class PDF::Reader::Buffer
A string tokeniser that recognises PDF grammar. When passed an IO stream or a string, repeated calls to token() will return the next token from the source.
This is very low level, and getting the raw tokens is not very useful in itself.
This will usually be used in conjunction with PDF:Reader::Parser, which converts the raw tokens into objects we can work with (strings, ints, arrays, etc)
Constants
- FWD_SLASH
- ID
- LEFT_PAREN
some strings for comparissons. Declaring them here avoids creating new strings that need GC over and over
- LESS_THAN
- NULL_BYTE
- STREAM
- TOKEN_DELIMITER
- TOKEN_WHITESPACE
Attributes
Public Class Methods
Creates a new buffer.
Params:
io - an IO stream or string with the raw data to tokenise
options:
:seek - a byte offset to seek to before starting to tokenise :content_stream - set to true if buffer will be tokenising a content stream. Defaults to false
# File lib/pdf/reader/buffer.rb, line 64 def initialize(io, opts = {}) @io = io @tokens = [] @in_content_stream = opts[:content_stream] @io.seek(opts[:seek]) if opts[:seek] @pos = @io.pos end
Public Instance Methods
return true if there are no more tokens left
# File lib/pdf/reader/buffer.rb, line 75 def empty? prepare_tokens if @tokens.size < 3 @tokens.empty? end
return the byte offset where the first XRef table in th source can be found.
# File lib/pdf/reader/buffer.rb, line 126 def find_first_xref_offset check_size_is_non_zero @io.seek(-1024, IO::SEEK_END) rescue @io.seek(0) data = @io.read(1024) # the PDF 1.7 spec (section #3.4) says that EOL markers can be either \r, \n, or both. lines = data.split(/[\n\r]+/).reverse eof_index = lines.index { |l| l.strip[/^%%EOF/] } raise MalformedPDFError, "PDF does not contain EOF marker" if eof_index.nil? raise MalformedPDFError, "PDF EOF marker does not follow offset" if eof_index >= lines.size-1 lines[eof_index+1].to_i end
return raw bytes from the underlying IO stream.
bytes - the number of bytes to read
options:
:skip_eol - if true, the IO stream is advanced past a CRLF or LF that is sitting under the io cursor.
# File lib/pdf/reader/buffer.rb, line 90 def read(bytes, opts = {}) reset_pos if opts[:skip_eol] @io.seek(-1, IO::SEEK_CUR) str = @io.read(2) if str.nil? return nil elsif str == "\r\n" # do nothing elsif str[0,1] == "\n" @io.seek(-1, IO::SEEK_CUR) else @io.seek(-2, IO::SEEK_CUR) end end bytes = @io.read(bytes) save_pos bytes end
return the next token from the source. Returns a string if a token is found, nil if there are no tokens left.
# File lib/pdf/reader/buffer.rb, line 115 def token reset_pos prepare_tokens if @tokens.size < 3 merge_indirect_reference prepare_tokens if @tokens.size < 3 @tokens.shift end
Private Instance Methods
# File lib/pdf/reader/buffer.rb, line 142 def check_size_is_non_zero @io.seek(-1, IO::SEEK_END) @io.seek(0) rescue Errno::EINVAL raise MalformedPDFError, "PDF file is empty" end
Returns true if this buffer is parsing a content stream
# File lib/pdf/reader/buffer.rb, line 151 def in_content_stream? @in_content_stream ? true : false end
detect a series of 3 tokens that make up an indirect object. If we find them, replace the tokens with a PDF::Reader::Reference instance.
Merging them into a single string was another option, but that would mean code further up the stack would need to check every token to see if it looks like an indirect object. For optimisation reasons, I'd rather avoid that extra check.
It's incredibly likely that the next 3 tokens in the buffer are NOT an indirect reference, so test for that case first and avoid the relatively expensive regexp checks if possible.
