1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4 5 // Package ascii85 implements the ascii85 data encoding
6 // as used in the btoa tool and Adobe's PostScript and PDF document formats.
7 package ascii85
8 9 import (
10 "io"
11 "strconv"
12 )
13 14 /*
15 * Encoder
16 */
17 18 // Encode encodes src into at most [MaxEncodedLen](len(src))
19 // bytes of dst, returning the actual number of bytes written.
20 //
21 // The encoding handles 4-byte chunks, using a special encoding
22 // for the last fragment, so Encode is not appropriate for use on
23 // individual blocks of a large data stream. Use [NewEncoder] instead.
24 //
25 // Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
26 // Encode does not add these.
27 func Encode(dst, src []byte) int {
28 if len(src) == 0 {
29 return 0
30 }
31 32 n := 0
33 for len(src) > 0 {
34 dst[0] = 0
35 dst[1] = 0
36 dst[2] = 0
37 dst[3] = 0
38 dst[4] = 0
39 40 // Unpack 4 bytes into uint32 to repack into base 85 5-byte.
41 var v uint32
42 switch len(src) {
43 default:
44 v |= uint32(src[3])
45 v |= uint32(src[2]) << 8
46 v |= uint32(src[1]) << 16
47 v |= uint32(src[0]) << 24
48 case 3:
49 v |= uint32(src[2]) << 8
50 v |= uint32(src[1]) << 16
51 v |= uint32(src[0]) << 24
52 case 2:
53 v |= uint32(src[1]) << 16
54 v |= uint32(src[0]) << 24
55 case 1:
56 v |= uint32(src[0]) << 24
57 }
58 59 // Special case: zero (!!!!!) shortens to z.
60 if v == 0 && len(src) >= 4 {
61 dst[0] = 'z'
62 dst = dst[1:]
63 src = src[4:]
64 n++
65 continue
66 }
67 68 // Otherwise, 5 base 85 digits starting at !.
69 for i := 4; i >= 0; i-- {
70 dst[i] = '!' + byte(v%85)
71 v /= 85
72 }
73 74 // If src was short, discard the low destination bytes.
75 m := 5
76 if len(src) < 4 {
77 m -= 4 - len(src)
78 src = nil
79 } else {
80 src = src[4:]
81 }
82 dst = dst[m:]
83 n += m
84 }
85 return n
86 }
87 88 // MaxEncodedLen returns the maximum length of an encoding of n source bytes.
89 func MaxEncodedLen(n int) int { return (n + 3) / 4 * 5 }
90 91 // NewEncoder returns a new ascii85 stream encoder. Data written to
92 // the returned writer will be encoded and then written to w.
93 // Ascii85 encodings operate in 32-bit blocks; when finished
94 // writing, the caller must Close the returned encoder to flush any
95 // trailing partial block.
96 func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} }
97 98 type encoder struct {
99 err error
100 w io.Writer
101 buf [4]byte // buffered data waiting to be encoded
102 nbuf int // number of bytes in buf
103 out [1024]byte // output buffer
104 }
105 106 func (e *encoder) Write(p []byte) (n int, err error) {
107 if e.err != nil {
108 return 0, e.err
109 }
110 111 // Leading fringe.
112 if e.nbuf > 0 {
113 var i int
114 for i = 0; i < len(p) && e.nbuf < 4; i++ {
115 e.buf[e.nbuf] = p[i]
116 e.nbuf++
117 }
118 n += i
119 p = p[i:]
120 if e.nbuf < 4 {
121 return
122 }
123 nout := Encode(e.out[0:], e.buf[0:])
124 if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
125 return n, e.err
126 }
127 e.nbuf = 0
128 }
129 130 // Large interior chunks.
131 for len(p) >= 4 {
132 nn := len(e.out) / 5 * 4
133 if nn > len(p) {
134 nn = len(p)
135 }
136 nn -= nn % 4
137 if nn > 0 {
138 nout := Encode(e.out[0:], p[0:nn])
139 if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil {
140 return n, e.err
141 }
142 }
143 n += nn
144 p = p[nn:]
145 }
146 147 // Trailing fringe.
148 copy(e.buf[:], p)
149 e.nbuf = len(p)
150 n += len(p)
151 return
152 }
153 154 // Close flushes any pending output from the encoder.
155 // It is an error to call Write after calling Close.
