runtime/doc/develop.txt

*develop.txt*   For Vim version 8.0.  Last change: 2018 Apr 17


		  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://vim.wikia.com/wiki/How_to_make_and_submit_a_patch.

==============================================================================
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 exist.
- 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.  It does provide a terminal
  window, in which you can run a shell or debugger.  E.g. to be able to do
  this over an ssh connection.  But if you don't need a text editor with that
  it is out of scope (use something like screen or tmux instead).
  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. Get the code from github.  That makes it easier to keep your changed
   version in sync with the main code base (it may be a while before your
   changes will be included).  You do need to spend some time learning git,
   it's not the most user friendly tool.
2. Adjust the documentation.  Doing this first gives you an impression of how
   your changes affect the user.
3. Make the source code changes.
4. Check ../doc/todo.txt if the change affects any listed item.
5. Make a patch with "git diff".  You can also create a pull request on
   github, but it's the diff that matters.
6. Make a note about what changed, preferably mentioning the problem and the
   solution.  Send an email to the |vim-dev| maillist with an explanation and
   include the diff. Or create a pull request on github.


C COMPILER						*style-compiler*

The minimal C compiler version supported is C89, also known as ANSI C.
Later standards, such as C99, are not widely supported, or at least not 100%
supported.  Therefore we use only some of the C99 features and disallow some
(at least for now).

Please don't make changes everywhere to use the C99 features, it causes merge
problems for existing patches.  Only use them for new and changed code.

Comments ~

Traditionally Vim uses /* comments */.  We intend to keep it that way,
especially for file and function headers.  For new code or lines of code that
change, it is allowed to use // comments.  Especially when it comes after
code:
	int some_var;  // single line comment useful here

Enums ~

The last item in an enum may have a trailing comma.  C89 didn't allow this.

Types ~

"long long" is allowed and can be expected to be 64 bits.  Use %lld in printf
formats.  Also "long long unsigned" with %llu.

Flexible array members ~

This is an array without size, used as the last member of a struct.  Vim used
to have an array of size one, which causes trouble with FORTIFY_SOURCE. Using
an "unsized array" is the intended use, we will change all of them.
	struct some_stuff {
	   size_t  length;
	   char    payload[];  // will have size "length"
	};

Not to be used ~

These C99 features are not to be used, because not enough compilers support
them:
- Declaration after Statements (MSVC 2012 does not support it).  All
  declarations need to be at the start of the block.
- Variable length arrays (even in C11 this is an optional feature).
- _Bool and _Complex types.
- "inline" (it's hardly ever needed, let the optimizer do its work)


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" or "this" 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.

clear		Mac curses.h
echo		Mac curses.h
instr		Mac curses.h
meta		Mac curses.h
newwin		Mac curses.h
nl		Mac curses.h
overwrite	Mac curses.h
refresh		Mac curses.h
scroll		Mac curses.h
typeahead	Mac curses.h

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);

Wrong:	    if (cond) {
               cmd;
               cmd;
	    } else {
               cmd;
               cmd;
	    }

OK:	    if (cond)
            {
               cmd;
               cmd;
	    }
	    else
	    {
               cmd;
               cmd;
	    }

Use ANSI (new style) function declarations with the return type on a separate
indented line.

Wrong:	int function_name(int arg1, int arg2)

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

	    local = arg1 * arg2;


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 easier 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 too 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 noticeable 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.

 vim:tw=78:ts=8:ft=help:norl: