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f8316638af
SourceForge doesn't choke on this batch :-)
I'm not entirely sure this is 100% correct. The patch changes an
\index{persistency} to \index{presistence}, and I don't know what \index{}
does. But it seems to do so persi--er, consistently, so I hope it isn't a
problem.
112 lines
4.5 KiB
TeX
112 lines
4.5 KiB
TeX
\section{\module{chunk} ---
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Read IFF chunked data}
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\declaremodule{standard}{chunk}
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\modulesynopsis{Module to read IFF chunks.}
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\moduleauthor{Sjoerd Mullender}{sjoerd@acm.org}
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\sectionauthor{Sjoerd Mullender}{sjoerd@acm.org}
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This module provides an interface for reading files that use EA IFF 85
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chunks.\footnote{``EA IFF 85'' Standard for Interchange Format Files,
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Jerry Morrison, Electronic Arts, January 1985.} This format is used
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in at least the Audio\index{Audio Interchange File
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Format}\index{AIFF}\index{AIFF-C} Interchange File Format
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(AIFF/AIFF-C) and the Real\index{Real Media File Format} Media File
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Format\index{RMFF} (RMFF). The WAVE audio file format is closely
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related and can also be read using this module.
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A chunk has the following structure:
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\begin{tableiii}{c|c|l}{textrm}{Offset}{Length}{Contents}
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\lineiii{0}{4}{Chunk ID}
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\lineiii{4}{4}{Size of chunk in big-endian byte order, not including the
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header}
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\lineiii{8}{\var{n}}{Data bytes, where \var{n} is the size given in
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the preceding field}
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\lineiii{8 + \var{n}}{0 or 1}{Pad byte needed if \var{n} is odd and
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chunk alignment is used}
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\end{tableiii}
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The ID is a 4-byte string which identifies the type of chunk.
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The size field (a 32-bit value, encoded using big-endian byte order)
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gives the size of the chunk data, not including the 8-byte header.
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Usually an IFF-type file consists of one or more chunks. The proposed
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usage of the \class{Chunk} class defined here is to instantiate an
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instance at the start of each chunk and read from the instance until
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it reaches the end, after which a new instance can be instantiated.
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At the end of the file, creating a new instance will fail with a
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\exception{EOFError} exception.
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\begin{classdesc}{Chunk}{file\optional{, align, bigendian, inclheader}}
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Class which represents a chunk. The \var{file} argument is expected
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to be a file-like object. An instance of this class is specifically
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allowed. The only method that is needed is \method{read()}. If the
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methods \method{seek()} and \method{tell()} are present and don't
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raise an exception, they are also used. If these methods are present
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and raise an exception, they are expected to not have altered the
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object. If the optional argument \var{align} is true, chunks are
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assumed to be aligned on 2-byte boundaries. If \var{align} is
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false, no alignment is assumed. The default value is true. If the
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optional argument \var{bigendian} is false, the chunk size is assumed
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to be in little-endian order. This is needed for WAVE audio files.
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The default value is true. If the optional argument \var{inclheader}
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is true, the size given in the chunk header includes the size of the
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header. The default value is false.
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\end{classdesc}
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A \class{Chunk} object supports the following methods:
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\begin{methoddesc}{getname}{}
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Returns the name (ID) of the chunk. This is the first 4 bytes of the
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chunk.
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\end{methoddesc}
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\begin{methoddesc}{getsize}{}
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Returns the size of the chunk.
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\end{methoddesc}
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\begin{methoddesc}{close}{}
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Close and skip to the end of the chunk. This does not close the
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underlying file.
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\end{methoddesc}
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The remaining methods will raise \exception{IOError} if called after
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the \method{close()} method has been called.
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\begin{methoddesc}{isatty}{}
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Returns \code{0}.
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\end{methoddesc}
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\begin{methoddesc}{seek}{pos\optional{, whence}}
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Set the chunk's current position. The \var{whence} argument is
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optional and defaults to \code{0} (absolute file positioning); other
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values are \code{1} (seek relative to the current position) and
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\code{2} (seek relative to the file's end). There is no return value.
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If the underlying file does not allow seek, only forward seeks are
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allowed.
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\end{methoddesc}
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\begin{methoddesc}{tell}{}
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Return the current position into the chunk.
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\end{methoddesc}
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\begin{methoddesc}{read}{\optional{size}}
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Read at most \var{size} bytes from the chunk (less if the read hits
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the end of the chunk before obtaining \var{size} bytes). If the
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\var{size} argument is negative or omitted, read all data until the
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end of the chunk. The bytes are returned as a string object. An
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empty string is returned when the end of the chunk is encountered
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immediately.
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\end{methoddesc}
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\begin{methoddesc}{skip}{}
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Skip to the end of the chunk. All further calls to \method{read()}
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for the chunk will return \code{''}. If you are not interested in the
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contents of the chunk, this method should be called so that the file
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points to the start of the next chunk.
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\end{methoddesc}
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