package snowflake

import (
	"base-framework/pkg/config"
	"sync"
	"sync/atomic"
	"time"
)

const (
	datacenterBits = uint64(5)
	workerBits     = uint64(5)
	sequenceBits   = uint64(12)

	maxSequence     = -1 ^ (-1 << sequenceBits)
	workerShift     = sequenceBits
	datacenterShift = sequenceBits + workerBits
	timestampShift  = sequenceBits + workerBits + datacenterBits

	epoch = int64(1672531200000) // 2023-01-01
)

type Snowflake struct {
	lastStamp uint64 // 上一次时间戳 (毫秒)
	sequence  uint64 // 序列号
	dcID      uint64
	workerID  uint64
}

var (
	sfInstance *Snowflake
	once       sync.Once
)

// lazy 初始化实例
func initInstance() {
	var dcID, wID uint64
	if config.Snowflake != nil {
		dcID = config.Snowflake.DatacenterID
		wID = config.Snowflake.WorkerID
	}
	sfInstance = &Snowflake{
		dcID:     dcID,
		workerID: wID,
	}
}

// NextID 生成全局唯一 ID，无锁版本
func NextID() uint64 {
	once.Do(initInstance)

	for {
		now := uint64(time.Now().UnixNano() / 1e6)

		last := atomic.LoadUint64(&sfInstance.lastStamp)
		seq := atomic.LoadUint64(&sfInstance.sequence)

		if now < last {
			// 系统时钟回拨，等待
			now = waitNextMillis(last)
		}

		if now == last {
			seq = (seq + 1) & uint64(maxSequence)
			if seq == 0 {
				now = waitNextMillis(last)
			}
		} else {
			seq = 0
		}

		if atomic.CompareAndSwapUint64(&sfInstance.lastStamp, last, now) {
			atomic.StoreUint64(&sfInstance.sequence, seq)
			id := ((now - uint64(epoch)) << timestampShift) |
				(sfInstance.dcID << datacenterShift) |
				(sfInstance.workerID << workerShift) |
				seq
			return id
		}
	}
}

// 等待下一个毫秒
func waitNextMillis(last uint64) uint64 {
	now := uint64(time.Now().UnixNano() / 1e6)
	for now <= last {
		now = uint64(time.Now().UnixNano() / 1e6)
	}
	return now
}