package uuid

import (
	"unsafe"

	"github.com/google/uuid"
)

const (
	letters    = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
	MIN_LENGTH = 2 // 最小长度为2的原因是Go中整型长度最大为64位，而UUID v4是128位长，所以最小长度为2。
	MAX_LENGTH = 128
)

// 将一个字节拆分成8个比特位，并以一个8位长字节数组输出。
func breakByte(b byte) []byte {
	bytes := make([]byte, 8)
	for i := byte(0); i < 8; i++ {
		mask := byte(0x1) << (7 - i)
		bytes[i] = (b & mask) >> (7 - i)
	}
	return bytes
}

// 根据UUID v4生成一个指定长度的短UUID字符串。
func ShortUUID(length uint) string {
	var uuidLength uint
	if length < MIN_LENGTH {
		uuidLength = MIN_LENGTH
	} else if length > MAX_LENGTH {
		uuidLength = MAX_LENGTH
	} else {
		uuidLength = length
	}
	// UUID是一个16字节长的字节数组，每个字节都可以拆分成8个比特位，共128比特位。
	newUUID := uuid.New()
	uuidBits := make([]byte, 0)
	for _, b := range newUUID[:] {
		uuidBits = append(uuidBits, breakByte(b)...)
	}
	var partitionLength uint = 0
	if 128%uuidLength == 0 {
		partitionLength = 128 / uuidLength
	} else {
		partitionLength = 128/uuidLength + 1
	}
	var paritionedUUID = make([][]byte, uuidLength)
	for i := uint(0); i < uuidLength; i++ {
		if i != uuidLength-1 {
			paritionedUUID[i] = uuidBits[i*partitionLength : (i+1)*partitionLength]
		} else {
			paritionedUUID[i] = uuidBits[i*partitionLength:]
		}
	}
	result := make([]byte, uuidLength)
	for i := uint(0); i < uuidLength; i++ {
		var assembledRune uint64 = 0
		for j := uint(0); j < uint(len(paritionedUUID[i])); j++ {
			assembledRune |= uint64(paritionedUUID[i][j]) << (partitionLength - j - 1)
		}
		result[i] = letters[assembledRune%uint64(len(letters))]
	}

	return *(*string)(unsafe.Pointer(&result))
}