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// Copyright (C) 2022 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package lowmemjson
import (
"encoding/base64"
"io"
"strings"
)
type base64Decoder struct {
dst io.Writer
err error
pos int64
buf [4]byte
bufLen int
}
func newBase64Decoder(w io.Writer) io.WriteCloser {
return &base64Decoder{
dst: w,
}
}
func (dec *base64Decoder) decodeByte(b byte) (byte, bool) {
const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
n := strings.IndexByte(alphabet, b)
if n < 0 {
return 0, false
}
dec.pos++
return byte(n), true
}
func (dec *base64Decoder) decodeTuple(a, b, c, d byte) error {
var decodedLen int
var encoded [4]byte
var ok bool
if a != '=' {
encoded[0], ok = dec.decodeByte(a)
if !ok {
return base64.CorruptInputError(dec.pos)
}
decodedLen++
}
if b != '=' {
encoded[1], ok = dec.decodeByte(b)
if !ok {
return base64.CorruptInputError(dec.pos)
}
// do NOT increment decodedLen here
}
if c != '=' {
encoded[2], ok = dec.decodeByte(c)
if !ok {
return base64.CorruptInputError(dec.pos)
}
decodedLen++
}
if d != '=' {
encoded[3], ok = dec.decodeByte(d)
if !ok {
return base64.CorruptInputError(dec.pos)
}
decodedLen++
}
val := 0 |
uint32(encoded[0])<<18 |
uint32(encoded[1])<<12 |
uint32(encoded[2])<<6 |
uint32(encoded[3])<<0
var decoded [3]byte
decoded[0] = byte(val >> 16)
decoded[1] = byte(val >> 8)
decoded[2] = byte(val >> 0)
_, err := dec.dst.Write(decoded[:decodedLen])
return err
}
func (dec *base64Decoder) Write(dat []byte) (int, error) {
if len(dat) == 0 {
return 0, nil
}
if dec.err != nil {
return 0, dec.err
}
var n int
if dec.bufLen > 0 {
n = copy(dec.buf[dec.bufLen:], dat)
dec.bufLen += n
if dec.bufLen < 4 {
return len(dat), nil
}
if err := dec.decodeTuple(dec.buf[0], dec.buf[1], dec.buf[2], dec.buf[3]); err != nil {
dec.err = err
return 0, dec.err
}
}
for ; n+3 < len(dat); n += 4 {
if err := dec.decodeTuple(dat[n], dat[n+1], dat[n+2], dat[n+3]); err != nil {
dec.err = err
return n, dec.err
}
}
dec.bufLen = copy(dec.buf[:], dat[n:])
return len(dat), nil
}
func (dec *base64Decoder) Close() error {
if dec.bufLen == 0 {
return nil
}
copy(dec.buf[:], "====")
return dec.decodeTuple(dec.buf[0], dec.buf[1], dec.buf[2], dec.buf[3])
}
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