# Vim documentation: develop

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    *develop.txt*   For Vim version 7.0e.  Last change: 2006 Mar 09

    		  VIM REFERENCE MANUAL    by Bram Moolenaar

    Development of Vim.					*development*

    This text is important for those who want to be involved in further developing
    Vim.

    1. Design goals		|design-goals|
    2. Coding style		|coding-style|
    3. Design decisions	|design-decisions|
    4. Assumptions		|design-assumptions|

    See the file README.txt in the "src" directory for an overview of the source
    code.

    Vim is open source software.  Everybody is encouraged to contribute to help
    improving Vim.  For sending patches a context diff "diff -c" is preferred.
     Also see	http://www.vim.org/tips/tip.php?tip_id=618.

    ==============================================================================

    1. Design goals						*design-goals*

    Most important things come first (roughly).

    Note that quite a few items are contradicting.  This is intentional.  A
    balance must be found between them.

    VIM IS... VI COMPATIBLE					*design-compatible*

    First of all, it should be possible to use Vim as a drop-in replacement for
    Vi.  When the user wants to, he can use Vim in compatible mode and hardly
    notice any difference with the original Vi.

    Exceptions:
    - We don't reproduce obvious Vi bugs in Vim.
    - There are different versions of Vi.  I am using Version 3.7 (6/7/85) as a
      reference.  But support for other versions is also included when possible.
      The Vi part of POSIX is not considered a definitive source.
    - Vim adds new commands, you cannot rely on some command to fail because it
      didn't exist in Vi.
    - Vim will have a lot of features that Vi doesn't have.  Going back from Vim
      to Vi will be a problem, this cannot be avoided.
    - Some things are hardly ever used (open mode, sending an e-mail when
      crashing, etc.).  Those will only be included when someone has a good reason
      why it should be included and it's not too much work.
    - For some items it is debatable whether Vi compatibility should be
      maintained.  There will be an option flag for these.

    VIM IS... IMPROVED					*design-improved*

    The IMproved bits of Vim should make it a better Vi, without becoming a
    completely different editor.  Extensions are done with a "Vi spirit".
    - Use the keyboard as much as feasible.  The mouse requires a third hand,
      which we don't have.  Many terminals don't have a mouse.
    - When the mouse is used anyway, avoid the need to switch back to the
      keyboard.  Avoid mixing mouse and keyboard handling.
    - Add commands and options in a consistent way.  Otherwise people will have a
      hard time finding and remembering them.  Keep in mind that more commands and
      options will be added later.
    - A feature that people do not know about is a useless feature.  Don't add
      obscure features, or at least add hints in documentation that they exists.
    - Minimize using CTRL and other modifiers, they are more difficult to type.
    - There are many first-time and inexperienced Vim users.  Make it easy for
      them to start using Vim and learn more over time.
    - There is no limit to the features that can be added.  Selecting new features
      is one based on (1) what users ask for, (2) how much effort it takes to
      implement and (3) someone actually implementing it.

    VIM IS... MULTI PLATFORM				*design-multi-platform*

    Vim tries to help as many users on as many platforms as possible.
    - Support many kinds of terminals.  The minimal demands are cursor positioning
      and clear-screen.  Commands should only use key strokes that most keyboards
      have.  Support all the keys on the keyboard for mapping.
    - Support many platforms.  A condition is that there is someone willing to do
      Vim development on that platform, and it doesn't mean messing up the code.
    - Support many compilers and libraries.  Not everybody is able or allowed to
      install another compiler or GUI library.
    - People switch from one platform to another, and from GUI to terminal
      version.  Features should be present in all versions, or at least in as many
      as possible with a reasonable effort.  Try to avoid that users must switch
      between platforms to accomplish their work efficiently.
    - That a feature is not possible on some platforms, or only possible on one
      platform, does not mean it cannot be implemented.  [This intentionally
      contradicts the previous item, these two must be balanced.]

    VIM IS... WELL DOCUMENTED				*design-documented*

    - A feature that isn't documented is a useless feature.  A patch for a new
      feature must include the documentation.
    - Documentation should be comprehensive and understandable.  Using examples is
      recommended.
    - Don't make the text unnecessarily long.  Less documentation means that an
      item is easier to find.

    VIM IS... HIGH SPEED AND SMALL IN SIZE			*design-speed-size*

    Using Vim must not be a big attack on system resources.  Keep it small and
    fast.
    - Computers are becoming faster and bigger each year.  Vim can grow too, but
      no faster than computers are growing.  Keep Vim usable on older systems.
    - Many users start Vim from a shell very often.  Startup time must be short.
    - Commands must work efficiently.  The time they consume must be as small as
      possible.  Useful commands may take longer.
    - Don't forget that some people use Vim over a slow connection.  Minimize the
      communication overhead.
    - Items that add considerably to the size and are not used by many people
      should be a feature that can be disabled.
    - Vim is a component among other components.  Don't turn it into a massive
      application, but have it work well together with other programs.

