// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2018 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 <uint256.h>

#include <stdint.h>
#include <limits>
#include <functional>

/* Seed OpenSSL PRNG with additional entropy data */
void RandAddSeed();

/**
 * Functions to gather random data via the OpenSSL PRNG
 */
void GetRandBytes(unsigned char* buf, int num);
uint64_t GetRand(uint64_t nMax);
int GetRandInt(int nMax);
uint256 GetRandHash();

/**
 * Add a little bit of randomness to the output of GetStrongRangBytes.
 * This sleeps for a millisecond, so should only be called when there is
 * no other work to be done.
 */
void RandAddSeedSleep();

/**
 * Function to gather random data from multiple sources, failing whenever any
 * of those sources fail to provide a result.
 */
void GetStrongRandBytes(unsigned char* buf, int num);

/**
 * Fast randomness source. This is seeded once with secure random data, but
 * is completely deterministic and insecure after that.
 * This class is not thread-safe.
 */
class FastRandomContext {
private:
    bool requires_seed;
    ChaCha20 rng;

    unsigned char bytebuf[64];
    int bytebuf_size;

    uint64_t bitbuf;
    int bitbuf_size;

    void RandomSeed();

    void FillByteBuffer()
    {
        if (requires_seed) {
            RandomSeed();
        }
        rng.Output(bytebuf, sizeof(bytebuf));
        bytebuf_size = sizeof(bytebuf);
    }

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

public:
    explicit FastRandomContext(bool fDeterministic = false);

    /** Initialize with explicit seed (only for testing) */
    explicit FastRandomContext(const uint256& seed);

    /** Generate a random 64-bit integer. */
    uint64_t rand64()
    {
        if (bytebuf_size < 8) FillByteBuffer();
        uint64_t ret = ReadLE64(bytebuf + 64 - bytebuf_size);
        bytebuf_size -= 8;
        return ret;
    }

    /** Generate a random (bits)-bit integer. */
    uint64_t randbits(int bits) {
        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;
        }
    }

    /** Generate a random integer in the range [0..range). */
    uint64_t randrange(uint64_t range)
    {
        --range;
        int bits = CountBits(range);
        while (true) {
            uint64_t ret = randbits(bits);
            if (ret <= range) return ret;
        }
    }

    /** Generate random bytes. */
    std::vector<unsigned char> randbytes(size_t len);

    /** Generate a random 32-bit integer. */
    uint32_t rand32() { return randbits(32); }

    /** generate a random uint256. */
    uint256 rand256();

    /** Generate a random boolean. */
    bool randbool() { return randbits(1); }

    // Compatibility with the C++11 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()() { return rand64(); }
};

/* 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;

/** Get 32 bytes of system entropy. Do not use this in application code: use
 * GetStrongRandBytes instead.
 */
void GetOSRand(unsigned char *ent32);

/** Check that OS randomness is available and returning the requested number
 * of bytes.
 */
bool Random_SanityCheck();

/** Initialize the RNG. */
void RandomInit();

/**
 * Identity function for MappedShuffle, so that elements retain their original order.
 */
int GenIdentity(int n);

/**
 * Rearranges the elements in the range [first,first+len) randomly, assuming
 * that gen is a uniform random number generator. Follows the same algorithm as
 * std::shuffle in C++11 (a Durstenfeld shuffle).
 *
 * The elements in the range [mapFirst,mapFirst+len) are rearranged according to
 * the same permutation, enabling the permutation to be tracked by the caller.
 *
 * gen takes an integer n and produces a uniform random output in [0,n).
 */
template <typename RandomAccessIterator, typename MapRandomAccessIterator>
    void MappedShuffle(RandomAccessIterator first,
                       MapRandomAccessIterator mapFirst,
                       size_t len,
                       std::function<int(int)> gen)
{
    for (size_t i = len-1; i > 0; --i) {
        auto r = gen(i+1);
        assert(r >= 0);
        assert(r <= i);
        std::swap(first[i], first[r]);
        std::swap(mapFirst[i], mapFirst[r]);
    }
}

#endif // BITCOIN_RANDOM_H