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