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.
7package ascii85
8
9import (
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.
27func 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.
86func 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.
93func NewEncoder(w io.Writer) io.WriteCloser { return &encoder{w: w} }
94
95type 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
103func (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	copy(e.buf[:], p)
146	e.nbuf = len(p)
147	n += len(p)
148	return
149}
150
151// Close flushes any pending output from the encoder.
152// It is an error to call Write after calling Close.
153func (e *encoder) Close() error {
154	// If there's anything left in the buffer, flush it out
155	if e.err == nil && e.nbuf > 0 {
156		nout := Encode(e.out[0:], e.buf[0:e.nbuf])
157		e.nbuf = 0
158		_, e.err = e.w.Write(e.out[0:nout])
159	}
160	return e.err
161}
162
163/*
164 * Decoder
165 */
166
167type CorruptInputError int64
168
169func (e CorruptInputError) Error() string {
170	return "illegal ascii85 data at input byte " + strconv.FormatInt(int64(e), 10)
171}
172
173// Decode decodes src into dst, returning both the number
174// of bytes written to dst and the number consumed from src.
175// If src contains invalid ascii85 data, Decode will return the
176// number of bytes successfully written and a [CorruptInputError].
177// Decode ignores space and control characters in src.
178// Often, ascii85-encoded data is wrapped in <~ and ~> symbols.
179// Decode expects these to have been stripped by the caller.
180//
181// If flush is true, Decode assumes that src represents the
182// end of the input stream and processes it completely rather
183// than wait for the completion of another 32-bit block.
184//
185// [NewDecoder] wraps an [io.Reader] interface around Decode.
186func Decode(dst, src []byte, flush bool) (ndst, nsrc int, err error) {
187	var v uint32
188	var nb int
189	for i, b := range src {
190		if len(dst)-ndst < 4 {
191			return
192		}
193		switch {
194		case b <= ' ':
195			continue
196		case b == 'z' && nb == 0:
197			nb = 5
198			v = 0
199		case '!' <= b && b <= 'u':
200			v = v*85 + uint32(b-'!')
201			nb++
202		default:
203			return 0, 0, CorruptInputError(i)
204		}
205		if nb == 5 {
206			nsrc = i + 1
207			dst[ndst] = byte(v >> 24)
208			dst[ndst+1] = byte(v >> 16)
209			dst[ndst+2] = byte(v >> 8)
210			dst[ndst+3] = byte(v)
211			ndst += 4
212			nb = 0
213			v = 0
214		}
215	}
216	if flush {
217		nsrc = len(src)
218		if nb > 0 {
219			// The number of output bytes in the last fragment
220			// is the number of leftover input bytes - 1:
221			// the extra byte provides enough bits to cover
222			// the inefficiency of the encoding for the block.
223			if nb == 1 {
224				return 0, 0, CorruptInputError(len(src))
225			}
226			for i := nb; i < 5; i++ {
227				// The short encoding truncated the output value.
228				// We have to assume the worst case values (digit 84)
229				// in order to ensure that the top bits are correct.
230				v = v*85 + 84
231			}
232			for i := 0; i < nb-1; i++ {
233				dst[ndst] = byte(v >> 24)
234				v <<= 8
235				ndst++
236			}
237		}
238	}
239	return
240}
241
242// NewDecoder constructs a new ascii85 stream decoder.
243func NewDecoder(r io.Reader) io.Reader { return &decoder{r: r} }
244
245type decoder struct {
246	err     error
247	readErr error
248	r       io.Reader
249	buf     [1024]byte // leftover input
250	nbuf    int
251	out     []byte // leftover decoded output
252	outbuf  [1024]byte
253}
254
255func (d *decoder) Read(p []byte) (n int, err error) {
256	if len(p) == 0 {
257		return 0, nil
258	}
259	if d.err != nil {
260		return 0, d.err
261	}
262
263	for {
264		// Copy leftover output from last decode.
265		if len(d.out) > 0 {
266			n = copy(p, d.out)
267			d.out = d.out[n:]
268			return
269		}
270
271		// Decode leftover input from last read.
272		var nn, nsrc, ndst int
273		if d.nbuf > 0 {
274			ndst, nsrc, d.err = Decode(d.outbuf[0:], d.buf[0:d.nbuf], d.readErr != nil)
275			if ndst > 0 {
276				d.out = d.outbuf[0:ndst]
277				d.nbuf = copy(d.buf[0:], d.buf[nsrc:d.nbuf])
278				continue // copy out and return
279			}
280			if ndst == 0 && d.err == nil {
281				// Special case: input buffer is mostly filled with non-data bytes.
282				// Filter out such bytes to make room for more input.
283				off := 0
284				for i := 0; i < d.nbuf; i++ {
285					if d.buf[i] > ' ' {
286						d.buf[off] = d.buf[i]
287						off++
288					}
289				}
290				d.nbuf = off
291			}
292		}
293
294		// Out of input, out of decoded output. Check errors.
295		if d.err != nil {
296			return 0, d.err
297		}
298		if d.readErr != nil {
299			d.err = d.readErr
300			return 0, d.err
301		}
302
303		// Read more data.
304		nn, d.readErr = d.r.Read(d.buf[d.nbuf:])
305		d.nbuf += nn
306	}
307}
308