mkvtoolnix/doc/development.md

# File system layout #

There's no dedicated include directory. Each include file is located
in the same directory its accompanying cpp file is located in as well.

Directories in alphabetical order:

* `ac`: Macro snippets used for the `configure` script.
* `contrib`: Spec files for RPM creation and other contributed files.
* `doc/man`: man pages. They're written as DocBook XML files and
  converted to HTML and man format automatically.
* `examples`: Examples for the end user.
* `installer`: Files for the Windows installer.
* `lib`: External libraries that are required for building. Should not
  be touched but can be used.
* `po`: Translation of the programs (not of the man pages).
* `share`: Image, desktop files etc.
* `src/common`: Common classes used by all tools. This library is
  linked to each program.
* `src/extract`: mkvextract's main files.
* `src/info`: mkvinfo's main files including its GUI.
* `src/input`: Readers (demux source files). Only used in mkvmerge.
* `src/merge`: mkvmerge's main files, e.g. command line handling,
  output control, file creating etc.
* `src/mkvtoolnix-gui`: the MKVToolNix GUI.
* `src/output`: Packetizers (create the tracks in the output
  file). Only used in mkvmerge.
* `src/propedit`: mkvpropedit's main files.
* `src/scripts` and `src/tools`: Scripts and compiled tools that may
  or may not be useful during development.
* `tests`: Test suite (requires external data package not distributed
  freely)

# Strings & encoding #

Internally all strings are encoded in UTF-8. Strings must be re-coded
during input/output.

# Timestamps #

All timestamps and durations are stored as `int64_t` variables with
nanosecond precision.

# Outputting messages to the user #

There are basically four functions that output stuff: `mxinfo()`,
`mxverb()`, `mxwarn()` and `mxerror()`. Each takes a
string. Formatting is done via the `fmt::format()` if required (see
[the fmt documentation](http://fmtlib.net/latest/index.html) if you're
not familiar with it), and `mxverb()` also takes a verbosity level
argument.

`mxinfo()` is supposed to be used for messages that are always
output. Its messages must be translatable by using the `Y()` macro,
Example:

    mxinfo(fmt::format(Y("Hello, {0}\n")) % user_name);

The same applies to `mxwarn()` and `mxerror()`. However, both prefix
their messages with "Warning:" and "Error:" respectively. Also they
modify mkvmerge's exit code -- "1" if `mxwarn()` has been used at
least once and "2" if `mxerror()` is used. `mxerror()` also instantly
exits the program. The GUI shows such warnings and errors in separate
areas as to draw the user's attention to them. Therefore `mxwarn()`
should be used for stuff the user really should know, and `mxerror()`
for stuff that prevents further processing altogether.

`mxverb()` is a debugging facility. mkvmerge starts out at verbosity
level 1. Therefore `mxverb(1, ...)` is equivalent to
`mxinfo(...)`. You can raise the verbosity level by adding the "-v" or
"--verbose" command line parameters multiple times. Messages output
with `mxverb()` should not be translatable, meaning you shouldn't use
the `_()` macro.

# Classes #

## General ##

Most classes are nameed `*_c`, and the corresponding counted pointer
implementation is called `*_cptr`. A counted pointer is a reference
counter implementation that automatically deletes its instance once
the counter reaches zero.

## `memory_c` (from `common/memory.h`) ##

Stores a pointer to a buffer and its size. Can free the memory if the
buffer has been malloced.

Allocating memory:

    memory_cptr mem = memory_c::alloc(12345);

Copying/reading something into it:

    memcpy(mem->get_buffer(), source_pointer, mem->get_size());
    file->read(mem, 20);

## `mm_io_c` (from `common/mm_io.h`) ##

Base class for input/output. Several derived classes implement access
to binary files (`mm_file_io_c`), text files (`mm_text_io_c`,
including BOM handling), memory buffers (`mm_mem_io_c`) etc.

## `mm_file_io_c` (from `common/mm_io.h`) ##

File input/output class. Opening a file for reading and retrieving its
size:

    mm_file_io_cptr file(new mm_file_io_c(file_name));
    int64_t size = file->get_size();

Seeking:

    // Seek to absolute position 123:
    file->setFilePointer(123, seek_beginning);
    // Seek forward 61 bytes:
    file->setFilePointer(61, seek_current);

Reading:

    size_t num_read = file->read(some_buffer, 12345);

## `track_info_c` (from `src/merge/pr_generic.h`) ##

A file and track information storage class. It is created by the
command line parser with the source file name and all the command line
options the user has passed in.

The instance is then passed to the reader which stores a copy in the
`m_ti` variable. The reader also passes that copy to each packetizer
it creates which in turn stores its own copy (again in the `m_ti`
variable).

# Readers #

A reader is a class that demuxes a certain file type. It has to parse
the file content, create one packetizer for each track, report the
progress etc. Each reader class is derived from `generic_reader_c`
(from `src/merge/pr_generic.h`).

An example for a rather minimal implementation is the MP3 reader in
`src/input/r_mp3.h`.

## Constructor & virtual destructor ##

The constructor usually only takes a `track_info_c` argument.  A
virtual destructor must be present, even if it is empty. Otherwise
you'll get linker errors.

The constructor must open the file and parse all its headers so that
all track information has been processed. This will most likely be
split up into a separate function in the future.

## `read(generic_packetizer_c *ptzr, bool force = false)` ##

Requests that the reader reads data for a specific track. mkvmerge
uses a pulling model: the core asks each packetizer to provide
data. The packetizers in turn ask the reader they've been created from
to read data by calling this function.

