random.h

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// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#ifndef BITCOIN_RANDOM_H
#define BITCOIN_RANDOM_H

#include <crypto/chacha20.h>
#include <crypto/common.h>
#include <span.h>
#include <uint256.h>

#include <cassert>
#include <chrono>
#include <cstdint>
#include <limits>
#include <vector>

void GetRandBytes(Span<unsigned char> bytes) noexcept;
uint64_t GetRandInternal(uint64_t nMax) noexcept;
template<typename T>
T GetRand(T nMax=std::numeric_limits<T>::max()) noexcept {
    static_assert(std::is_integral<T>(), "T must be integral");
    static_assert(std::numeric_limits<T>::max() <= std::numeric_limits<uint64_t>::max(), "GetRand only supports up to uint64_t");
    return T(GetRandInternal(nMax));
}
template <typename D>
D GetRandomDuration(typename std::common_type<D>::type max) noexcept
// Having the compiler infer the template argument from the function argument
// is dangerous, because the desired return value generally has a different
// type than the function argument. So std::common_type is used to force the
// call site to specify the type of the return value.
{
    assert(max.count() > 0);
    return D{GetRand(max.count())};
};
constexpr auto GetRandMicros = GetRandomDuration<std::chrono::microseconds>;
constexpr auto GetRandMillis = GetRandomDuration<std::chrono::milliseconds>;

std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval);

uint256 GetRandHash() noexcept;

void GetStrongRandBytes(Span<unsigned char> bytes) noexcept;

void RandAddPeriodic() noexcept;

void RandAddEvent(const uint32_t event_info) noexcept;

class FastRandomContext
{
private:
    bool requires_seed;
    ChaCha20 rng;

    uint64_t bitbuf;
    int bitbuf_size;

    void RandomSeed();

    void FillBitBuffer()
    {
        bitbuf = rand64();
        bitbuf_size = 64;
    }

public:
    explicit FastRandomContext(bool fDeterministic = false) noexcept;

    explicit FastRandomContext(const uint256& seed) noexcept;

    // Do not permit copying a FastRandomContext (move it, or create a new one to get reseeded).
    FastRandomContext(const FastRandomContext&) = delete;
    FastRandomContext(FastRandomContext&&) = delete;
    FastRandomContext& operator=(const FastRandomContext&) = delete;

    FastRandomContext& operator=(FastRandomContext&& from) noexcept;

    uint64_t rand64() noexcept
    {
        if (requires_seed) RandomSeed();
        std::array<std::byte, 8> buf;
        rng.Keystream(buf);
        return ReadLE64(UCharCast(buf.data()));
    }

    uint64_t randbits(int bits) noexcept
    {
        if (bits == 0) {
            return 0;
        } else if (bits > 32) {
            return rand64() >> (64 - bits);
        } else {
            if (bitbuf_size < bits) FillBitBuffer();
            uint64_t ret = bitbuf & (~uint64_t{0} >> (64 - bits));
            bitbuf >>= bits;
            bitbuf_size -= bits;
            return ret;
        }
    }

    uint64_t randrange(uint64_t range) noexcept
    {
        assert(range);
        --range;
        int bits = CountBits(range);
        while (true) {
            uint64_t ret = randbits(bits);
            if (ret <= range) return ret;
        }
    }

    template <typename B = unsigned char>
    std::vector<B> randbytes(size_t len);

    void fillrand(Span<std::byte> output);

    uint32_t rand32() noexcept { return randbits(32); }

    uint256 rand256() noexcept;

    bool randbool() noexcept { return randbits(1); }

    template <typename Tp>
    Tp rand_uniform_delay(const Tp& time, typename Tp::duration range)
    {
        return time + rand_uniform_duration<Tp>(range);
    }

    template <typename Chrono>
    typename Chrono::duration rand_uniform_duration(typename Chrono::duration range) noexcept
    {
        using Dur = typename Chrono::duration;
        return range.count() > 0 ? /* interval [0..range) */ Dur{randrange(range.count())} :
               range.count() < 0 ? /* interval (range..0] */ -Dur{randrange(-range.count())} :
                                   /* interval [0..0] */ Dur{0};
    };

    // Compatibility with the UniformRandomBitGenerator concept
    typedef uint64_t result_type;
    static constexpr uint64_t min() { return 0; }
    static constexpr uint64_t max() { return std::numeric_limits<uint64_t>::max(); }
    inline uint64_t operator()() noexcept { return rand64(); }
};

template <typename I, typename R>
void Shuffle(I first, I last, R&& rng)
{
    while (first != last) {
        size_t j = rng.randrange(last - first);
        if (j) {
            using std::swap;
            swap(*first, *(first + j));
        }
        ++first;
    }
}

/* Number of random bytes returned by GetOSRand.
 * When changing this constant make sure to change all call sites, and make
 * sure that the underlying OS APIs for all platforms support the number.
 * (many cap out at 256 bytes).
 */
static const int NUM_OS_RANDOM_BYTES = 32;

void GetOSRand(unsigned char* ent32);

bool Random_SanityCheck();

void RandomInit();

#endif // BITCOIN_RANDOM_H