//  Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.

//  This source code is licensed under both the GPLv2 (found in the

//  COPYING file in the root directory) and Apache 2.0 License

//  (found in the LICENSE.Apache file in the root directory).

// This file is for functions that generate unique identifiers by

// (at least in part) by extracting novel entropy or sources of uniqueness

// from the execution environment. (By contrast, random.h is for algorithmic

// pseudorandomness.)

//

// These functions could eventually migrate to public APIs, such as in Env.

#pragma once

#include <array>

#include <atomic>

#include <cstdint>

#include <type_traits>

#include "port/port.h"

#include "rocksdb/rocksdb_namespace.h"

namespace ROCKSDB_NAMESPACE {

// Generates a new 128-bit identifier that is universally unique

// (with high probability) for each call. The result is split into

// two 64-bit pieces. This function has NOT been validated for use in

// cryptography.

//

// This is used in generating DB session IDs and by Env::GenerateUniqueId

// (used for DB IDENTITY) if the platform does not provide a generator of

// RFC 4122 UUIDs or fails somehow. (Set exclude_port_uuid=true if this

// function is used as a fallback for GenerateRfcUuid, because no need

// trying it again.)

void GenerateRawUniqueId(uint64_t* a, uint64_t* b,

                         bool exclude_port_uuid = false);

#ifndef NDEBUG

// A version of above with options for challenge testing

void TEST_GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid,

                              bool exclude_env_details,

                              bool exclude_random_device);

#endif

// Generates globally unique ids with lower probability of any collisions

// vs. each unique id being independently random (GenerateRawUniqueId).

// We call this "semi-structured" because between different

// SemiStructuredUniqueIdGen objects, the IDs are separated by random

// intervals (unstructured), but within a single SemiStructuredUniqueIdGen

// object, the generated IDs are trivially related (structured). See

// https://github.com/pdillinger/unique_id for how this improves probability

// of no collision. In short, if we have n SemiStructuredUniqueIdGen

// objects each generating m IDs, the first collision is expected at

// around n = sqrt(2^128 / m), equivalently n * sqrt(m) = 2^64,

// rather than n * m = 2^64 for fully random IDs.

class SemiStructuredUniqueIdGen {

 public:

  // Initializes with random starting state (from GenerateRawUniqueId)

  SemiStructuredUniqueIdGen() { Reset(); }

  // Re-initializes, but not thread safe

  void Reset();

  // Assuming no fork(), `lower` is guaranteed unique from one call

  // to the next (thread safe).

  void GenerateNext(uint64_t* upper, uint64_t* lower);

  // For generating smaller values. Will cycle through all the possibilities

  // before repeating.

  template <typename T>

  T GenerateNext() {

    static_assert(sizeof(T) <= sizeof(uint64_t));

    static_assert(std::is_integral_v<T>);

    uint64_t ignore;

    uint64_t val;

    GenerateNext(&ignore, &val);

    return static_cast<T>(val);

  }

  uint64_t GetBaseUpper() const { return base_upper_; }

 private:

  uint64_t base_upper_;

  uint64_t base_lower_;

  std::atomic<uint64_t> counter_;

  int64_t saved_process_id_;

};

// A unique id generator that should provide reasonable security against

// predicting the output from previous outputs, but is NOT known to be

// cryptographically secure. Unlike std::random_device, this is guaranteed

// not to block once initialized.

class ALIGN_AS(CACHE_LINE_SIZE) UnpredictableUniqueIdGen {

 public:

  // Initializes with random starting state (from several GenerateRawUniqueId)

  UnpredictableUniqueIdGen() { Reset(); }

  // Re-initializes, but not thread safe

  void Reset();

  // Generate next probabilistically unique value. Thread safe. Uses timing

  // information to add to the entropy pool.

  void GenerateNext(uint64_t* upper, uint64_t* lower);

  // Explicitly include given value for entropy pool instead of timing

  // information.

  void GenerateNextWithEntropy(uint64_t* upper, uint64_t* lower,

                               uint64_t extra_entropy);

#ifndef NDEBUG

  struct TEST_ZeroInitialized {};

  explicit UnpredictableUniqueIdGen(TEST_ZeroInitialized);

  std::atomic<uint64_t>& TEST_counter() { return counter_; }

#endif

 private:

  // 256 bit entropy pool

  std::array<std::atomic<uint64_t>, 4> pool_;

  // Counter to ensure unique hash inputs

  std::atomic<uint64_t> counter_;

};

}  // namespace ROCKSDB_NAMESPACE