/*
** aacgain - modifications to mp3gain to support mp4/m4a files
** Copyright (C) 2004-2010 David Lasker, Altos Design, Inc.
**
** This program is free software; you can redistribute it and/or modify
** it under the terms of the GNU General Public License as published by
** the Free Software Foundation; either version 2 of the License, or
** (at your option) any later version.
**
** This program is distributed in the hope that it will be useful,
** but WITHOUT ANY WARRANTY; without even the implied warranty of
** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
** GNU General Public License for more details.
**
** You should have received a copy of the GNU General Public License
** along with this program; if not, write to the Free Software
** Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
**/

AACGAIN VERSION 2.0.0

AACgain is a modification to Glen Sawyer's mp3gain, to support AAC 
(mp4/m4a/QuickTime) files as well as mp3 files. If you are not familiar with
mp3gain, stop reading this, and go to http://mp3gain.sourceforge.net

USAGE NOTES

1. BACK UP YOUR MUSIC FILES BEFORE USING AACGAIN! UNLIKE MP3GAIN, AACGAIN IS
   NOT COMPLETELY REVERSABLE. THE UNDO OPTION IS INTENDED TO RESTORE A MUSIC
   FILE TO BE FUNCTIONALLY EQUIVALENT TO ITS ORIGINAL STATE, BUT IT WILL NOT
   BE BIT-FOR-BIT IDENTICAL TO THE ORIGINAL FILE. IT IS POSSIBLE FOR APPLE
   TO CHANGE THE ITUNES MUSIC FILE FORMAT IN WAYS THAT ARE INCOMPATIBLE
   WITH AACGAIN, RESULTING IN CORRUPTED MUSIC FILES THAT CANNOT BE RESTORED
   TO THEIR ORIGINAL STATE. I AM NOT RESPONSIBLE FOR DAMAGED FILES.
2. AACGain supports files in the mp4/Quicktime file format. These files
   usually have a file extension of .mp4, .m4a, or m4v. Raw AAC files,
   files encoded with HE-AAC/SBR, or Apple Lossless files are not supported.
3. DRM'ed files purchased from the iTunes music store are not supported. You
   may convert these to .m4a format by burning them to a CD-ROM and importing
   them back into iTunes, or by using one of the underground methods to
   losslessly convert them to .m4a.
4. AACGain takes the same command line options as mp3gain - no
   changes have been made to the command usage.
5. If you rename the AACGain.exe executable to mp3gain.exe, it will work 
   with the Visual Basic front-end MP3GainGUI.exe.
6. When calculating or applying album gain, you may mix aac and mp3 files
   in the same command.
7. The /t (use temp file) option is ignored for aac files. A temp file
   is always used. The mp4v2 Optimize function is used to copy the
   processed file back to the original name. Experience has shown that this
   is required for processed music files to play on older iPods.
8. The /w (wrap) flag is ignored for aac files, since allowing an aac file
   to wrap will leave it in an unplayable state.
9. The /f (assume input file is an MPEG 2 Layer III) option is ignored
   for mp4/m4a/m4v files. If a file starts with a 'ftyp' atom, it is assumed 
   to be an mp4 container, even if /f was specified.
10. The /s i (id2v3) option is ignored for mp4/m4a/m4v files. The tags for
    these files are stored in iTunes metadata format as described below.
11. The following metadata tags will be added to your mp4/m4a files:
     - replaygain_track_gain
     - replaygain_album_gain
     - replaygain_track_peak
     - replaygain_album_peak
     - replaygain_track_minmax
     - replaygain_album_minmax
     - replaygain_undo
    These are all free-form metadata tags (moov.udta.meta.ilst.----)
    with 'data' text fields in the same format as the mp3gain
    equivalents. You may use AtomicParsley (download from sourceforge)
    to view the tags.
    
SUPPPORT
This is free software. No support is provided. 

You may try to post questions on the Hydrogen Audio AAC Tech forum:
http://www.hydrogenaudio.org/forums/index.php?showforum=14
I regularly monitor this forum, and use it to announce new releases
of aacgain.

You may email me questions, but there is no guarantee of a response.
If you have what you believe to be a valid .m4a/.mp4 file that is 
not processed correctly by aacgain, please attach it to your email.
I do not support the original mp3gain code.

WHAT'S NEW IN VERSION 2.0
a) Fixed a bug where mp4 files would be silently deleted on error.
   Thanks to Sappharad for the fix. http://www.sappharad.com/
b) Add support for files with multiple audio tracks.
   Thanks to Carter Thaxton for the fix.
a) Rewrote the build system
b) Updated faad2 to 2.9.1, mp4v2 to 3.0.4

WHAT'S NEW IN VERSION 1.9
a) Fixed a bug in mp4v2 which caused iTunes music video files
   to fail with "not a valid mp4/m4a file" error.
b) Gives a more meaningful error message when attempting to
   process Apple Lossless files.
c) Built with updated mp4v2 from Google Code, which should build
   error-free on modern Windows and Linux systems.
d) Built with updated faad2 code, which should build error-free
   on modern Windows and Linux systems.
d) Built with latest mp3gain version 1.5.2 sources checked into CVS,
   which include id3v2 tag support.
e) MSVC++ solution and project files have been converted to Visual
   C++ 2010 Express Edition.
    
