path: root/notes/coding_standard.rst

libmaple Coding Standards
=========================

Author: Marti Bolivar (mbolivar@leaflabs.com)

LeafLabs team members are required to follow these when producing new
code.  Community contributors to libmaple are strongly encouraged to
do so; following these rules will greatly increase the probability
that your patches will be folded in.

In general, do it like this unless there's a really good reason why
not.  You being lazy doesn't count as a good reason.  Most, if not
all, of these decisions are entirely arbitrary, but it's important for
readability that we be consistent.

The file ``.dir-locals.el`` in the libmaple root directory already
ensures that many of these standards are followed by default in Emacs
(but not on Windows, where it would need to be named
``_dir_locals.el``, and no way, man).  There's also some elisp
scattered about this file which will provide you additional help.

Vim customizations to do the same thing would be nice (hint, hint)!

.. contents:: Contents
   :local:

License
-------

.. highlight:: scheme

- Put an MIT license at the beginning of the file (look at any of our
  source files for an example).  Copyright should go either to you or
  to LeafLabs, LLC.

  Emacs: if you don't like seeing the license, you should use
  elide-head (which will hide it for you).  You can use the following::

    (require 'elide-head)
    (setq programming-mode-hooks '(c-mode-hook c++-mode-hook))
    (add-to-list 'elide-head-headers-to-hide
                 '("The MIT License" . "DEALINGS IN\n [*] THE SOFTWARE"))
    (add-to-list 'elide-head-headers-to-hide
                 '("The MIT License" . "DEALINGS IN THE\n...SOFTWARE"))
    (dolist (hook programming-mode-hooks)
      (add-hook hook (lambda () (elide-head))))

Whitespace/Indentation
----------------------

- 4 space indents.  [Set in ``.dir-locals.el``]

- Unix newlines. [Some exceptions are currently grandfathered in;
  these will go away in time.]

- No tab characters. [Set in ``.dir-locals.el``]

- No trailing whitespace.  For help getting this (and no tab
  characters) done automatically in Emacs, you can use this:

      http://github.com/mbolivar/code-fascism

  I hear tell you can get something similar in Vim; ask around, I
  guess.

- Files end in exactly one newline. [The presence of a newline at EOF
  is already done by ``c-require-final-newline`` in recent versions of
  Emacs.]

- Exactly two newlines separate source paragraphs (you do separate
  your code into paragraphs, don't you?).

- The first line in a function is non-blank.

.. highlight:: cpp

- Exactly one space after ``if``, ``else``, ``for``, and ``while``,
  before the following ``{`` or ``(``.  One space before ``else``,
  after the preceding ``}``.  For example::

      // This is good; we like this:
      if (foo) {
          while (quux) {
              bar();
          }
      } else {
          baz();
      }

      // THIS IS BAD! DON'T DO THIS:
      if(foo){
          while(quux){
              bar();
          }
      }else{
          baz();
      }

- Exactly one space in between binary arithmetic, logical, and
  comparison operators and their operands.  This doesn't apply to the
  . and -> operators.  Examples::

      // This is good:
      int x = a + b * (c - d);
      if (x != 0 && a > 7) {
          SerialUSB.println(x);
      }

      // THIS IS BAD!
      int x = a+b*(c-d);
      if (x!=0 && a>7) {
          SerialUSB.println(x);
      }

      // This is good:
      uint32 adc_data = ADC1_BASE->DR;
      SerialUSB.println(adc_data);

      // THIS IS BAD!
      uint32 adc_data = ADC1_BASE -> DR;
      SerialUSB . println(adc_data);

- No space between a unary operator and its operand.  Examples::

      // Good:
      x++;

      // BAD!
      x ++;

      // Good:
      y = -x;

      // BAD!
      y = - x;

- If you need to break up a long line:

  * Prefer to break up long expressions after a binary operator.  Example::

      // Good:
      if (some_really_long_conditional_wow_this_really_goes_on_forever ||
          maybe_something_else_could_happen_too) {
          ...
      }

      // BAD!
      if (some_really_long_conditional_wow_this_really_goes_on_forever
          || maybe_something_else_could_happen_too) {
          ...
      }

