From: Florian Gross <flgr@...>
Date: 2005-01-16T07:41:10+09:00
Subject: Re: Ogg Vorbis

asenchi@asenchi.com wrote:

> I've consider using external apps, but I would really like the comment
> manipluation to happen in Ruby.

See below. (I could not attach this because my news server complained 
about it being an attachment, sorry.) It should be a good starting point...

It would probably be a good idea to refactor the repetitive parts into 
private methods and being able to actually manipulate the data would 
also be nifty.

----

> class OggFile
>   class OggError < IOError; end
>   class MagicError < OggError; end
>   class VersionError < OggError; end
>   class PageTypeError < OggError; end
>   class PageOrderError < OggError; end
>   class PacketTypeError < OggError; end
>   class BlocksizeError < OggError; end
>   class ValueError < OggError; end
> 
>   class << self
>     alias :open :new
>   end
> 
>   module HeaderFlags
>     Continued, First, Last = *(0 .. 2).map { |i| 1 << i }
>   end
> 
>   module PacketTypes
>     Identification, Comment, Setup = *1 .. 2
>   end
> 
>   AllowedBlocksizes = [64, 128, 256, 512, 1024, 2048, 4096, 8192]
> 
>   attr_accessor :vorbis_version, :audio_channels, :audio_sample_rate,
>     :bitrate_maximum, :bitrate_nominal, :bitrate_minimum, :blocksize_0,
>     :blocksize_1, :comments
> 
>   def initialize(filename)
>     data = File.open(filename, "rb") { |file| file.read }
> 
>     magic, version, header_type, granule_position, serial_number,
>     page_number, checksum, segment_size, rest = *data.unpack("a4CCQLLLCa*")
> 
>     raise(MagicError, "File magic bytes are not 'OggS'") if magic != "OggS"
>     raise(VersionError, "Unknown structure version") if version != 0
>     raise(PageOrderError, "Out of order page") if page_number != 0
>     # Checksum is ignored for now
> 
>     if (header_type & HeaderFlags::First).zero? then
>       raise(PageTypeError, "Unexpected non-first page at file beginning") 
>     end
> 
>     # Rest of initial page, identification header
>     segment, id_packet_type, id_magic, @vorbis_version, @audio_channels,
>       @audio_sample_rate, @bitrate_maximum, @bitrate_nominal, @bitrate_minimum,
>       blocksize, framing_bit, rest = *rest.unpack(
>       "a#{segment_size}Ca6LCLlllCCa*")
> 
>     @blocksize_0 = 2 ** (blocksize & 0b1111) # first 4 bits are 2 exponent
>     @blocksize_1 = 2 ** (blocksize >> 4)     # last 4 bits are 2 exponent
> 
>     #if id_packet_type != PacketTypes::Identification then
>     #  raise(PacketTypeError, "Out of order packet. Expected Identification, " +
>     #    "but got #{id_packet_type}")
>     #end
> 
>     if id_magic != "vorbis" then
>       raise(MagicError, "Header magic needs to be 'vorbis'") 
>     end
> 
>     raise(VersionError, "Unknown vorbis version") if @vorbis_version != 0
>     raise(ValueError, "Audio channels need to be > 0") if @audio_channels == 0
> 
>     if @audio_sample_rate == 0 then
>       raise(ValueError, "Audio sample rate needs to be > 0")
>     end
> 
>     [@blocksize_0, @blocksize_1].each do |bs|
>       unless AllowedBlocksizes.include?(bs)
>         raise(ValueError, "Unallowed blocksize #{bs}")
>       end 
>     end
> 
>     if @blocksize_0 > @blocksize_1 then
>       raise(ValueError, "Blocksize 0 has to be > blocksize 1")
>     end
> 
>     raise(ValueError, "Framing bit has to be non-zero") if framing_bit.zero?
> 
>     # Second page
>     magic, version, header_type, granule_position, serial_number,
>     page_number, checksum, segment_size, rest = *rest.unpack("a4CCQLLLCa*")
> 
>     raise(MagicError, "File magic bytes are not 'OggS'") if magic != "OggS"
>     raise(VersionError, "Unknown structure version") if version != 0
>     raise(PageOrderError, "Out of order page") if page_number != 1
>     # Checksum is ignored for now
> 
>     # Rest of second page, comment header
>     segment, cmt_packet_type, cmt_magic, vendor_length, rest = *rest.unpack(
>       "a#{segment_size}Ca6LA*")
> 
>     #if cmt_packet_type != PacketTypes::Comment then
>     #  raise(PacketTypeError, "Out of order packet. Expected Comment, " +
>     #    "but got #{id_packet_type}")
>     #end
> 
>     if cmt_magic != "vorbis" then
>       raise(MagicError, "Header magic needs to be 'vorbis'")
>     end
> 
>     vendor, comment_count, rest = *rest.unpack("a#{vendor_length}LA*")
> 
>     @comments = Hash.new { |hash, key| hash[key] = Array.new }
>     comment_count.times do
>       length, rest = *rest.unpack("La*")
>       string, rest = *rest.unpack("a#{length}a*")
>       key, value = string.split("=", 2)
>       @comments[key.upcase] << value
>     end
> 
>     framing_bit, rest = *rest.unpack("Ca*")
> 
>     raise(ValueError, "Framing bit has to be non-zero") if framing_bit.zero?
>   end
> end