cpython/Lib/aifc.py

"""Stuff to parse AIFF-C and AIFF files.

Unless explicitly stated otherwise, the description below is true
both for AIFF-C files and AIFF files.

An AIFF-C file has the following structure.

  +-----------------+
  | FORM            |
  +-----------------+
  | <size>          |
  +----+------------+
  |    | AIFC       |
  |    +------------+
  |    | <chunks>   |
  |    |    .       |
  |    |    .       |
  |    |    .       |
  +----+------------+

An AIFF file has the string "AIFF" instead of "AIFC".

A chunk consists of an identifier (4 bytes) followed by a size (4 bytes,
big endian order), followed by the data.  The size field does not include
the size of the 8 byte header.

The following chunk types are recognized.

  FVER
      <version number of AIFF-C defining document> (AIFF-C only).
  MARK
      <# of markers> (2 bytes)
      list of markers:
          <marker ID> (2 bytes, must be > 0)
          <position> (4 bytes)
          <marker name> ("pstring")
  COMM
      <# of channels> (2 bytes)
      <# of sound frames> (4 bytes)
      <size of the samples> (2 bytes)
      <sampling frequency> (10 bytes, IEEE 80-bit extended
          floating point)
      in AIFF-C files only:
      <compression type> (4 bytes)
      <human-readable version of compression type> ("pstring")
  SSND
      <offset> (4 bytes, not used by this program)
      <blocksize> (4 bytes, not used by this program)
      <sound data>

A pstring consists of 1 byte length, a string of characters, and 0 or 1
byte pad to make the total length even.

Usage.

Reading AIFF files:
  f = aifc.open(file, 'r')
where file is either the name of a file or an open file pointer.
The open file pointer must have methods read(), seek(), and close().
In some types of audio files, if the setpos() method is not used,
the seek() method is not necessary.

This returns an instance of a class with the following public methods:
  getnchannels()  -- returns number of audio channels (1 for
             mono, 2 for stereo)
  getsampwidth()  -- returns sample width in bytes
  getframerate()  -- returns sampling frequency
  getnframes()    -- returns number of audio frames
  getcomptype()   -- returns compression type ('NONE' for AIFF files)
  getcompname()   -- returns human-readable version of
             compression type ('not compressed' for AIFF files)
  getparams() -- returns a tuple consisting of all of the
             above in the above order
  getmarkers()    -- get the list of marks in the audio file or None
             if there are no marks
  getmark(id) -- get mark with the specified id (raises an error
             if the mark does not exist)
  readframes(n)   -- returns at most n frames of audio
  rewind()    -- rewind to the beginning of the audio stream
  setpos(pos) -- seek to the specified position
  tell()      -- return the current position
  close()     -- close the instance (make it unusable)
The position returned by tell(), the position given to setpos() and
the position of marks are all compatible and have nothing to do with
the actual position in the file.
The close() method is called automatically when the class instance
is destroyed.

Writing AIFF files:
  f = aifc.open(file, 'w')
where file is either the name of a file or an open file pointer.
The open file pointer must have methods write(), tell(), seek(), and
close().

This returns an instance of a class with the following public methods:
  aiff()      -- create an AIFF file (AIFF-C default)
  aifc()      -- create an AIFF-C file
  setnchannels(n) -- set the number of channels
  setsampwidth(n) -- set the sample width
  setframerate(n) -- set the frame rate
  setnframes(n)   -- set the number of frames
  setcomptype(type, name)
          -- set the compression type and the
             human-readable compression type
  setparams(tuple)
          -- set all parameters at once
  setmark(id, pos, name)
          -- add specified mark to the list of marks
  tell()      -- return current position in output file (useful
             in combination with setmark())
  writeframesraw(data)
          -- write audio frames without pathing up the
             file header
  writeframes(data)
          -- write audio frames and patch up the file header
  close()     -- patch up the file header and close the
             output file
You should set the parameters before the first writeframesraw or
writeframes.  The total number of frames does not need to be set,
but when it is set to the correct value, the header does not have to
be patched up.
It is best to first set all parameters, perhaps possibly the
compression type, and then write audio frames using writeframesraw.
When all frames have been written, either call writeframes('') or
close() to patch up the sizes in the header.
Marks can be added anytime.  If there are any marks, you must call
close() after all frames have been written.
The close() method is called automatically when the class instance
is destroyed.

