package uuidv7

import (
	"crypto/rand"
	"fmt"
	"time"
)

// New returns a UUIDv7 string.
func New() (string, error) {
	var b [16]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", err
	}
	ms := uint64(time.Now().UnixMilli())
	b[0] = byte(ms >> 40)
	b[1] = byte(ms >> 32)
	b[2] = byte(ms >> 24)
	b[3] = byte(ms >> 16)
	b[4] = byte(ms >> 8)
	b[5] = byte(ms)
	b[6] = (b[6] & 0x0f) | 0x70
	b[8] = (b[8] & 0x3f) | 0x80
	return fmt.Sprintf("%08x-%04x-%04x-%04x-%012x",
		uint32(b[0])<<24|uint32(b[1])<<16|uint32(b[2])<<8|uint32(b[3]),
		uint16(b[4])<<8|uint16(b[5]),
		uint16(b[6])<<8|uint16(b[7]),
		uint16(b[8])<<8|uint16(b[9]),
		uint64(b[10])<<40|uint64(b[11])<<32|uint64(b[12])<<24|uint64(b[13])<<16|uint64(b[14])<<8|uint64(b[15]),
	), nil
}