  * When breaking up a function's arguments over multiple lines, align
    the arguments on subsequent lines with the first argument.
    Example::

      // Good:
      return_type value_i_got = function_with_a_really_long_name(argument1,
                                                                 argument2,
                                                                 argument3);

      // BAD!
      return_type value_i_got = function_with_a_really_long_name(argument1,
          argument2,
          argument3);

      // BAD!
      return_type value_i_got = function_with_a_really_long_name(argument1,
                                                                    argument2,
                                                                    argument3);

- In function invocations, no space in between the function name and
  the opening parenthesis.  Example::

      // Good:
      SerialUSB.println("Hello, world!");

      // BAD!
      SerialUSB.println ("Hello, world!");

- Don't indent C code within a conditionally-compiled ``extern "C"``
  block.  Example::

      // Good:
      #ifdef __cplusplus
      extern "C"{
      #endif

      void some_c_function(void);

      #ifdef __cplusplus
      } // extern "C"
      #endif

      // BAD!
      #ifdef __cplusplus
      extern "C"{
      #endif

          void some_c_function(void);

      #ifdef __cplusplus
      } // extern "C"
      #endif

.. highlight:: scheme

  Emacs does the "bad" behavior by default, which can be very
  annoying.  You can turn this off with::

      (defun c-mode-inextern-lang-hook ()
          (setcdr (assq 'inextern-lang c-offsets-alist) '-))
      (add-hook 'c-mode-hook c-mode-inextern-lang-hook)

Comments
--------

.. highlight:: c++

- Multi-line comments are pretty flexible.  Any of these is fine::

    /* Comment starts here.
     * Continued lines have a '*' before them.
     * The comment can end after the last line.
     */

    /* Comment starts here.
     * The comment can end on the same line. */

    /*
     * You can also place a newline after the opening "/*".
     */

- Doxygen comments are multi-line comments that begin with ``/**``
  instead.

- Single-line comments on the same line are ``//`` in C++. (That's OK
  in C as well).

- Single-line comments on their own source line should be ``/* */`` in
  C, but can also be ``//`` in C++.  (This isn't of great importance).
  In Emacs, you can use M-; (comment-dwim), and it'll Do What You
  Mean.

Braces
------

- Mostly 1TBS:

      http://en.wikipedia.org/wiki/Indent_style#Variant:_1TBS

  The only difference is that the opening brace of a function's
  definition occurs exactly one space character after the closing
  parenthesis in that function's parameter list.  Example::

      void func(void) {
          ...
      }

Naming conventions
------------------

There's always a fight about upper and lower case vs. underscores.
We'll handle this as follows.

- First, ``Dont_Mix_Like_This``, because ``It_Looks_Really_Ugly``, ok?
  [There's been some debate about this, and some exceptions are
  already grandfathered in, so in order to settle it, let's call this
  a "recommendation" instead of "requirement".]

- Variables: Use underscores to separate words in C identifiers::

    int some_example_name;

  User-facing C++ variables should be camel cased
  (``thisIsAnExample``, ``boardPWMPins``, etc.), for consistency with
  the Arduino style.  It's probably a good idea for you to case
  non-user facing C++ variables in the C style; this will help
  disambiguate what's part of the Wirish API and what's not.

- Classes: Pascal case.  So ``ThisIsAClassName``, but ``thisIsNot``,
  ``this_is_not``, and like I said,
  ``Dont_You_DareTryANYTHING_STUPID``.

- Functions: C functions are all lowercase, and words are separated by
  underscores.  C++ method names are camel cased.

- Structs: Usually like variables (``adc_dev``, ``adc_reg_map``,
  etc.), but it's not crucial.  Don't feel obliged to put ``_t`` at
  the end of the type name; we don't.

- Macros and constants: all caps, separated by underscores.  C++
  variables with the ``const`` qualifier generally aren't considered
  "constants" for the purposes of this rule; i.e., they are cased
  according to the rules for variables.  We make an exception for
  ``PIN_MAP``, because it's the central Wirish data structure.

- foo.h gets ``#ifdef``\ 'ed to ``_FOO_H_``.

- Acronyms: The case of letters in an acronym is determined by the
  case of the first letter in the acronym, which is determined by
  following the above rules.  Examples::

      // Good:
      void usb_func() { ... }
      void frob_usb_disc() { ... }
      class SomethingUSB {
          void usbInit();
          void initUSB();
      };

      // BAD:
      class BadUsb { ... }; // say "GoodUSB" instead
      void swizzle_USB_disc() { ... } // say "swizzle_usb_disc" instead

Documentation
-------------

- You **must** document your code.  At a bare minimum, this means
  Doxygen comments on every user-facing function and type.
  Additionally, you need to individually document the fields and
  enumerator values of ``struct``\ s and ``enum``\ s.  See any
  register map type's definition for an example.

- For libmaple proper, you don't need comments for each register bit
  definition (for now).

- Doxygen comments generally just belong on types, functions,
  etc. that are part of the public user-facing API.  This generally
  means that if there's ReST documentation for it under docs/source/,
  it needs Doxygen comments, and that ReST should use Breathe to pull
  that Doxygen comment out. (For more info on this, see docs/README).

  There are some exceptions to this rule since Breathe isn't totally
  mature yet and Sphinx's C++ domain is still in flux.  In these
  cases, document the code "manually" in ReST.

  This should be avoided if at all possible, since it creates a
  maintenance burden of documenting things in two places at once, and
  makes it easier for documentation to go stale.

  If you do have to document something manually, put a comment in the
  source file informing future maintainers about it, so they'll pay
  extra attention when making changes.

- When adding peripheral support, it would be nice if you put
  longer-form comments into the /notes/ directory, with a comment in
  the corresponding .h file referring to it.  See /libmaple/dac.h for
  an example.

  This lets us keep the source files relatively free of "introductory"
  material, while allowing new readers a convenient starting point.
  These longer-form notes also have a habit of turning into
  user-facing documentation.

- For libmaple proper (the pure C library under libmaple/); the
  convention is to document any user-facing function at the point where
  it is defined.  In particular, this means you should document an
  externally-linked function defined in a .c file in that .c file, not
  in the header file where it is declared to the user.

General Formatting
------------------

.. highlight:: scheme

- Keep it 80-column clean.  That means Emacs says the largest column
  number=79.  You should turn on column number mode to help you out:

    (column-number-mode 1)

  You can get more help from lineker-mode:

    http://www.helsinki.fi/~sjpaavol/programs/lineker.el

  Just put lineker.el somewhere in your load-path, and:

    (require 'lineker)
    (dolist (hook '(c-mode-hook c++-mode-hook))
      (add-hook hook (lambda () (lineker-mode 1))))

.. highlight:: cpp

Language Features and Compiler Extensions
-----------------------------------------

- In libmaple proper, aim for C99 compatibility.  Some GCC extensions
  are OK, but let's not go crazy.  

- If you'd like to get code into libmaple which uses a GCC extension
  not already in use elsewhere, ask a LeafLabs developer (or another
  one, if you are one) what they think about it first.

- Explicitly approved GCC extensions:

  * ``asm volatile``:
    http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

  * ``Nested functions``:
    http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html

- In wirish, generally be very conservative when using C++ features
  that aren't part of C.  We are forced to use C++ for Arduino
  compatibility (and the general Arduino style of pretending that an
  object is a library), but it's an angry beast, and we don't want to
  provoke it.  The mantra is "C with classes".

- Explicitly approved C++ features:

  * Initializers that aren't constant; e.g. the ``gpio_dev*`` values
    in a ``PIN_MAP``.

  * Default arguments: e.g., the timeout argument defaulting to 0
    (meaning to wait forever) in ``waitForButtonPress()``.

- Explicitly forbidden C++ features:

  * Templates

- C++ features that are conditionally allowed, but require explicit
  approval from at least two libmaple developers (one of which may be
  yourself):

  * Operator overloading: Never allowed when it's just for style.
    Potentially allowed when you're implementing a class that models a
    mathematical structure, and you'd like to implement
    e.g. ``operator+()``.