# File lib/pdf/reader/buffer.rb, line 214 def merge_indirect_reference return if @tokens.size < 3 return if @tokens[2] != "R" if @tokens[0].match(/\d+/) && @tokens[1].match(/\d+/) @tokens[0] = PDF::Reader::Reference.new(@tokens[0].to_i, @tokens[1].to_i) @tokens[1] = nil @tokens[2] = nil @tokens.compact! end end
peek at the next character in the io stream, leaving the stream position untouched
# File lib/pdf/reader/buffer.rb, line 383 def peek_byte byte = @io.getbyte @io.seek(-1, IO::SEEK_CUR) if byte byte end
if we're currently inside a hex string, read hex nibbles until we find a closing >
# File lib/pdf/reader/buffer.rb, line 249 def prepare_hex_token str = "" finished = false while !finished byte = @io.getbyte if byte.nil? finished = true # unbalanced params elsif (48..57).include?(byte) || (65..90).include?(byte) || (97..122).include?(byte) str << byte elsif byte <= 32 # ignore it else @tokens << str if str.size > 0 @tokens << ">" if byte != 0x3E # '>' @tokens << byte.chr finished = true end end end
# File lib/pdf/reader/buffer.rb, line 226 def prepare_inline_token str = "" buffer = [] until buffer[0] =~ /\s|\0/ && buffer[1, 2] == ["E", "I"] chr = @io.read(1) buffer << chr if buffer.length > 3 str << buffer.shift end end str << NULL_BYTE if buffer.first == NULL_BYTE @tokens << string_token(str) @io.seek(-3, IO::SEEK_CUR) unless chr.nil? end
if we're currently inside a literal string we more or less just read bytes until we find the closing ) delimiter. Lots of bytes that would otherwise indicate the start of a new token in regular mode are left untouched when inside a literal string.
The entire literal string will be returned as a single token. It will need further processing to fix things like escaped new lines, but that's someone else's problem.
# File lib/pdf/reader/buffer.rb, line 279 def prepare_literal_token str = "" count = 1 while count > 0 byte = @io.getbyte if byte.nil? count = 0 # unbalanced params elsif byte == 0x5C str << byte << @io.getbyte elsif byte == 0x28 # "(" str << "(" count += 1 elsif byte == 0x29 # ")" count -= 1 str << ")" unless count == 0 else str << byte unless count == 0 end end @tokens << str if str.size > 0 @tokens << ")" end
Extract the next regular token and stock it in our buffer, ready to be returned.
What each byte means is complex, check out section “3.1.1 Character Set” of the 1.7 spec to read up on it.
# File lib/pdf/reader/buffer.rb, line 309 def prepare_regular_token tok = "" while byte = @io.getbyte case byte when 0x25 # comment, ignore everything until the next EOL char done = false while !done byte = @io.getbyte done = true if byte.nil? || byte == 0x0A || byte == 0x0D end when *TOKEN_WHITESPACE # white space, token finished @tokens << tok if tok.size > 0 #If the token was empty, chomp the rest of the whitespace too while TOKEN_WHITESPACE.include?(peek_byte) && tok.size == 0 @io.getbyte end tok = "" break when 0x3C # opening delimiter '<', start of new token @tokens << tok if tok.size > 0 if peek_byte == 0x3C # check if token is actually '<<' @io.getbyte @tokens << "<<" else @tokens << "<" end tok = "" break when 0x3E # closing delimiter '>', start of new token @tokens << tok if tok.size > 0 if peek_byte == 0x3E # check if token is actually '>>' @io.getbyte @tokens << ">>" else @tokens << ">" end tok = "" break when 0x28, 0x5B, 0x7B # opening delimiter, start of new token @tokens << tok if tok.size > 0 @tokens << byte.chr tok = "" break when 0x29, 0x5D, 0x7D # closing delimiter @tokens << tok if tok.size > 0 @tokens << byte.chr tok = "" break when 0x2F # PDF name, start of new token @tokens << tok if tok.size > 0 @tokens << byte.chr @tokens << "" if byte == 0x2F && ([nil, 0x20, 0x0A] + TOKEN_DELIMITER).include?(peek_byte) tok = "" break else tok << byte end end @tokens << tok if tok.size > 0 end
attempt to prime the buffer with the next few tokens.
# File lib/pdf/reader/buffer.rb, line 170 def prepare_tokens 10.times do case state when :literal_string then prepare_literal_token when :hex_string then prepare_hex_token when :regular then prepare_regular_token when :inline then prepare_inline_token end end save_pos end
Some bastard moved our IO stream cursor. Restore it.
# File lib/pdf/reader/buffer.rb, line 157 def reset_pos @io.seek(@pos) if @io.pos != @pos end
save the current position of the source IO stream. If someone else (like another buffer) moves the cursor, we can then restore it.
# File lib/pdf/reader/buffer.rb, line 164 def save_pos @pos = @io.pos end
tokenising behaves slightly differently based on the current context. Determine the current context/state by examining the last token we found
# File lib/pdf/reader/buffer.rb, line 186 def state case @tokens.last when LEFT_PAREN then :literal_string when LESS_THAN then :hex_string when STREAM then :stream when ID if in_content_stream? && @tokens[-2] != FWD_SLASH :inline else :regular end else :regular end end
for a handful of tokens we want to tell the parser how to convert them into higher level tokens. This methods adds a to_token() method to tokens that should remain as strings.
# File lib/pdf/reader/buffer.rb, line 393 def string_token(token) def token.to_token to_s end token end