Class FastRandomContext

src/random.h:131–230

* Fast randomness source. This is seeded once with secure random data, but * is completely deterministic and does not gather more entropy after that. * * This class is not thread-safe. */

 * 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.Keystream(bytebuf, sizeof(bytebuf));
        bytebuf_size = sizeof(bytebuf);
    }

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

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

    /** Initialize with explicit seed (only for testing) */
    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;

    /** Move a FastRandomContext. If the original one is used again, it will be reseeded. */
    FastRandomContext& operator=(FastRandomContext&& from) noexcept;

    /** Generate a random 64-bit integer. */
    uint64_t rand64() noexcept
    {
        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) noexcept
    {
        if (bits == 0) {
            return 0;
        } else if (bits > 32) {