If the reader supports arbitrary access to track data (e.g. for AVI
and MP4/MOV files) then the reader should only read data for the
requested track in order not to consume too much memory. If the file
format doesn't allow for direct access to that data then the reader
can simply read the next packet regardless which track that packet
belongs to. The packetizer will call the reader's `read()` function as
often as necessary until it has enough data.

The reader must return `FILE_STATUS_MOREDATA` if more data is
available and `FILE_STATUS_DONE` when the end has been reached.

Each data packet is stored in an instance of `packet_c`. If the source
container provides a timestamp then that timestamp should be set in the
packet as well.

## `create_packetizer(int64_t track_id)` ##

Has to create a packetizer for the track with the specific ID. This ID
is the same number that the user uses on the command line as well as
the number used in the call to `id_result_track()` (see below).

This function has to verify that the user actually wants this track
processed. This is checked with the `demuxing_requested()`
function. Example from the MP3 reader (as the MP3 file format can only
contain one track the ID is always 0; see the Matroska reader for more
complex examples):

    // Check if the audio demuxer with ID 0 is requested. Also make
    // sure that the number of packetizers this reader has created is
    // still 0.
    if (!demuxing_requested('a', 0) || (NPTZR() != 0))
      return;

    // Create the actual packetizer.
    add_packetizer(new mp3_packetizer_c(this, m_ti, mp3header.sampling_frequency, mp3header.channels, false));

    // Inform the user.
    show_packetizer_info(0, PTZR0);

A lot of packetizers expect their codec private data to be constructed
completely by the reader. This often requires that the reader
processes at least a few packets. For a rather complex case see the
MPEG PS reader's handling of h264 tracks.

## `get_progress()` ##

Returns the demuxing progress, a value between 0 and 100. Usually
based on the number of bytes processed.

## `identify()` ##

File identification. Has to call `id_result_container()`
once for the container type.

For each supported track the function must call `id_result_track(...)`
once.

Both the container and the track identification can contain extended
information. For an extensive example see the Matroska reader's
identification function in `src/input/r_matroska.cpp`.

# Translations #

These are general hints for anyone wishing to translate
MKVToolNix.

There are basically three things that can be translated in the
MKVToolNix package:

1. The programs and their messages themselves. This is the most
   important part.
2. The manual pages.

All three are translated by the `gettext` system. This system uses
.po-files which map the original English version to the translated
version.  There are several (OpenSource) applications available that
can be used to translate such files,
e.g. [poedit](http://www.poedit.net/) if you're using Windows.

Alternatively you can use the [Transifex](https://www.transifex.com/)
web service for translating MKVToolNix. Please apply to join the
[MKVToolNix project](https://www.transifex.com/moritz-bunkus/mkvtoolnix/dashboard/)
and drop me a line via [email](mailto:moritz@bunkus.org). I'll add you
to the project as soon as possible.

Please ask the author for an up-to-date template of the untranslated
strings before you start doing anything else. He will attach the
untranslated translation file (xyz.po) for the programs themselves (the
first point of the three mentioned above). Take a look at it and at
the poedit software. If you feel up to the task then simply start
translating and send the translated .po file back to the author. The
author can notify you whenever phrases change or are added.

If you want to test your translations you can do that as well:

1. Install mkvtoolnix.
2. Translate the xyz.po file.
3. Whenever you want to test something compile the xyz.po to a .mo file
   with your po editor application.
4. Replace one of the installed translation files
   (e.g. `mkvtoolnix\locale\de\mkvtoolnix.mo`) with your compiled file.
5. Start the GUI, go to the preferences dialog and select 'German' as the
   language (the 'de' file you've replaced in step 4 is for German).

Please note that a lot of strings must not be translated, especially
command line options and their values. If a value to an option is a
fixed string then it must not be translated, but if it is a placeholder
e.g. for a file name then you can of course translate the `filename`
part. Example:

    --stereo-mode <TID:[none|both]>

Here neither `--stereo-mode` nor `none` or `both` may be translated
because mkvmerge expects all those in English. But in

    --attach-file filename

you can of course translate `filename`.

Some other hints:

* `TID` = track ID, a number
* `FID` = file ID, again a number
* `SFID`, `STID`, `DFID`, `DTID`: source file ID, source track ID, destination
  file ID, destination track ID

If you're unsure what the author is trying to say (the author is not a
native English speaker) or what you can and what you must not
translate just drop him a note.

You can send the author your work in progress whenever you feel like
it -- you don't have to finish everything at once.

Once you have shown that you are comfortable with translating you can
also get write access to the project's Git repository and push updates
there yourself.

# Adding new translations #

This is a TODO list for adding a new translation (a .po file) to MKVToolNix:

* Add new entry to `translation_c::initialize_available_translations()`
  in `src/common/translation.cpp`
* Copy `.po` file to `po/` sub-directory
* Add entry in `packaging/centos-fedora-opensuse/mkvtoolnix.spec`
* Add installation and removal entries in `packaging/windows/installer/mkvtoolnix.nsi`
* Add new language file to `packaging/windows/installer/translations/`
* Include translation's author in `AUTHORS`
* Include message in `NEWS.md`
* Either re-run `configure` or add the new translation in `build-config`
  to `TRANSLATIONS`; afterwards verify the format strings with `rake
  translations:verify-format-strings`