When a file is opened with the extension '.aiff', an AIFF file is
written, otherwise an AIFF-C file is written.  This default can be
changed by calling aiff() or aifc() before the first writeframes or
writeframesraw.
"""

import struct
import builtins
import warnings

__all__ = ["Error", "open", "openfp"]

class Error(Exception):
    pass

_AIFC_version = 0xA2805140     # Version 1 of AIFF-C

def _read_long(file):
    try:
        return struct.unpack('>l', file.read(4))[0]
    except struct.error:
        raise EOFError

def _read_ulong(file):
    try:
        return struct.unpack('>L', file.read(4))[0]
    except struct.error:
        raise EOFError

def _read_short(file):
    try:
        return struct.unpack('>h', file.read(2))[0]
    except struct.error:
        raise EOFError

def _read_ushort(file):
    try:
        return struct.unpack('>H', file.read(2))[0]
    except struct.error:
        raise EOFError

def _read_string(file):
    length = ord(file.read(1))
    if length == 0:
        data = b''
    else:
        data = file.read(length)
    if length & 1 == 0:
        dummy = file.read(1)
    return data

_HUGE_VAL = 1.79769313486231e+308 # See <limits.h>

def _read_float(f): # 10 bytes
    expon = _read_short(f) # 2 bytes
    sign = 1
    if expon < 0:
        sign = -1
        expon = expon + 0x8000
    himant = _read_ulong(f) # 4 bytes
    lomant = _read_ulong(f) # 4 bytes
    if expon == himant == lomant == 0:
        f = 0.0
    elif expon == 0x7FFF:
        f = _HUGE_VAL
    else:
        expon = expon - 16383
        f = (himant * 0x100000000 + lomant) * pow(2.0, expon - 63)
    return sign * f

def _write_short(f, x):
    f.write(struct.pack('>h', x))

def _write_ushort(f, x):
    f.write(struct.pack('>H', x))

def _write_long(f, x):
    f.write(struct.pack('>l', x))

def _write_ulong(f, x):
    f.write(struct.pack('>L', x))

def _write_string(f, s):
    if len(s) > 255:
        raise ValueError("string exceeds maximum pstring length")
    f.write(struct.pack('B', len(s)))
    f.write(s)
    if len(s) & 1 == 0:
        f.write(b'\x00')

def _write_float(f, x):
    import math
    if x < 0:
        sign = 0x8000
        x = x * -1
    else:
        sign = 0
    if x == 0:
        expon = 0
        himant = 0
        lomant = 0
    else:
        fmant, expon = math.frexp(x)
        if expon > 16384 or fmant >= 1 or fmant != fmant: # Infinity or NaN
            expon = sign|0x7FFF
            himant = 0
            lomant = 0
        else:                   # Finite
            expon = expon + 16382
            if expon < 0:           # denormalized
                fmant = math.ldexp(fmant, expon)
                expon = 0
            expon = expon | sign
            fmant = math.ldexp(fmant, 32)
            fsmant = math.floor(fmant)
            himant = int(fsmant)
            fmant = math.ldexp(fmant - fsmant, 32)
            fsmant = math.floor(fmant)
            lomant = int(fsmant)
    _write_ushort(f, expon)
    _write_ulong(f, himant)
    _write_ulong(f, lomant)

from chunk import Chunk

class Aifc_read:
    # Variables used in this class:
    #
    # These variables are available to the user though appropriate
    # methods of this class:
    # _file -- the open file with methods read(), close(), and seek()
    #       set through the __init__() method
    # _nchannels -- the number of audio channels
    #       available through the getnchannels() method
    # _nframes -- the number of audio frames
    #       available through the getnframes() method
    # _sampwidth -- the number of bytes per audio sample
    #       available through the getsampwidth() method
    # _framerate -- the sampling frequency
    #       available through the getframerate() method
    # _comptype -- the AIFF-C compression type ('NONE' if AIFF)
    #       available through the getcomptype() method
    # _compname -- the human-readable AIFF-C compression type
    #       available through the getcomptype() method
    # _markers -- the marks in the audio file
    #       available through the getmarkers() and getmark()
    #       methods
    # _soundpos -- the position in the audio stream
    #       available through the tell() method, set through the
    #       setpos() method
    #
    # These variables are used internally only:
    # _version -- the AIFF-C version number
    # _decomp -- the decompressor from builtin module cl
    # _comm_chunk_read -- 1 iff the COMM chunk has been read
    # _aifc -- 1 iff reading an AIFF-C file
    # _ssnd_seek_needed -- 1 iff positioned correctly in audio
    #       file for readframes()
    # _ssnd_chunk -- instantiation of a chunk class for the SSND chunk
    # _framesize -- size of one frame in the file

    def initfp(self, file):
        self._version = 0
        self._convert = None
        self._markers = []
        self._soundpos = 0
        self._file = file
        chunk = Chunk(file)
        if chunk.getname() != b'FORM':
            raise Error('file does not start with FORM id')
        formdata = chunk.read(4)
        if formdata == b'AIFF':
            self._aifc = 0
        elif formdata == b'AIFC':
            self._aifc = 1
        else:
            raise Error('not an AIFF or AIFF-C file')
        self._comm_chunk_read = 0
        while 1:
            self._ssnd_seek_needed = 1
            try:
                chunk = Chunk(self._file)
            except EOFError:
                break
            chunkname = chunk.getname()
            if chunkname == b'COMM':
                self._read_comm_chunk(chunk)
                self._comm_chunk_read = 1
            elif chunkname == b'SSND':
                self._ssnd_chunk = chunk
                dummy = chunk.read(8)
                self._ssnd_seek_needed = 0
            elif chunkname == b'FVER':
                self._version = _read_ulong(chunk)
            elif chunkname == b'MARK':
                self._readmark(chunk)
            chunk.skip()
        if not self._comm_chunk_read or not self._ssnd_chunk:
            raise Error('COMM chunk and/or SSND chunk missing')

    def __init__(self, f):
        if isinstance(f, str):
            f = builtins.open(f, 'rb')
        # else, assume it is an open file object already
        self.initfp(f)

    #
    # User visible methods.
    #
    def getfp(self):
        return self._file

    def rewind(self):
        self._ssnd_seek_needed = 1
        self._soundpos = 0

    def close(self):
        self._file.close()

    def tell(self):
        return self._soundpos

    def getnchannels(self):
        return self._nchannels

    def getnframes(self):
        return self._nframes

    def getsampwidth(self):
        return self._sampwidth

    def getframerate(self):
        return self._framerate

    def getcomptype(self):
        return self._comptype

    def getcompname(self):
        return self._compname

##  def getversion(self):
##      return self._version

    def getparams(self):
        return self.getnchannels(), self.getsampwidth(), \
              self.getframerate(), self.getnframes(), \
              self.getcomptype(), self.getcompname()

    def getmarkers(self):
        if len(self._markers) == 0:
            return None
        return self._markers

    def getmark(self, id):
        for marker in self._markers:
            if id == marker[0]:
                return marker
        raise Error('marker {0!r} does not exist'.format(id))

    def setpos(self, pos):
        if pos < 0 or pos > self._nframes:
            raise Error('position not in range')
        self._soundpos = pos
        self._ssnd_seek_needed = 1

    def readframes(self, nframes):
        if self._ssnd_seek_needed:
            self._ssnd_chunk.seek(0)
            dummy = self._ssnd_chunk.read(8)
            pos = self._soundpos * self._framesize
            if pos:
                self._ssnd_chunk.seek(pos + 8)
            self._ssnd_seek_needed = 0
        if nframes == 0:
            return b''
        data = self._ssnd_chunk.read(nframes * self._framesize)
        if self._convert and data:
            data = self._convert(data)
        self._soundpos = self._soundpos + len(data) // (self._nchannels
                                                        * self._sampwidth)
        return data

