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// Copyright 2015 The etcd Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package idutil implements utility functions for generating unique,
// randomized ids.
package idutil
import (
"math"
"sync/atomic"
"time"
)
const (
tsLen = 5 * 8
cntLen = 8
suffixLen = tsLen + cntLen
)
// Generator generates unique identifiers based on counters, timestamps, and
// a node member ID.
//
// The initial id is in this format:
// High order 2 bytes are from memberID, next 5 bytes are from timestamp,
// and low order one byte is a counter.
// | prefix | suffix |
// | 2 bytes | 5 bytes | 1 byte |
// | memberID | timestamp | cnt |
//
// The timestamp 5 bytes is different when the machine is restart
// after 1 ms and before 35 years.
//
// It increases suffix to generate the next id.
// The count field may overflow to timestamp field, which is intentional.
// It helps to extend the event window to 2^56. This doesn't break that
// id generated after restart is unique because etcd throughput is <<
// 256req/ms(250k reqs/second).
type Generator struct {
// high order 2 bytes
prefix uint64
// low order 6 bytes
suffix uint64
}
func NewGenerator(memberID uint16, now time.Time) *Generator {
prefix := uint64(memberID) << suffixLen
unixMilli := uint64(now.UnixNano()) / uint64(time.Millisecond/time.Nanosecond)
suffix := lowbit(unixMilli, tsLen) << cntLen
return &Generator{
prefix: prefix,
suffix: suffix,
}
}
// Next generates a id that is unique.
func (g *Generator) Next() uint64 {
suffix := atomic.AddUint64(&g.suffix, 1)
id := g.prefix | lowbit(suffix, suffixLen)
return id
}
func lowbit(x uint64, n uint) uint64 {
return x & (math.MaxUint64 >> (64 - n))
}
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