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// Copyright 2023 Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package uuid
import (
"io"
)
// UUID version 7 features a time-ordered value field derived from the widely
// implemented and well known Unix Epoch timestamp source,
// the number of milliseconds seconds since midnight 1 Jan 1970 UTC, leap seconds excluded.
// As well as improved entropy characteristics over versions 1 or 6.
//
// see https://datatracker.ietf.org/doc/html/draft-peabody-dispatch-new-uuid-format-03#name-uuid-version-7
//
// Implementations SHOULD utilize UUID version 7 over UUID version 1 and 6 if possible.
//
// NewV7 returns a Version 7 UUID based on the current time(Unix Epoch).
// Uses the randomness pool if it was enabled with EnableRandPool.
// On error, NewV7 returns Nil and an error
func NewV7() (UUID, error) {
uuid, err := NewRandom()
if err != nil {
return uuid, err
}
makeV7(uuid[:])
return uuid, nil
}
// NewV7FromReader returns a Version 7 UUID based on the current time(Unix Epoch).
// it use NewRandomFromReader fill random bits.
// On error, NewV7FromReader returns Nil and an error.
func NewV7FromReader(r io.Reader) (UUID, error) {
uuid, err := NewRandomFromReader(r)
if err != nil {
return uuid, err
}
makeV7(uuid[:])
return uuid, nil
}
// makeV7 fill 48 bits time (uuid[0] - uuid[5]), set version b0111 (uuid[6])
// uuid[8] already has the right version number (Variant is 10)
// see function NewV7 and NewV7FromReader
func makeV7(uuid []byte) {
/*
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms | ver | rand_a (12 bit seq) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rand_b |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*/
_ = uuid[15] // bounds check
t, s := getV7Time()
uuid[0] = byte(t >> 40)
uuid[1] = byte(t >> 32)
uuid[2] = byte(t >> 24)
uuid[3] = byte(t >> 16)
uuid[4] = byte(t >> 8)
uuid[5] = byte(t)
uuid[6] = 0x70 | (0x0F & byte(s>>8))
uuid[7] = byte(s)
}
// lastV7time is the last time we returned stored as:
//
// 52 bits of time in milliseconds since epoch
// 12 bits of (fractional nanoseconds) >> 8
var lastV7time int64
const nanoPerMilli = 1000000
// getV7Time returns the time in milliseconds and nanoseconds / 256.
// The returned (milli << 12 + seq) is guarenteed to be greater than
// (milli << 12 + seq) returned by any previous call to getV7Time.
func getV7Time() (milli, seq int64) {
timeMu.Lock()
defer timeMu.Unlock()
nano := timeNow().UnixNano()
milli = nano / nanoPerMilli
// Sequence number is between 0 and 3906 (nanoPerMilli>>8)
seq = (nano - milli*nanoPerMilli) >> 8
now := milli<<12 + seq
if now <= lastV7time {
now = lastV7time + 1
milli = now >> 12
seq = now & 0xfff
}
lastV7time = now
return milli, seq
}