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 fallthrough 46 case 3: 47 v |= uint32(src[2]) << 8 48 fallthrough 49 case 2: 50 v |= uint32(src[1]) << 16 51 fallthrough 52 case 1: 53 v |= uint32(src[0]) << 24 54 } 55 56 // Special case: zero (!!!!!) shortens to z. 57 if v == 0 && len(src) >= 4 { 58 dst[0] = 'z' 59 dst = dst[1:] 60 src = src[4:] 61 n++ 62 continue 63 } 64 65 // Otherwise, 5 base 85 digits starting at !. 66 for i := 4; i >= 0; i-- { 67 dst[i] = '!' + byte(v%85) 68 v /= 85 69 } 70 71 // If src was short, discard the low destination bytes. 72 m := 5 73 if len(src) < 4 { 74 m -= 4 - len(src) 75 src = nil 76 } else { 77 src = src[4:] 78 } 79 dst = dst[m:] 80 n += m 81 } 82 return n 83 } 84 85 // MaxEncodedLen returns the maximum length of an encoding of n source bytes. 86 func MaxEncodedLen(n int) int { return (n + 3) / 4 * 5 } 87 88 // NewEncoder returns a new ascii85 stream encoder. Data written to 89 // the returned writer will be encoded and then written to w. 90 // Ascii85 encodings operate in 32-bit blocks; when finished 91 // writing, the caller must Close the returned encoder to flush any 92 // trailing partial block. 93 func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} } 94 95 type encoder struct { 96 err error 97 w io.Writer 98 buf [4]byte // buffered data waiting to be encoded 99 nbuf int // number of bytes in buf 100 out [1024]byte // output buffer 101 } 102 103 func (e *encoder) Write(p []byte) (n int, err error) { 104 if e.err != nil { 105 return 0, e.err 106 } 107 108 // Leading fringe. 109 if e.nbuf > 0 { 110 var i int 111 for i = 0; i < len(p) && e.nbuf < 4; i++ { 112 e.buf[e.nbuf] = p[i] 113 e.nbuf++ 114 } 115 n += i 116 p = p[i:] 117 if e.nbuf < 4 { 118 return 119 } 120 nout := Encode(e.out[0:], e.buf[0:]) 121 if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil { 122 return n, e.err 123 } 124 e.nbuf = 0 125 } 126 127 // Large interior chunks. 128 for len(p) >= 4 { 129 nn := len(e.out) / 5 * 4 130 if nn > len(p) { 131 nn = len(p) 132 } 133 nn -= nn % 4 134 if nn > 0 { 135 nout := Encode(e.out[0:], p[0:nn]) 136 if _, e.err = e.w.Write(e.out[0:nout]); e.err != nil { 137 return n, e.err 138 } 139 } 140 n += nn 141 p = p[nn:] 142 } 143 144 // Trailing fringe. 145 for i := 0; i < len(p); i++ { 146 e.buf[i] = p[i] 147 } 148 e.nbuf = len(p) 149 n += len(p) 150 return 151 } 152 153 // Close flushes any pending output from the encoder. 154 // It is an error to call Write after calling Close. 155 func (e *encoder) Close() error { 156 // If there's anything left in the buffer, flush it out 157 if e.err == nil && e.nbuf > 0 { 158 nout := Encode(e.out[0:], e.buf[0:e.nbuf]) 159 e.nbuf = 0 160 _, e.err = e.w.Write(e.out[0:nout]) 161 } 162 return e.err 163 } 164 165 /* 166 * Decoder 167 */ 168 169 type CorruptInputError int64 170 171 func (e CorruptInputError) Error() string { 172 return "illegal ascii85 data at input byte " + strconv.FormatInt(int64(e), 10) 173 } 174 175 // Decode decodes src into dst, returning both the number 176 // of bytes written to dst and the number consumed from src. 177 // If src contains invalid ascii85 data, Decode will return the 178 // number of bytes successfully written and a CorruptInputError. 179 // Decode ignores space and control characters in src. 180 // Often, ascii85-encoded data is wrapped in <~ and ~> symbols. 181 // Decode expects these to have been stripped by the caller. 182 // 183 // If flush is true, Decode assumes that src represents the 184 // end of the input stream and processes it completely rather 185 // than wait for the completion of another 32-bit block. 186 // 187 // NewDecoder wraps an io.Reader interface around Decode. 188 // 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