    #
    # Internal methods.
    #

    def _alaw2lin(self, data):
        import audioop
        return audioop.alaw2lin(data, 2)

    def _ulaw2lin(self, data):
        import audioop
        return audioop.ulaw2lin(data, 2)

    def _adpcm2lin(self, data):
        import audioop
        if not hasattr(self, '_adpcmstate'):
            # first time
            self._adpcmstate = None
        data, self._adpcmstate = audioop.adpcm2lin(data, 2, self._adpcmstate)
        return data

    def _read_comm_chunk(self, chunk):
        self._nchannels = _read_short(chunk)
        self._nframes = _read_long(chunk)
        self._sampwidth = (_read_short(chunk) + 7) // 8
        self._framerate = int(_read_float(chunk))
        self._framesize = self._nchannels * self._sampwidth
        if self._aifc:
            #DEBUG: SGI's soundeditor produces a bad size :-(
            kludge = 0
            if chunk.chunksize == 18:
                kludge = 1
                warnings.warn('Warning: bad COMM chunk size')
                chunk.chunksize = 23
            #DEBUG end
            self._comptype = chunk.read(4)
            #DEBUG start
            if kludge:
                length = ord(chunk.file.read(1))
                if length & 1 == 0:
                    length = length + 1
                chunk.chunksize = chunk.chunksize + length
                chunk.file.seek(-1, 1)
            #DEBUG end
            self._compname = _read_string(chunk)
            if self._comptype != b'NONE':
                if self._comptype == b'G722':
                    self._convert = self._adpcm2lin
                elif self._comptype in (b'ulaw', b'ULAW'):
                    self._convert = self._ulaw2lin
                elif self._comptype in (b'alaw', b'ALAW'):
                    self._convert = self._alaw2lin
                else:
                    raise Error('unsupported compression type')
                self._sampwidth = 2
        else:
            self._comptype = b'NONE'
            self._compname = b'not compressed'

    def _readmark(self, chunk):
        nmarkers = _read_short(chunk)
        # Some files appear to contain invalid counts.
        # Cope with this by testing for EOF.
        try:
            for i in range(nmarkers):
                id = _read_short(chunk)
                pos = _read_long(chunk)
                name = _read_string(chunk)
                if pos or name:
                    # some files appear to have
                    # dummy markers consisting of
                    # a position 0 and name ''
                    self._markers.append((id, pos, name))
        except EOFError:
            w = ('Warning: MARK chunk contains only %s marker%s instead of %s' %
                 (len(self._markers), '' if len(self._markers) == 1 else 's',
                  nmarkers))
            warnings.warn(w)

class Aifc_write:
    # Variables used in this class:
    #
    # These variables are user settable through appropriate methods
    # of this class:
    # _file -- the open file with methods write(), close(), tell(), seek()
    #       set through the __init__() method
    # _comptype -- the AIFF-C compression type ('NONE' in AIFF)
    #       set through the setcomptype() or setparams() method
    # _compname -- the human-readable AIFF-C compression type
    #       set through the setcomptype() or setparams() method
    # _nchannels -- the number of audio channels
    #       set through the setnchannels() or setparams() method
    # _sampwidth -- the number of bytes per audio sample
    #       set through the setsampwidth() or setparams() method
    # _framerate -- the sampling frequency
    #       set through the setframerate() or setparams() method
    # _nframes -- the number of audio frames written to the header
    #       set through the setnframes() or setparams() method
    # _aifc -- whether we're writing an AIFF-C file or an AIFF file
    #       set through the aifc() method, reset through the
    #       aiff() method
    #
    # These variables are used internally only:
    # _version -- the AIFF-C version number
    # _comp -- the compressor from builtin module cl
    # _nframeswritten -- the number of audio frames actually written
    # _datalength -- the size of the audio samples written to the header
    # _datawritten -- the size of the audio samples actually written

