// 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 (
	"encoding/binary"
	"time"
)

// UUID version 6 is a field-compatible version of UUIDv1, reordered for improved DB locality.
// It is expected that UUIDv6 will primarily be used in contexts where there are existing v1 UUIDs.
// Systems that do not involve legacy UUIDv1 SHOULD consider using UUIDv7 instead.
//
// see https://datatracker.ietf.org/doc/html/rfc9562#uuidv6
//
// NewV6 returns a Version 6 UUID based on the current NodeID and clock
// sequence, and the current time. If the NodeID has not been set by SetNodeID
// or SetNodeInterface then it will be set automatically. If the NodeID cannot
// be set NewV6 set NodeID is random bits automatically . If clock sequence has not been set by
// SetClockSequence then it will be set automatically. If GetTime fails to
// return the current NewV6 returns Nil and an error.
func NewV6() (UUID, error) {
	now, seq, err := GetTime()
	if err != nil {
		return Nil, err
	}
	return generateV6(now, seq), nil
}

// NewV6WithTime returns a Version 6 UUID based on the current NodeID, clock
// sequence, and a specified time. It is similar to the NewV6 function, but allows
// you to specify the time. If time is passed as nil, then the current time is used.
//
// There is a limit on how many UUIDs can be generated for the same time, so if you
// are generating multiple UUIDs, it is recommended to increment the time.
// If getTime fails to return the current NewV6WithTime returns Nil and an error.
func NewV6WithTime(customTime *time.Time) (UUID, error) {
	now, seq, err := getTime(customTime)
	if err != nil {
		return Nil, err
	}

	return generateV6(now, seq), nil
}

func generateV6(now Time, seq uint16) UUID {
	var uuid UUID

	/*
	    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
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |                           time_high                           |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |           time_mid            |      time_low_and_version     |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |clk_seq_hi_res |  clk_seq_low  |         node (0-1)            |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |                         node (2-5)                            |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	*/

	timeHigh := uint32((now >> 28) & 0xffffffff)
	timeMid := uint16((now >> 12) & 0xffff)
	timeLow := uint16(now & 0x0fff)
	timeLow |= 0x6000 // Version 6

	binary.BigEndian.PutUint32(uuid[0:], timeHigh)
	binary.BigEndian.PutUint16(uuid[4:], timeMid)
	binary.BigEndian.PutUint16(uuid[6:], timeLow)
	binary.BigEndian.PutUint16(uuid[8:], seq)

	nodeMu.Lock()
	if nodeID == zeroID {
		setNodeInterface("")
	}
	copy(uuid[10:], nodeID[:])
	nodeMu.Unlock()

	return uuid
}