/*
 *  Copyright 2018 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef PC_UNIQUE_ID_GENERATOR_H_
#define PC_UNIQUE_ID_GENERATOR_H_

#include <limits>
#include <set>
#include <string>

#include "api/array_view.h"

namespace webrtc {

// This class will generate numbers. A common use case is for identifiers.
// The generated numbers will be unique, in the local scope of the generator.
// This means that a generator will never generate the same number twice.
// The generator can also be initialized with a sequence of known ids.
// In such a case, it will never generate an id from that list.
// Recommendedations:
//  * Prefer unsigned types.
//  * Prefer larger types (uint8_t will run out quickly).
template <typename TIntegral>
class UniqueNumberGenerator {
 public:
  typedef TIntegral value_type;
  UniqueNumberGenerator();
  // Creates a generator that will never return any value from the given list.
  explicit UniqueNumberGenerator(rtc::ArrayView<TIntegral> known_ids);
  ~UniqueNumberGenerator();

  // Generates a number that this generator has never produced before.
  // If there are no available numbers to generate, this method will fail
  // with an |RTC_CHECK|.
  TIntegral GenerateNumber();
  TIntegral operator()() { return GenerateNumber(); }

 private:
  static_assert(std::is_integral<TIntegral>::value, "Must be integral type.");
  TIntegral counter_;
  // This class can be further optimized by removing the known_ids_ set when
  // the generator was created without a sequence of ids to ignore.
  // In such a case, the implementation uses a counter which is sufficient to
  // prevent repetitions of the generated values.
  std::set<TIntegral> known_ids_;
};

// This class will generate unique ids. Ids are 32 bit unsigned integers.
// The generated ids will be unique, in the local scope of the generator.
// This means that a generator will never generate the same id twice.
// The generator can also be initialized with a sequence of known ids.
// In such a case, it will never generate an id from that list.
class UniqueRandomIdGenerator {
 public:
  typedef uint32_t value_type;
  UniqueRandomIdGenerator();
  // Create a generator that will never return any value from the given list.
  explicit UniqueRandomIdGenerator(rtc::ArrayView<uint32_t> known_ids);
  ~UniqueRandomIdGenerator();

  // Generates a random id that this generator has never produced before.
  // This method becomes more expensive with each use, as the probability of
  // collision for the randomly generated numbers increases.
  uint32_t GenerateId();
  uint32_t operator()() { return GenerateId(); }

 private:
  std::set<uint32_t> known_ids_;
};

// This class will generate strings. A common use case is for identifiers.
// The generated strings will be unique, in the local scope of the generator.
// This means that a generator will never generate the same string twice.
// The generator can also be initialized with a sequence of known ids.
// In such a case, it will never generate an id from that list.
class UniqueStringGenerator {
 public:
  typedef std::string value_type;
  UniqueStringGenerator();
  explicit UniqueStringGenerator(rtc::ArrayView<std::string> known_ids);
  ~UniqueStringGenerator();

  std::string GenerateString();
  std::string operator()() { return GenerateString(); }

 private:
  // This implementation will be simple and will generate "0", "1", ...
  UniqueNumberGenerator<uint32_t> unique_number_generator_;
};

template <typename TIntegral>
UniqueNumberGenerator<TIntegral>::UniqueNumberGenerator() : counter_(0) {}

template <typename TIntegral>
UniqueNumberGenerator<TIntegral>::UniqueNumberGenerator(
    rtc::ArrayView<TIntegral> known_ids)
    : counter_(0), known_ids_(known_ids.begin(), known_ids.end()) {}

template <typename TIntegral>
UniqueNumberGenerator<TIntegral>::~UniqueNumberGenerator() {}

template <typename TIntegral>
TIntegral UniqueNumberGenerator<TIntegral>::GenerateNumber() {
  while (true) {
    RTC_CHECK_LT(counter_, std::numeric_limits<TIntegral>::max());
    auto pair = known_ids_.insert(counter_++);
    if (pair.second) {
      return *pair.first;
    }
  }
}

}  // namespace webrtc

#endif  // PC_UNIQUE_ID_GENERATOR_H_