    def __init__(self, f):
        if isinstance(f, str):
            filename = f
            f = builtins.open(f, 'wb')
        else:
            # else, assume it is an open file object already
            filename = '???'
        self.initfp(f)
        if filename[-5:] == '.aiff':
            self._aifc = 0
        else:
            self._aifc = 1

    def initfp(self, file):
        self._file = file
        self._version = _AIFC_version
        self._comptype = b'NONE'
        self._compname = b'not compressed'
        self._convert = None
        self._nchannels = 0
        self._sampwidth = 0
        self._framerate = 0
        self._nframes = 0
        self._nframeswritten = 0
        self._datawritten = 0
        self._datalength = 0
        self._markers = []
        self._marklength = 0
        self._aifc = 1      # AIFF-C is default

    def __del__(self):
        self.close()

    #
    # User visible methods.
    #
    def aiff(self):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        self._aifc = 0

    def aifc(self):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        self._aifc = 1

    def setnchannels(self, nchannels):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        if nchannels < 1:
            raise Error('bad # of channels')
        self._nchannels = nchannels

    def getnchannels(self):
        if not self._nchannels:
            raise Error('number of channels not set')
        return self._nchannels

    def setsampwidth(self, sampwidth):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        if sampwidth < 1 or sampwidth > 4:
            raise Error('bad sample width')
        self._sampwidth = sampwidth

    def getsampwidth(self):
        if not self._sampwidth:
            raise Error('sample width not set')
        return self._sampwidth

    def setframerate(self, framerate):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        if framerate <= 0:
            raise Error('bad frame rate')
        self._framerate = framerate

    def getframerate(self):
        if not self._framerate:
            raise Error('frame rate not set')
        return self._framerate

    def setnframes(self, nframes):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        self._nframes = nframes

    def getnframes(self):
        return self._nframeswritten

    def setcomptype(self, comptype, compname):
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        if comptype not in (b'NONE', b'ulaw', b'ULAW',
                            b'alaw', b'ALAW', b'G722'):
            raise Error('unsupported compression type')
        self._comptype = comptype
        self._compname = compname

    def getcomptype(self):
        return self._comptype

    def getcompname(self):
        return self._compname

##  def setversion(self, version):
##      if self._nframeswritten:
##          raise Error, 'cannot change parameters after starting to write'
##      self._version = version

    def setparams(self, params):
        nchannels, sampwidth, framerate, nframes, comptype, compname = params
        if self._nframeswritten:
            raise Error('cannot change parameters after starting to write')
        if comptype not in (b'NONE', b'ulaw', b'ULAW',
                            b'alaw', b'ALAW', b'G722'):
            raise Error('unsupported compression type')
        self.setnchannels(nchannels)
        self.setsampwidth(sampwidth)
        self.setframerate(framerate)
        self.setnframes(nframes)
        self.setcomptype(comptype, compname)

    def getparams(self):
        if not self._nchannels or not self._sampwidth or not self._framerate:
            raise Error('not all parameters set')
        return self._nchannels, self._sampwidth, self._framerate, \
              self._nframes, self._comptype, self._compname

    def setmark(self, id, pos, name):
        if id <= 0:
            raise Error('marker ID must be > 0')
        if pos < 0:
            raise Error('marker position must be >= 0')
        if not isinstance(name, bytes):
            raise Error('marker name must be bytes')
        for i in range(len(self._markers)):
            if id == self._markers[i][0]:
                self._markers[i] = id, pos, name
                return
        self._markers.append((id, pos, name))