    VIM IS... MAINTAINABLE					*design-maintain*

    - The source code should not become a mess.  It should be reliable code.
    - Use the same layout in all files to make it easy to read |coding-style|.
    - Use comments in a useful way!  Quoting the function name and argument names
      is NOT useful.  Do explain what they are for.
    - Porting to another platform should be made easy, without having to change
      too much platform-independent code.
    - Use the object-oriented spirit: Put data and code together.  Minimize the
      knowledge spread to other parts of the code.

    VIM IS... FLEXIBLE					*design-flexible*

    Vim should make it easy for users to work in their preferred styles rather
    than coercing its users into particular patterns of work.  This can be for
    items with a large impact (e.g., the 'compatible' option) or for details.  The
    defaults are carefully chosen such that most users will enjoy using Vim as it
    is.  Commands and options can be used to adjust Vim to the desire of the user
    and its environment.

    VIM IS... NOT						*design-not*

    - Vim is not a shell or an Operating System.  You will not be able to run a
      shell inside Vim or use it to control a debugger.  This should work the
      other way around: Use Vim as a component from a shell or in an IDE.
      A satirical way to say this: "Unlike Emacs, Vim does not attempt to include
      everything but the kitchen sink, but some people say that you can clean one
      with it.  ;-)"
     To use Vim with gdb see:	http://www.agide.org and	http://clewn.sf.net.
    - Vim is not a fancy GUI editor that tries to look nice at the cost of
      being less consistent over all platforms.  But functional GUI features are
      welcomed.

    ==============================================================================

    2. Coding style						*coding-style*

    These are the rules to use when making changes to the Vim source code.  Please
    stick to these rules, to keep the sources readable and maintainable.

    This list is not complete.  Look in the source code for more examples.

    MAKING CHANGES						*style-changes*

    The basic steps to make changes to the code:
    1. Adjust the documentation.  Doing this first gives you an impression of how
       your changes affect the user.
    2. Make the source code changes.
    3. Check ../doc/todo.txt if the change affects any listed item.
    4. Make a patch with "diff -c" against the unmodified code and docs.
    5. Make a note about what changed and include it with the patch.

    USE OF COMMON FUNCTIONS					*style-functions*

    Some functions that are common to use, have a special Vim version.  Always
    consider using the Vim version, because they were introduced with a reason.

    NORMAL NAME	VIM NAME	DIFFERENCE OF VIM VERSION
    free()		vim_free()	Checks for freeing NULL
    malloc()	alloc()		Checks for out of memory situation
    malloc()	lalloc()	Like alloc(), but has long argument
    strcpy()	STRCPY()	Includes cast to (char *), for char_u * args
    strchr()	vim_strchr()	Accepts special characters
    strrchr()	vim_strrchr()	Accepts special characters
    isspace()	vim_isspace()	Can handle characters > 128
    iswhite()	vim_iswhite()	Only TRUE for Tab and space
    memcpy()	mch_memmove()	Handles overlapped copies
    bcopy()		mch_memmove()	Handles overlapped copies
    memset()	vim_memset()	Uniform for all systems

    NAMES							*style-names*

    Function names can not be more than 31 characters long (because of VMS).

    Don't use "delete" as a variable name, C++ doesn't like it.

    Because of the requirement that Vim runs on as many systems as possible, we
    need to avoid using names that are already defined by the system.  This is a
    list of names that are known to cause trouble.  The name is given as a regexp
    pattern.

    is.*()		POSIX, ctype.h
    to.*()		POSIX, ctype.h

    d_.*		POSIX, dirent.h
    l_.*		POSIX, fcntl.h
    gr_.*		POSIX, grp.h
    pw_.*		POSIX, pwd.h
    sa_.*		POSIX, signal.h
    mem.*		POSIX, string.h
    str.*		POSIX, string.h
    wcs.*		POSIX, string.h
    st_.*		POSIX, stat.h
    tms_.*		POSIX, times.h
    tm_.*		POSIX, time.h
    c_.*		POSIX, termios.h
    MAX.*		POSIX, limits.h
    __.*		POSIX, system
    _[A-Z].*	POSIX, system
    E[A-Z0-9]*	POSIX, errno.h

    *_t		POSIX, for typedefs.  Use *_T instead.

    wait		don't use as argument to a function, conflicts with types.h
    index		shadows global declaration
    time		shadows global declaration
    new		C++ reserved keyword
    try		Borland C++ doesn't like it to be used as a variable.

    basename()	GNU string function
    dirname()	GNU string function
    get_env_value()	Linux system function

    VARIOUS							*style-various*

    Typedef'ed names should end in "_T":
        typedef int some_T;
    Define'ed names should be uppercase:
        #define SOME_THING
    Features always start with "FEAT_":
        #define FEAT_FOO

    Don't use '\"', some compilers can't handle it.  '"'' works fine.

