blog review 第六期

  std::vector<char> buffer_;
  size_t readerIndex_;
  size_t writerIndex_;
const char* peek() const
 { return begin() + readerIndex_; }
size_t writableBytes() const
  { return buffer_.size() - writerIndex_; }
size_t readableBytes() const
  { return writerIndex_ - readerIndex_; }
 size_t prependableBytes() const
  { return readerIndex_; }

#pragma once
#include <cstdint>
#include <chrono>
#include <stdexcept>
#include <mutex>

class snowflake_nonlock
{
public:
    void lock()
    {
    }
    void unlock()
    {
    }
};

template<int64_t Twepoch, typename Lock = snowflake_nonlock>
class snowflake
{
    using lock_type = Lock;
    static constexpr int64_t TWEPOCH = Twepoch;
    static constexpr int64_t WORKER_ID_BITS = 5L;
    static constexpr int64_t DATACENTER_ID_BITS = 5L;
    static constexpr int64_t MAX_WORKER_ID = (1 << WORKER_ID_BITS) - 1;
    static constexpr int64_t MAX_DATACENTER_ID = (1 << DATACENTER_ID_BITS) - 1;
    static constexpr int64_t SEQUENCE_BITS = 12L;
    static constexpr int64_t WORKER_ID_SHIFT = SEQUENCE_BITS;
    static constexpr int64_t DATACENTER_ID_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS;
    static constexpr int64_t TIMESTAMP_LEFT_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS + DATACENTER_ID_BITS;
    static constexpr int64_t SEQUENCE_MASK = (1 << SEQUENCE_BITS) - 1;

    using time_point = std::chrono::time_point<std::chrono::steady_clock>;

    time_point start_time_point_ = std::chrono::steady_clock::now();
    int64_t start_millsecond_ = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count();

    int64_t last_timestamp_ = -1;
    int64_t workerid_ = 0;
    int64_t datacenterid_ = 0;
    int64_t sequence_ = 0;
    lock_type lock_;
public:
    snowflake() = default;

    snowflake(const snowflake&) = delete;

    snowflake& operator=(const snowflake&) = delete;

    void init(int64_t workerid, int64_t datacenterid)
    {
        if (workerid > MAX_WORKER_ID || workerid < 0) {
            throw std::runtime_error("worker Id can't be greater than 31 or less than 0");
        }

        if (datacenterid > MAX_DATACENTER_ID || datacenterid < 0) {
            throw std::runtime_error("datacenter Id can't be greater than 31 or less than 0");
        }

        workerid_ = workerid;
        datacenterid_ = datacenterid;
    }

    int64_t nextid()
    {
        std::lock_guard<lock_type> lock(lock_);
        //std::chrono::steady_clock  cannot decrease as physical time moves forward
        auto timestamp = millsecond();
        if (last_timestamp_ == timestamp)
        {
          // 当同一时间戳（精度：毫秒）下多次生成id会增加序列号
            sequence_ = (sequence_ + 1)&SEQUENCE_MASK;
            if (sequence_ == 0)
            {
                // 如果当前序列超出12bit长度，则需要等待下一毫秒
                // 下一毫秒将使用sequence:0
                // 为什么这里不直接+1
                timestamp = wait_next_millis(last_timestamp_);
            }
        }
        else
        {
            sequence_ = 0;
        }

        last_timestamp_ = timestamp;

        return ((timestamp - TWEPOCH) << TIMESTAMP_LEFT_SHIFT)
            | (datacenterid_ << DATACENTER_ID_SHIFT)
            | (workerid_ << WORKER_ID_SHIFT)
            | sequence_;
    }

private:
    int64_t millsecond() const noexcept
    {
        auto diff = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::steady_clock::now() - start_time_point_);
        return start_millsecond_ + diff.count();
    }

    int64_t wait_next_millis(int64_t last) const noexcept
    {
        auto timestamp = millsecond();
        while (timestamp <= last)
        {
            timestamp = millsecond();
        }
        return timestamp;
    }
};