Circular Array Loop

class Solution {
public:
    bool circularArrayLoop(vector<int>& nums) {
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] == 0) {
                continue;
            }
            int back = i, ahead = i;
            while (nums[next(nums, back)] * nums[i] > 0 &&
                   nums[next(nums, ahead)] * nums[i] > 0 &&
                   nums[next(nums, next(nums, ahead))] * nums[i] > 0) {

                back = next(nums, back);
                ahead = next(nums, next(nums, ahead));
                if (back == ahead) {
                    if (back == next(nums, back)) {
                        break;
                    }
                    return true;
                }
            }
            back = i;
            while (nums[back] * nums[i] > 0) {
                nums[back] = 0;
                back = next(nums, back);
            }
        }
        return false;
    }

private:
    int next(const vector<int>& nums, int i) {
        return ((i + nums[i]) + nums.size()) % nums.size();
    }
};