1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
|
--
-- pdf.lua: Portable Document Format
--
-- Based on PDF Reference, version 1.7
-- In practice almost useless, I just wanted to learn about the file format.
-- FIXME: it's also not very robust and doesn't support all documents.
--
-- Copyright (c) 2017, Přemysl Eric Janouch <p@janouch.name>
--
-- Permission to use, copy, modify, and/or distribute this software for any
-- purpose with or without fee is hereby granted.
--
-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
-- WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
-- MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
-- SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-- WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
-- OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
-- CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
--
local oct_alphabet = "01234567"
local dec_alphabet = "0123456789"
local hex_alphabet = "0123456789abcdefABCDEF"
local whitespace = "\x00\t\n\f\r "
local delimiters = "()<>[]{}/%"
local strchr = function (s, ch) return s:find (ch, 1, true) end
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
local Lexer = {}
Lexer.__index = Lexer
function Lexer:new (c)
return setmetatable ({ c = c }, self)
end
-- TODO: make it possible to follow a string, we should probably be able to
-- supply callbacks to the constructor, or a wrapper object;
-- this will be used for object streams
function Lexer:getc ()
if self.c.eof then return nil end
return self.c:read (1)
end
function Lexer:ungetc ()
self.c.position = self.c.position - 1
end
function Lexer:token (type, value, description)
if description then
self.c (self.start, self.c.position - 1):mark (description)
end
return { type=type, value=value,
start=self.start, stop=self.c.position - 1 }
end
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
function Lexer:eat_newline (ch)
if ch == '\r' then
ch = self:getc ()
if ch and ch ~= '\n' then self:ungetc () end
return true
elseif ch == '\n' then
return true
end
end
function Lexer:string ()
local value, level, ch = "", 1
::continue::
while true do
ch = self:getc ()
if not ch then return nil
elseif ch == '\\' then
ch = self:getc ()
if not ch then return nil
elseif ch == 'n' then ch = '\n'
elseif ch == 'r' then ch = '\r'
elseif ch == 't' then ch = '\t'
elseif ch == 'b' then ch = '\b'
elseif ch == 'f' then ch = '\f'
elseif self:eat_newline (ch) then goto continue
elseif strchr (oct_alphabet, ch) then
local buf, i = ch
for i = 1, 2 do
ch = self:getc ()
if not ch then return nil
elseif not strchr (oct_alphabet, ch) then
self:ungetc ()
break
end
buf = buf .. ch
end
ch = string.char (tonumber (buf, 8))
end
elseif self:eat_newline (ch) then
ch = '\n'
elseif ch == '(' then
level = level + 1
elseif ch == ')' then
level = level - 1
if level == 0 then break end
end
value = value .. ch
end
return self:token ('string', value, "string literal")
end
function Lexer:string_hex ()
local value, buf, ch = ""
while true do
ch = self:getc ()
if not ch then return nil
elseif ch == '>' then
break
elseif not strchr (hex_alphabet, ch) then
return nil
elseif buf then
value = value .. string.char (tonumber (buf .. ch, 16))
buf = nil
else
buf = ch
end
end
if buf then value = value .. string.char (tonumber (buf .. '0', 16)) end
return self:token ('string', value, "string hex")
end
function Lexer:name ()
local value, ch = ""
while true do
ch = self:getc ()
if not ch then break
elseif ch == '#' then
local ch1, ch2 = self:getc (), self:getc ()
if not ch1 or not ch2
or not strchr (hex_alphabet, ch1)
or not strchr (hex_alphabet, ch2) then
return nil
end
ch = string.char (tonumber (ch1 .. ch2, 16))
elseif strchr (whitespace .. delimiters, ch) then
self:ungetc ()
break
end
value = value .. ch
end
if value == "" then return nil end
return self:token ('name', value, "name")
end
function Lexer:comment ()
local value, ch = ""
while true do
ch = self:getc ()
if not ch then break
elseif ch == '\r' or ch == '\n' then
self:ungetc ()
break
end
value = value .. ch
end
return self:token ('comment', value, "comment")
end
function Lexer:number (ch)
local value, real, digits = "", false, false
if ch == '-' then
value = ch
ch = self:getc ()
end
while ch do
if strchr (dec_alphabet, ch) then
digits = true
elseif ch == '.' and not real then
real = true
else
self:ungetc ()
break
end
value = value .. ch
ch = self:getc ()
end
-- XXX: perhaps we should instead let it be interpreted as a keyword
if not digits then return nil end
-- XXX: maybe we should differentiate between integers and real values
return self:token ('number', tonumber (value, 10), "number")
end
function Lexer:get_token ()
::restart::
self.start = self.c.position
local ch = self:getc ()
if not ch then return nil
elseif ch == '(' then return self:string ()
elseif ch == '[' then return self:token ('begin_array')
elseif ch == ']' then return self:token ('end_array')
elseif ch == '<' then
-- It seems they ran out of paired characters, yet {} is unused