    def getmark(self, id):
        for marker in self._markers:
            if id == marker[0]:
                return marker
        raise Error('marker {0!r} does not exist'.format(id))

    def getmarkers(self):
        if len(self._markers) == 0:
            return None
        return self._markers

    def tell(self):
        return self._nframeswritten

    def writeframesraw(self, data):
        self._ensure_header_written(len(data))
        nframes = len(data) // (self._sampwidth * self._nchannels)
        if self._convert:
            data = self._convert(data)
        self._file.write(data)
        self._nframeswritten = self._nframeswritten + nframes
        self._datawritten = self._datawritten + len(data)

    def writeframes(self, data):
        self.writeframesraw(data)
        if self._nframeswritten != self._nframes or \
              self._datalength != self._datawritten:
            self._patchheader()

    def close(self):
        if self._file is None:
            return
        try:
            self._ensure_header_written(0)
            if self._datawritten & 1:
                # quick pad to even size
                self._file.write(b'\x00')
                self._datawritten = self._datawritten + 1
            self._writemarkers()
            if self._nframeswritten != self._nframes or \
                  self._datalength != self._datawritten or \
                  self._marklength:
                self._patchheader()
        finally:
            # Prevent ref cycles
            self._convert = None
            f = self._file
            self._file = None
            f.close()

    #
    # Internal methods.
    #

    def _lin2alaw(self, data):
        import audioop
        return audioop.lin2alaw(data, 2)

    def _lin2ulaw(self, data):
        import audioop
        return audioop.lin2ulaw(data, 2)

    def _lin2adpcm(self, data):
        import audioop
        if not hasattr(self, '_adpcmstate'):
            self._adpcmstate = None
        data, self._adpcmstate = audioop.lin2adpcm(data, 2, self._adpcmstate)
        return data

    def _ensure_header_written(self, datasize):
        if not self._nframeswritten:
            if self._comptype in (b'ULAW', b'ulaw', b'ALAW', b'alaw', b'G722'):
                if not self._sampwidth:
                    self._sampwidth = 2
                if self._sampwidth != 2:
                    raise Error('sample width must be 2 when compressing '
                                'with ulaw/ULAW, alaw/ALAW or G7.22 (ADPCM)')
            if not self._nchannels:
                raise Error('# channels not specified')
            if not self._sampwidth:
                raise Error('sample width not specified')
            if not self._framerate:
                raise Error('sampling rate not specified')
            self._write_header(datasize)

    def _init_compression(self):
        if self._comptype == b'G722':
            self._convert = self._lin2adpcm
        elif self._comptype in (b'ulaw', b'ULAW'):
            self._convert = self._lin2ulaw
        elif self._comptype in (b'alaw', b'ALAW'):
            self._convert = self._lin2alaw

    def _write_header(self, initlength):
        if self._aifc and self._comptype != b'NONE':
            self._init_compression()
        self._file.write(b'FORM')
        if not self._nframes:
            self._nframes = initlength // (self._nchannels * self._sampwidth)
        self._datalength = self._nframes * self._nchannels * self._sampwidth
        if self._datalength & 1:
            self._datalength = self._datalength + 1
        if self._aifc:
            if self._comptype in (b'ulaw', b'ULAW', b'alaw', b'ALAW'):
                self._datalength = self._datalength // 2
                if self._datalength & 1:
                    self._datalength = self._datalength + 1
            elif self._comptype == b'G722':
                self._datalength = (self._datalength + 3) // 4
                if self._datalength & 1:
                    self._datalength = self._datalength + 1
        try:
            self._form_length_pos = self._file.tell()
        except (AttributeError, OSError):
            self._form_length_pos = None
        commlength = self._write_form_length(self._datalength)
        if self._aifc:
            self._file.write(b'AIFC')
            self._file.write(b'FVER')
            _write_ulong(self._file, 4)
            _write_ulong(self._file, self._version)
        else:
            self._file.write(b'AIFF')
        self._file.write(b'COMM')
        _write_ulong(self._file, commlength)
        _write_short(self._file, self._nchannels)
        if self._form_length_pos is not None:
            self._nframes_pos = self._file.tell()
        _write_ulong(self._file, self._nframes)
        if self._comptype in (b'ULAW', b'ulaw', b'ALAW', b'alaw', b'G722'):
            _write_short(self._file, 8)
        else:
            _write_short(self._file, self._sampwidth * 8)
        _write_float(self._file, self._framerate)
        if self._aifc:
            self._file.write(self._comptype)
            _write_string(self._file, self._compname)
        self._file.write(b'SSND')
        if self._form_length_pos is not None:
            self._ssnd_length_pos = self._file.tell()
        _write_ulong(self._file, self._datalength + 8)
        _write_ulong(self._file, 0)
        _write_ulong(self._file, 0)

