#ifndef OBSERVERS_HH
#define OBSERVERS_HH

#include <memory>
#include <vector>

template<typename T>
class Observers {
public:
  Observers()
    : current_(0), protected_(0), removed_(0) {
  }

  void add(T* observer) {
    if (!observer) return;
    for (auto it = observers_.begin(); it != observers_.end(); ++it) {
      if (*it == observer) return;
    }
    observers_.push_back(observer);
  }

  void remove(T* observer) {
    if (!observer) return;
    size_t index = 0;
    for (; index < observers_.size(); ++index) {
      if (observers_[index] == observer) {
        if (index < protected_) {
          observers_[index] = nullptr;
          ++removed_;
        } else {
          observers_.erase(observers_.begin() + index);
        }
        break;
      }
    }
  }

  T* first() {
    if (protected_) return nullptr;
    if (observers_.empty()) return nullptr;
    protected_ = observers_.size();
    return observers_[current_];
  }

  T* next() {
    if (current_ >= protected_) return nullptr;
    ++current_;
    if (current_ >= protected_) {
      if (removed_) {
        for (size_t i = protected_; i > 0; --i) {
          if (!observers_[i - 1]) {
            observers_.erase(observers_.begin() + i - 1);
            if (--removed_ == 0) break;
          }
        }
      }
      protected_ = current_ = 0;
      return nullptr;
    }
    auto ret = observers_[current_];
    return ret ? ret : next();
  }

private:
  size_t current_;
  size_t protected_;
  size_t removed_;
  std::vector<T*> observers_;
};

#endif  // OBSERVERS_HH