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/*
* Copyright 2017 Google
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_SECURE_RANDOM_H_
#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_SECURE_RANDOM_H_
#include <cstdint>
#include <limits>
namespace firebase {
namespace firestore {
namespace util {
// A "secure" pseudorandom number generator, suitable for generating
// unguessable identifiers. This exists because
//
// * std::mt19937 is blazing fast but its outputs can be guessed once enough
// previous outputs have been observed.
// * std::random_device isn't guaranteed to come from a secure PRNG or be
// fast.
//
// The implementation satisfies the C++11 UniformRandomBitGenerator concept and
// delegates to an implementation that generates high quality random values
// quickly with periodic reseeding.
class SecureRandom {
public:
// C++11 UniformRandomBitGenerator interface
using result_type = uint32_t;
static constexpr result_type min() {
return std::numeric_limits<result_type>::min();
}
static constexpr result_type max() {
return std::numeric_limits<result_type>::max();
}
result_type operator()();
/** Returns a uniformly distributed pseudorandom integer in [0, n). */
inline result_type Uniform(result_type n) {
// Divides the range into buckets of size n plus leftovers.
const result_type rem = (max() - min()) % n + min() + 1;
result_type rnd;
// Generates random number until the number falls into a bucket.
do {
rnd = (*this)();
} while (rnd < rem);
return rnd % n;
}
inline bool OneIn(result_type n) {
return Uniform(n) == 0;
}
};
} // namespace util
} // namespace firestore
} // namespace firebase
#endif // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_SECURE_RANDOM_H_
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