    def _write_form_length(self, datalength):
        if self._aifc:
            commlength = 18 + 5 + len(self._compname)
            if commlength & 1:
                commlength = commlength + 1
            verslength = 12
        else:
            commlength = 18
            verslength = 0
        _write_ulong(self._file, 4 + verslength + self._marklength + \
                     8 + commlength + 16 + datalength)
        return commlength

    def _patchheader(self):
        curpos = self._file.tell()
        if self._datawritten & 1:
            datalength = self._datawritten + 1
            self._file.write(b'\x00')
        else:
            datalength = self._datawritten
        if datalength == self._datalength and \
              self._nframes == self._nframeswritten and \
              self._marklength == 0:
            self._file.seek(curpos, 0)
            return
        self._file.seek(self._form_length_pos, 0)
        dummy = self._write_form_length(datalength)
        self._file.seek(self._nframes_pos, 0)
        _write_ulong(self._file, self._nframeswritten)
        self._file.seek(self._ssnd_length_pos, 0)
        _write_ulong(self._file, datalength + 8)
        self._file.seek(curpos, 0)
        self._nframes = self._nframeswritten
        self._datalength = datalength

    def _writemarkers(self):
        if len(self._markers) == 0:
            return
        self._file.write(b'MARK')
        length = 2
        for marker in self._markers:
            id, pos, name = marker
            length = length + len(name) + 1 + 6
            if len(name) & 1 == 0:
                length = length + 1
        _write_ulong(self._file, length)
        self._marklength = length + 8
        _write_short(self._file, len(self._markers))
        for marker in self._markers:
            id, pos, name = marker
            _write_short(self._file, id)
            _write_ulong(self._file, pos)
            _write_string(self._file, name)

def open(f, mode=None):
    if mode is None:
        if hasattr(f, 'mode'):
            mode = f.mode
        else:
            mode = 'rb'
    if mode in ('r', 'rb'):
        return Aifc_read(f)
    elif mode in ('w', 'wb'):
        return Aifc_write(f)
    else:
        raise Error("mode must be 'r', 'rb', 'w', or 'wb'")

openfp = open # B/W compatibility

if __name__ == '__main__':
    import sys
    if not sys.argv[1:]:
        sys.argv.append('/usr/demos/data/audio/bach.aiff')
    fn = sys.argv[1]
    f = open(fn, 'r')
    try:
        print("Reading", fn)
        print("nchannels =", f.getnchannels())
        print("nframes   =", f.getnframes())
        print("sampwidth =", f.getsampwidth())
        print("framerate =", f.getframerate())
        print("comptype  =", f.getcomptype())
        print("compname  =", f.getcompname())
        if sys.argv[2:]:
            gn = sys.argv[2]
            print("Writing", gn)
            g = open(gn, 'w')
            try:
                g.setparams(f.getparams())
                while 1:
                    data = f.readframes(1024)
                    if not data:
                        break
                    g.writeframes(data)
            finally:
                g.close()
            print("Done.")
    finally:
        f.close()