    Don't use:
        #if HAVE_SOME
    Some compilers can't handle that and complain that "HAVE_SOME" is not defined.
    Use
        #ifdef HAVE_SOME
    or
        #if defined(HAVE_SOME)

    STYLE							*style-examples*

    General rule: One statement per line.

    Wrong:	    if (cond) a = 1;

    OK:	    if (cond)
    		a = 1;

    Wrong:	    while (cond);

    OK:	    while (cond)
    		;

    Wrong:	    do a = 1; while (cond);

    OK:	    do
    		a = 1;
    	    while (cond);

    Functions start with:

    Wrong:	int function_name(int arg1, int arg2)

    OK:	/*
    	 * Explanation of what this function is used for.
    	 *
    	 * Return value explanation.
    	 */
    	    int
    	function_name(arg1, arg2)
    	    int		arg1;		/* short comment about arg1 */
    	    int		arg2;		/* short comment about arg2 */
    	{
    	    int		local;		/* comment about local */

    	    local = arg1 * arg2;

    NOTE: Don't use ANSI style function declarations.  A few people still have to
    use a compiler that doesn't support it.

    SPACES AND PUNCTUATION					*style-spaces*

    No space between a function name and the bracket:

    Wrong:  func (arg);
    OK:	func(arg);

    Do use a space after if, while, switch, etc.

    Wrong:	if(arg)		for(;;)
    OK:	if (arg)	for (;;)

    Use a space after a comma and semicolon:

    Wrong:  func(arg1,arg2);	for (i = 0;i < 2;++i)
    OK:	func(arg1, arg2);	for (i = 0; i < 2; ++i)

    Use a space before and after '=', '+', '/', etc.

    Wrong:	var=a*5;
    OK:	var = a * 5;

    In general: Use empty lines to group lines of code together.  Put a comment
    just above the group of lines.  This makes it more easy to quickly see what is
    being done.

    OK:	/* Prepare for building the table. */
    	get_first_item();
    	table_idx = 0;

    	/* Build the table */
    	while (has_item())
    	    table[table_idx++] = next_item();

    	/* Finish up. */
    	cleanup_items();
    	generate_hash(table);

    ==============================================================================

    3. Design decisions					*design-decisions*

    Folding

    Several forms of folding should be possible for the same buffer.  For example,
    have one window that shows the text with function bodies folded, another
    window that shows a function body.

    Folding is a way to display the text.  It should not change the text itself.
    Therefore the folding has been implemented as a filter between the text stored
    in a buffer (buffer lines) and the text displayed in a window (logical lines).

    Naming the window

    The word "window" is commonly used for several things: A window on the screen,
    the xterm window, a window inside Vim to view a buffer.
    To avoid confusion, other items that are sometimes called window have been
    given another name.  Here is an overview of the related items:

    screen		The whole display.  For the GUI it's something like 1024x768
    		pixels.  The Vim shell can use the whole screen or part of it.
    shell		The Vim application.  This can cover the whole screen (e.g.,
    		when running in a console) or part of it (xterm or GUI).
    window		View on a buffer.  There can be several windows in Vim,
    		together with the command line, menubar, toolbar, etc. they
    		fit in the shell.

    Spell checking						*develop-spell*

    When spell checking was going to be added to Vim a survey was done over the
    available spell checking libraries and programs.  Unfortunately, the result
    was that none of them provided sufficient capabilities to be used as the spell
    checking engine in Vim, for various reasons:

    - Missing support for multi-byte encodings.  At least UTF-8 must be supported,
      so that more than one language can be used in the same file.
      Doing on-the-fly conversion is not always possible (would require iconv
      support).
    - For the programs and libraries: Using them as-is would require installing
      them separately from Vim.  That's mostly not impossible, but a drawback.
    - Performance: A few tests showed that it's possible to check spelling on the
      fly (while redrawing), just like syntax highlighting.  But the mechanisms
      used by other code are much slower.  Myspell uses a hashtable, for example.
      The affix compression that most spell checkers use makes it slower too.
    - For using an external program like aspell a communication mechanism would
      have to be setup.  That's complicated to do in a portable way (Unix-only
      would be relatively simple, but that's not good enough).  And performance
      will become a problem (lots of process switching involved).
    - Missing support for words with non-word characters, such as "Etten-Leur" and
      "et al.", would require marking the pieces of them OK, lowering the
      reliability.
    - Missing support for regions or dialects.  Makes it difficult to accept
      all English words and highlight non-Canadian words differently.
    - Missing support for rare words.  Many words are correct but hardly ever used
      and could be a misspelled often-used word.
    - For making suggestions the speed is less important and requiring to install
      another program or library would be acceptable.  But the word lists probably
      differ, the suggestions may be wrong words.