156 func (e *encoder) Close() error {
157 // If there's anything left in the buffer, flush it out
158 if e.err == nil && e.nbuf > 0 {
159 nout := Encode(e.out[0:], e.buf[0:e.nbuf])
160 e.nbuf = 0
161 _, e.err = e.w.Write(e.out[0:nout])
162 }
163 return e.err
164 }
165 166 /*
167 * Decoder
168 */
169 170 type CorruptInputError int64
171 172 func (e CorruptInputError) Error() string {
173 return "illegal ascii85 data at input byte " + strconv.FormatInt(int64(e), 10)
174 }
175 176 // Decode decodes src into dst, returning both the number
177 // of bytes written to dst and the number consumed from src.
178 // If src contains invalid ascii85 data, Decode will return the
179 // number of bytes successfully written and a [CorruptInputError].
180 // Decode ignores space and control characters in src.
181 // Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
182 // Decode expects these to have been stripped by the caller.
183 //
184 // If flush is true, Decode assumes that src represents the
185 // end of the input stream and processes it completely rather
186 // than wait for the completion of another 32-bit block.
187 //
188 // [NewDecoder] wraps an [io.Reader] interface around Decode.
189 func Decode(dst, src []byte, flush bool) (ndst, nsrc int, err error) {
190 var v uint32
191 var nb int
192 for i, b := range src {
193 if len(dst)-ndst < 4 {
194 return
195 }
196 switch {
197 case b <= ' ':
198 continue
199 case b == 'z' && nb == 0:
200 nb = 5
201 v = 0
202 case '!' <= b && b <= 'u':
203 v = v*85 + uint32(b-'!')
204 nb++
205 default:
206 return 0, 0, CorruptInputError(i)
207 }
208 if nb == 5 {
209 nsrc = i + 1
210 dst[ndst] = byte(v >> 24)
211 dst[ndst+1] = byte(v >> 16)
212 dst[ndst+2] = byte(v >> 8)
213 dst[ndst+3] = byte(v)
214 ndst += 4
215 nb = 0
216 v = 0
217 }
218 }
219 if flush {
220 nsrc = len(src)
221 if nb > 0 {
222 // The number of output bytes in the last fragment
223 // is the number of leftover input bytes - 1:
224 // the extra byte provides enough bits to cover
225 // the inefficiency of the encoding for the block.
226 if nb == 1 {
227 return 0, 0, CorruptInputError(len(src))
228 }
229 for i := nb; i < 5; i++ {
230 // The short encoding truncated the output value.
231 // We have to assume the worst case values (digit 84)
232 // in order to ensure that the top bits are correct.
233 v = v*85 + 84
234 }
235 for i := 0; i < nb-1; i++ {
236 dst[ndst] = byte(v >> 24)
237 v <<= 8
238 ndst++
239 }
240 }
241 }
242 return
243 }
244 245 // NewDecoder constructs a new ascii85 stream decoder.
246 func NewDecoder(r io.Reader) io.Reader { return &decoder{r: r} }
247 248 type decoder struct {
249 err error
250 readErr error
251 r io.Reader
252 buf [1024]byte // leftover input
253 nbuf int
254 out []byte // leftover decoded output
255 outbuf [1024]byte
256 }
257 258 func (d *decoder) Read(p []byte) (n int, err error) {
259 if len(p) == 0 {
260 return 0, nil
261 }
262 if d.err != nil {
263 return 0, d.err
264 }
265 266 for {
267 // Copy leftover output from last decode.
268 if len(d.out) > 0 {
269 n = copy(p, d.out)
270 d.out = d.out[n:]
271 return
272 }
273 274 // Decode leftover input from last read.
275 var nn, nsrc, ndst int
276 if d.nbuf > 0 {
277 ndst, nsrc, d.err = Decode(d.outbuf[0:], d.buf[0:d.nbuf], d.readErr != nil)
278 if ndst > 0 {
279 d.out = d.outbuf[0:ndst]
280 d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
281 continue // copy out and return
282 }
283 if ndst == 0 && d.err == nil {
284 // Special case: input buffer is mostly filled with non-data bytes.
285 // Filter out such bytes to make room for more input.
286 off := 0
287 for i := 0; i < d.nbuf; i++ {
288 if d.buf[i] > ' ' {
289 d.buf[off] = d.buf[i]
290 off++
291 }
292 }
293 d.nbuf = off
294 }
295 }
296 297 // Out of input, out of decoded output. Check errors.
298 if d.err != nil {
299 return 0, d.err
300 }
301 if d.readErr != nil {
302 d.err = d.readErr
303 return 0, d.err
304 }
305 306 // Read more data.
307 nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
308 d.nbuf += nn
309 }
310 }
311