WHAT'S NEW IN VERSION 1.8
a) Built with mpeg4ip/mp4v2 version 1.6.1. aacgain 1.7.1 was built with
   version 1.5.20. mp4v2 version 1.6.1 corrects problems that corrupted 
   video files containing Quicktime chapter markers.
b) Built with latest mp3gain version 1.4.7 sources checked into CVS. 
   The mp3gain changes made since 1.7.1 support sample rates up to 96000.
   Previous versions had a maxmimum sample rate of 48000.
c) Ignores faad2 "scalefactor out of range" errors. These errors resulted
   in "invalid file format" errors on apparently good files.

WHAT'S NEW IN VERSION 1.7.1
a) MSVC++ solution and project files have been converted to Visual
   C++ 2005 Express Edition. Other than the version number, no other 
   source code changes have been made.

WHAT'S NEW IN VERSION 1.7
a) If the total length of the directory plus file name path exceeded
   64 bytes, memory corruption would occur. This has been corrected.

WHAT'S NEW IN VERSION 1.6
a) Files with a 'cprt' (copyright) tag are now processed correctly.
   Previous versions of aacgain corrupted the metadata of files with a
   copyright tag.
b) AACGain now works on video or other files where the first audio track 
   has a track number of other than 1. Previous versions required the
   audio track to have a number of 1. AACGain will still only process
   the first audio track found on a file. Thanks to Arthur Yarwood at
   Sony Computer Entertainment Europe for this fix.

WHAT'S NEW IN VERSION 1.5
a) Running AACGain on Apple lossless or other unsupported file format 
   resulted in a crash with an unhandled exception. This is now fixed.

WHAT'S NEW IN VERSION 1.4
a) Temporary files are created in the same directory as the input file.
   Formerly they were created in the same directory that the program was run
   in. This solves a problem only seen on Unix systems where the temporary 
   file could not be renamed back to the original input filename, if the 
   input file was located on a different filesystem from where the program 
   was run. Windows users are unaffected by this problem and can continue to
   use 1.3.

WHAT'S NEW IN VERSION 1.3
a) Always uses the /t (use temp file) option, even if it is not specified
   on the command line. This resolves issues where processed files would
   not play on the iPod Shuffle.
b) Preserves original values of MP4 properties bufferSizeDB, maxBitrate and 
   avgBitrate. Without this change, mpeg4ip/mp4vs was recalulating the
   actual values of these parameters. This caused bitrate as displayed by
   iTunes to change slightly, i.e. songs ripped at 320KB would display as
   319KB.
c) Incorporates recent changes to mp3gain which prevent crashes when
   working with 48KB files on Linux.
d) Source code directory structure was reorganized to be compatible with mp3gain 
   CVS on sourceforge. Thanks to Glen Sawyer, the AACGain source code is now 
   hosted on sourceforge's CVS repository as part of the mp3gain project.

WHAT'S NEW IN VERSION 1.2
a) Fixes compatibility issue with MP3GainGUI version 1.2.5 or 1.3.4.
b) All temporary files created are erased.
   
WHAT'S NEW IN VERSION 1.1
a) Multi-channel and mono files now work.
b) Fixes compatibility issues with MP3GainGUI:
   - /o option always outputs Album gain
c) /f option is ignored for AAC files - see usage notes below.
d) Uses portable versions of all library functions, and has been
   successfully compiled under Linux. Thanks to Prakash Punnoor
   for his help with this.

HOW AACGAIN WORKS

mp4v2 is used to parse the mp4/QuickTime file format, to
read individual samples so that they can be passed to faad2, and to
read and write replaygain metadata tags.

faad2 is used to de-compress and decode the aac frames into linear 
PCM for analysis by the replaygain algorithm.

Files syntax.c and decoder.c are slightly modified versions of the
corresponding files from faad2. The modifications allow the offsets
and original values of the global_gain fields to be saved. Since the
modified functions were declared static in libfaad, I am able to use
my own modified copies of these functions without getting duplicate
definition link errors.

mp4v2 is used to apply any gain changes needed. It does this as follows: 
a) creates a new audio track
b) copyies the samples from the original audio track to the new track
   with the global_gain fields appropriately modified
c) deletes the original audio track
d) modifies the trackId property of the new track, and the nextTrackId
   property of the moov atom, to be the same as in the input file.

aacgain.cpp is a wrapper for the above functionality.

mp3gain.c has been modified to call aacgain when the symbol AACGAIN is
defined. mp3gain versions 1.5 and later have this modification included.

ACKNOWLEDGEMENTS

I am a total newbie to digital audio, and I created AACGain primarily
as a learning exercise. I couldn't have done it without the help of others:

1. Thanks to the user community at hydrogenaudio.org and audiocoding.com,
   especially Menno and Hans-Jürgen.
2. Thanks to Bill May for helping me get started with mp4v2.
3. Thanks to Prakash Punnoor, Hardloaf, Ben Allison, and Miguel Angel Alvarez
   for their help with Linux and Mac.
4. Last, but not least, thanks to Glen Sawyer for creating mp3gain.

Dave Lasker
dave@altosdesign.com