    Spelling suggestions				*develop-spell-suggestions*

    For making suggestions there are two basic mechanisms:
    1. Try changing the bad word a little bit and check for a match with a good
       word.  Or go through the list of good words, change them a little bit and
       check for a match with the bad word.  The changes are deleting a character,
       inserting a character, swapping two characters, etc.
    2. Perform soundfolding on both the bad word and the good words and then find
       matches, possibly with a few changes like with the first mechanism.

    The first is good for finding typing mistakes.  After experimenting with
    hashtables and looking at solutions from other spell checkers the conclusion
    was that a trie (a kind of tree structure) is ideal for this.  Both for
    reducing memory use and being able to try sensible changes.  For example, when
    inserting a character only characters that lead to good words need to be
    tried.  Other mechanisms (with hashtables) need to try all possible letters at
    every position in the word.  Also, a hashtable has the requirement that word
    boundaries are identified separately, while a trie does not require this.
    That makes the mechanism a lot simpler.

    Soundfolding is useful when someone knows how the words sounds but doesn't
    know how it is spelled.  For example, the word "dictionary" might be written
    as "daktonerie".  The number of changes that the first method would need to
    try is very big, it's hard to find the good word that way.  After soundfolding
    the words become "tktnr" and "tkxnry", these differ by only two letters.

    To find words by their soundfolded equivalent (soundalike word) we need a list
    of all soundfolded words.  A few experiments have been done to find out what
    the best method is.  Alternatives:
    1. Do the sound folding on the fly when looking for suggestions.  This means
       walking through the trie of good words, soundfolding each word and
       checking how different it is from the bad word.  This is very efficient for
       memory use, but takes a long time.  On a fast PC it takes a couple of
       seconds for English, which can be acceptable for interactive use.  But for
       some languages it takes more than ten seconds (e.g., German, Catalan),
       which is unacceptable slow.  For batch processing (automatic corrections)
       it's to slow for all languages.
    2. Use a trie for the soundfolded words, so that searching can be done just
       like how it works without soundfolding.  This requires remembering a list
       of good words for each soundfolded word.  This makes finding matches very
       fast but requires quite a lot of memory, in the order of 1 to 10 Mbyte.
       For some languages more than the original word list.
    3. Like the second alternative, but reduce the amount of memory by using affix
       compression and store only the soundfolded basic word.  This is what Aspell
       does.  Disadvantage is that affixes need to be stripped from the bad word
       before soundfolding it, which means that mistakes at the start and/or end
       of the word will cause the mechanism to fail.  Also, this becomes slow when
       the bad word is quite different from the good word.

    The choice made is to use the second mechanism and use a separate file.  This
    way a user with sufficient memory can get very good suggestions while a user
    who is short of memory or just wants the spell checking and no suggestions
    doesn't use so much memory.

    Word frequency

    For sorting suggestions it helps to know which words are common.  In theory we
    could store a word frequency with the word in the dictionary.  However, this
    requires storing a count per word.  That degrades word tree compression a lot.
    And maintaining the word frequency for all languages will be a heavy task.
    Also, it would be nice to prefer words that are already in the text.  This way
    the words that appear in the specific text are preferred for suggestions.

    What has been implemented is to count words that have been seen during
    displaying.  A hashtable is used to quickly find the word count.  The count is
    initialized from words listed in COMMON items in the affix file, so that it
    also works when starting a new file.

    This isn't ideal, because the longer Vim is running the higher the counts
    become.  But in practice it is a noticable improvement over not using the word
    count.

    ==============================================================================

    4. Assumptions						*design-assumptions*

    Size of variables:
    char	    8 bit signed
    char_u	    8 bit unsigned
    int	    32 or 64 bit signed (16 might be possible with limited features)
    unsigned    32 or 64 bit unsigned (16 as with ints)
    long	    32 or 64 bit signed, can hold a pointer

    Note that some compilers cannot handle long lines or strings.  The C89
    standard specifies a limit of 509 characters.

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