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package base64vlq
import (
"io"
)
const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
const (
vlqBaseShift = 5
vlqBase = 1 << vlqBaseShift
vlqBaseMask = vlqBase - 1
vlqSignBit = 1
vlqContinuationBit = vlqBase
)
var decodeMap [256]byte
func init() {
for i := 0; i < len(encodeStd); i++ {
decodeMap[encodeStd[i]] = byte(i)
}
}
func toVLQSigned(n int) int {
if n < 0 {
return -n<<1 + 1
}
return n << 1
}
func fromVLQSigned(n int) int {
isNeg := n&vlqSignBit != 0
n >>= 1
if isNeg {
return -n
}
return n
}
type Encoder struct {
w io.ByteWriter
}
func NewEncoder(w io.ByteWriter) *Encoder {
return &Encoder{
w: w,
}
}
func (enc Encoder) Encode(n int) error {
n = toVLQSigned(n)
for digit := vlqContinuationBit; digit&vlqContinuationBit != 0; {
digit = n & vlqBaseMask
n >>= vlqBaseShift
if n > 0 {
digit |= vlqContinuationBit
}
err := enc.w.WriteByte(encodeStd[digit])
if err != nil {
return err
}
}
return nil
}
type Decoder struct {
r io.ByteReader
}
func NewDecoder(r io.ByteReader) *Decoder {
return &Decoder{
r: r,
}
}
func (dec Decoder) Decode() (n int, err error) {
shift := uint(0)
for continuation := true; continuation; {
c, err := dec.r.ReadByte()
if err != nil {
return 0, err
}
c = decodeMap[c]
continuation = c&vlqContinuationBit != 0
n += int(c&vlqBaseMask) << shift
shift += vlqBaseShift
}
return fromVLQSigned(n), nil
}
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