ch = self:getc ()
if not ch then return nil
elseif ch == '<' then return self:token ('begin_dictionary')
else
self:ungetc ()
return self:string_hex ()
end
elseif ch == '>' then
ch = self:getc ()
if not ch then return nil
elseif ch == '>' then return self:token ('end_dictionary')
else return nil end
elseif ch == '/' then return self:name ()
elseif ch == '%' then return self:comment ()
elseif strchr ("-0123456789.", ch) then return self:number (ch)
elseif self:eat_newline (ch) then return self:token ('newline')
elseif strchr (whitespace, ch) then goto restart
else
-- {} end up being keywords but we should probably error out
local value = ch
while true do
ch = self:getc ()
if not ch then break
elseif strchr (whitespace .. delimiters, ch) then
self:ungetc ()
break
end
value = value .. ch
end
if value == "null" then
return self:token ('null', nil, "null")
elseif value == "true" then
return self:token ('boolean', true, "boolean")
elseif value == "false" then
return self:token ('boolean', false, "boolean")
end
return self:token ('keyword', value, "keyword")
end
end
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
local is_value = function (t)
return t == 'null' or t == 'boolean' or t == 'name'
or t == 'number' or t == 'string'
end
-- Retrieve the next thing in the stream, possibly popping values from the stack
local function get_object (lex, stack, deref)
::restart::
local token = lex:get_token ()
if token == nil then return nil
elseif token.type == 'begin_array' then
local array = {}
repeat
local object = get_object (lex, array, deref)
if not object then error ("array doesn't end") end
table.insert (array, object)
until object.type == 'end_array'
local stop = table.remove (array)
return { type='array', value=array, start=token.start, stop=stop.stop }
elseif token.type == 'begin_dictionary' then
local dict = {}
repeat
local object = get_object (lex, dict, deref)
if not object then error ("dictionary doesn't end") end
table.insert (dict, object)
until object.type == 'end_dictionary'
local stop, kv = table.remove (dict), {}
if #dict % 2 == 1 then error ("unbalanced dictionary") end
for i = 1, #dict, 2 do
local k, v = dict[i], dict[i + 1]
if k.type ~= 'name' then error ("invalid dictionary key type") end
kv[k.value] = v
end
return { type='dict', value=kv, start=token.start, stop=stop.stop }
elseif token.type == 'keyword' and token.value == 'stream' then
if #stack < 1 then error ("no dictionary for stream") end
local d = table.remove (stack)
if d.type ~= 'dict' then error ("stream not preceded by dictionary") end
if not lex:eat_newline (lex:getc ()) then
error ("'stream' not followed by newline")
end
local len = deref (d.value['Length'])
if not len or len.type ~= 'number' then
error ("missing stream length")
end
local data, stop = lex.c:read (len.value), get_object (lex, {}, deref)
if not stop or stop.type ~= 'keyword' or stop.value ~= 'endstream' then
error ("missing 'endstream'")
end
return { type='stream', value={ dict=dict, data=data },
start=token.start, stop=stop.stop }
elseif token.type == 'keyword' and token.value == 'obj' then
if #stack < 2 then error ("missing object ID pair") end
local gen, n = table.remove (stack), table.remove (stack)
if n.type ~= 'number' or gen.type ~= 'number' then
error ("object ID pair must be two integers")
end
local tmp = {}
repeat
local object = get_object (lex, tmp, deref)
if not object then error ("object doesn't end") end
table.insert (tmp, object)
until object.type == 'keyword' and object.value == 'endobj'
local stop = table.remove (tmp)
if #tmp ~= 1 then error ("objects must contain exactly one value") end
local value = table.remove (tmp)
return { type='object', n=n.value, gen=gen.value, value=value,
start=n.start, stop=stop.stop }
elseif token.type == 'keyword' and token.value == 'R' then
if #stack < 2 then error ("missing reference ID pair") end
local gen, n = table.remove (stack), table.remove (stack)
if n.type ~= 'number' or gen.type ~= 'number' then
error ("reference ID pair must be two integers")
end
return { type='reference', value={ n.value, gen.value } }
elseif token.type == 'newline' or token.type == 'comment' then
-- These are not objects and our callers aren't interested
goto restart
else
return token
end
end
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
local detect = function (c)
return c:read (5) == "%PDF-"
end
local decode_xref_subsection = function (lex, start, count, result)
if not lex:eat_newline (lex:getc ()) then
error ("xref subsection must start on a new line")
end
for i = 0, count - 1 do
local entry = lex.c:read (20)
local off, gen, typ = entry:match
("^(%d%d%d%d%d%d%d%d%d%d) (%d%d%d%d%d) ([fn])[\r ][\r\n]$")
if not off then error ("invalid xref entry") end
-- Translated to the extended XRefStm format
result[start + i] = {
t = typ == 'n' and 1 or 0,
o = math.tointeger (off),
g = math.tointeger (gen),
}
end
end
-- A deref that can't actually resolve anything, for early stages of processing
local deref_nil = function (x)
if not x or x.type == 'reference' then return nil end
