        return point{-p.y, p.x };
    }


    int memory_value_for_point(const memory_map &memory, const point &p) {
        if(memory.find(p) == memory.end()) {
            return 0;
        }
        return memory.at(p);
    }


    int solve(int num) {
        point direction {1,0};
        point location {0,0};
        bool expand = false;
        int target_distance = 1;
        int current_distance = 0;
        memory_map spiral_memory;
        spiral_memory[location] = 1;
        while(spiral_memory[location] <= num) {
            location.x += direction.x;
            location.y += direction.y;
            spiral_memory[location] = memory_value_for_point(spiral_memory, {location.x + 1, location.y + 1})
                                + memory_value_for_point(spiral_memory, {location.x, location.y + 1})
                                + memory_value_for_point(spiral_memory, {location.x - 1, location.y + 1})
                                + memory_value_for_point(spiral_memory, {location.x + 1, location.y})
                                + memory_value_for_point(spiral_memory, {location.x - 1, location.y})
                                + memory_value_for_point(spiral_memory, {location.x + 1, location.y - 1})
                                + memory_value_for_point(spiral_memory, {location.x, location.y - 1})
                                + memory_value_for_point(spiral_memory, {location.x - 1, location.y - 1});
            current_distance++;
            //cerr << "spiral_memory[(" << location.x << "," << location.y << ")] = " << memory_value_for_point(spiral_memory, location) << endl;
            if(current_distance == target_distance) {
                current_distance = 0;
                direction = rotate(direction);
                if(expand) {
                    target_distance++;
                    expand = false;
                } else {
                    expand = true;
                }
            }

        }
        return spiral_memory[location];
    }