return x
end
-- Creates a table with named indexes from the trailer and items indexed by
-- object numbers containing { XRefStm fields... }
local decode_xref_normal = function (lex)
local result = {}
while true do
local a = get_object (lex, {}, deref_nil)
local b = get_object (lex, {}, deref_nil)
if not a or not b then
error ("xref section ends too soon")
elseif a.type == 'number' and b.type == 'number' then
decode_xref_subsection (lex, a.value, b.value, result)
elseif a.type == 'keyword' and a.value == 'trailer'
and b.type == 'dict' then
for k, v in pairs (b.value) do
result[k] = v
end
return result
else
error ("invalid xref contents")
end
end
end
local decode_xref_stream = function (lex, stream)
if stream.dict['Type'] ~= 'XRef' then error ("expected an XRef stream") end
-- TODO: decode a cross-reference stream from stream.{dict,data};
-- the compression filter, if present, is always going to be FlateDecode,
-- which we'll have to import or implement
-- TODO: take care to also cache cross-reference streams by offset when
-- they're actually implemented
error ("cross-reference streams not implemented")
end
local decode_xref = function (c)
local lex, stack = Lexer:new (c), {}
while true do
local object = get_object (lex, stack, deref_nil)
if object == nil then
return nil
elseif object.type == 'keyword' and object.value == 'xref' then
return decode_xref_normal (lex)
elseif object.type == 'stream' then
return decode_xref_stream (lex, object)
end
table.insert (stack, object)
end
end
-- Return all objects found in xref tables as a table indexed by object number,
-- pointing to a list of generations and overwrites, from newest to oldest.
local read_all_xrefs = function (c, start_offset)
local loaded, result, offset = {}, {}, start_offset
while true do
-- Prevent an infinite loop with malicious files
if loaded[offset] then error ("cyclic cross-reference sections") end
local xref = decode_xref (c (1 + offset, #c))
if not xref then break end
for k, v in pairs (xref) do
if type (k) == 'number' then
if not result[k] then result[k] = {} end
table.insert (result[k], v)
end
end
loaded[offset] = true
-- TODO: when 'XRefStm' is found, it has precedence over this 'Prev',
-- and also has its own 'Prev' chain
local prev = xref['Prev']
if not prev or prev.type ~= 'number' then break end
offset = prev.value
end
return result
end
local decode = function (c)
assert (c.position == 1)
if not detect (c ()) then error ("not a PDF file") end
-- Look for a pointer to the xref section within the last kibibyte
-- NOTE: we could probably look backwards for the "trailer" line from here
-- but we don't know how long the trailer is and we don't want to regex
-- scan the whole file (ignoring that dictionary contents might, possibly
-- legally, include the word "trailer" at the beginning of a new line)
local tail_len = math.min (1024, #c)
local tail = c (#c - tail_len, #c):read (tail_len)
local xref_loc = tail:match (".*%sstartxref%s+(%d+)%s+%%%%EOF")
if not xref_loc then error ("cannot find trailer") end
-- We need to decode xref sections in order to be able to resolve indirect
-- references to stream lengths
local xref = read_all_xrefs (c, math.tointeger (xref_loc))
local deref
-- We have to make sure that we don't decode objects twice as that would
-- duplicate all marks, so we simply cache all objects by offset.
-- This may be quite the memory load but it seems to be the best thing.
local cache = {}
local read_object = function (offset)
if cache[offset] then return cache[offset] end
local lex, stack = Lexer:new (c (1 + offset, #c)), {}
repeat
local object = get_object (lex, stack, deref)
if not object then error ("object doesn't end") end
table.insert (stack, object)
until object.type == 'object'
local object = table.remove (stack)
cache[offset] = object
c (offset + object.start, offset + object.stop)
:mark ("object " .. object.n .. " " .. object.gen)
return object
end
-- Resolve an object -- if it's a reference, look it up in "xref",
-- otherwise just return the object as it was passed
deref = function (x)
if not x or x.type ~= 'reference' then return x end
local n, gen = x.value[1], x.value[2]
-- TODO: we should also ignore object numbers >= trailer /Size
local bin = xref[n]
if not bin then return nil end
local entry = bin[1]
if not entry or entry.t ~= 1 or entry.g ~= gen then return nil end
local object = read_object (entry.o)
if not object or object.n ~= n or object.gen ~= gen then return nil end
return object.value
end
-- Read all objects accessible from the current version of the document
for n, bin in pairs (xref) do
local entry = bin[1]
if entry and entry.t == 1 then
read_object (entry.o)
end
end
-- TODO: we should actually try to decode even unreferenced objects.
-- The problem with decoding content from previous versions of the
-- document is that we must ignore xref updates from newer versions.
-- The version information needs to be propagated everywhere.
end
hex.register { type="pdf", detect=detect, decode=decode }
|