wlroots/CONTRIBUTING.md

# Contributing to wlroots

Contributing just involves sending a pull request. You will probably be more
successful with your contribution if you visit the [IRC
channel](http://webchat.freenode.net/?channels=sway-devel&uio=d4) upfront and discuss
your plans.

## Pull Requests

If you already have your own pull request habits, feel free to use them. If you
don't, however, allow me to make a suggestion: feature branches pulled from
upstream. Try this:

1. Fork sway
2. Clone your fork
3. git remote add upstream https://github.com/SirCmpwn/wlroots

You only need to do this once. You're never going to use your fork's master
branch. Instead, when you start working on a feature, do this:

1. git fetch upstream
2. git checkout -b add-so-and-so-feature upstream/master
3. work
4. git push -u origin add-so-and-so-feature
5. Make pull request from your feature branch

## Commit Messages

Please strive to write good commit messages. Here's some guidelines to follow:

The first line should be limited to 50 characters and should be a sentence that
completes the thought [When applied, this commit will...] "Implement cmd_move"
or "Fix #742" or "Improve performance of arrange_windows on ARM" or similar.

The subsequent lines should be seperated from the subject line by a single
blank line, and include optional details. In this you can give justification
for the change, [reference Github
issues](https://help.github.com/articles/closing-issues-via-commit-messages/),
or explain some of the subtler details of your patch. This is important because
when someone finds a line of code they don't understand later, they can use the
`git blame` command to find out what the author was thinking when they wrote
it. It's also easier to review your pull requests if they're seperated into
logical commits that have good commit messages and justify themselves in the
extended commit description.

As a good rule of thumb, anything you might put into the pull request
description on Github is probably fair game for going into the extended commit
message as well.

## Coding Style

wlroots is written in C with the same style as Sway. The style guidelines is
[kernel
style](https://www.kernel.org/doc/Documentation/process/coding-style.rst), but
all braces go on the same line (*"but K&R says so!" is a silly way of justifying
something*). Some points to note:

* Do not use typedefs unless you have a good reason
* Do not use macros unless you have a *really* good reason
* Align `case` with `switch`
* Tabs, not spaces
* `char *pointer` - note position of `*`
* Use logging with reckless abandon
* Always include braces for if/for/while/etc, even for one-liners

An example of well formatted code:

```C
#include <stdio.h>
#include <stdlib.h>
#include "log.h"
#include "example.h"

struct foobar {
	char *foo;
	int bar;
	long baz;
}; // Do not typedef without a good reason

int main(int argc, const char **argv) {
	if (argc != 4) {
		sway_abort("Do not run this program manually. See man 5 sway and look for output options.");
	}

	if (!registry->desktop_shell) {
		sway_abort("swaybg requires the compositor to support the desktop-shell extension.");
	}

	int desired_output = atoi(argv[1]);
	sway_log(L_INFO, "Using output %d of %d", desired_output, registry->outputs->length);
	int i;
	struct output_state *output = registry->outputs->items[desired_output];
	struct window *window = window_setup(registry, 100, 100, false);
	if (!window) {
		sway_abort("Failed to create surfaces.");
	}
	window->width = output->width;
	window->height = output->height;
	desktop_shell_set_background(registry->desktop_shell, output->output, window->surface);
	list_add(surfaces, window);

	cairo_surface_t *image = cairo_image_surface_create_from_png(argv[2]);
	double width = cairo_image_surface_get_width(image);
	double height = cairo_image_surface_get_height(image);

	const char *scaling_mode_str = argv[3];
	enum scaling_mode scaling_mode;
	if (strcmp(scaling_mode_str, "stretch") == 0) {
		scaling_mode = SCALING_MODE_STRETCH;
	} else if (strcmp(scaling_mode_str, "fill") == 0) {
		scaling_mode = SCALING_MODE_FILL;
	} else if (strcmp(scaling_mode_str, "fit") == 0) {
		scaling_mode = SCALING_MODE_FIT;
	} else if (strcmp(scaling_mode_str, "center") == 0) {
		scaling_mode = SCALING_MODE_CENTER;
	} else if (strcmp(scaling_mode_str, "tile") == 0) {
		scaling_mode = SCALING_MODE_TILE;
	} else {
		sway_abort("Unsupported scaling mode: %s", scaling_mode_str);
	}

	for (i = 0; i < surfaces->length; ++i) {
		struct window *window = surfaces->items[i];
		if (window_prerender(window) && window->cairo) {
			switch (scaling_mode) {
			case SCALING_MODE_STRETCH:
				cairo_scale(window->cairo,
						(double) window->width / width,
						(double) window->height / height);
				cairo_set_source_surface(window->cairo, image, 0, 0);
				break;
			case SCALING_MODE_FILL:
			{
				double window_ratio = (double) window->width / window->height;
				double bg_ratio = width / height;

				if (window_ratio > bg_ratio) {
					double scale = (double) window->width / width;
					cairo_scale(window->cairo, scale, scale);
					cairo_set_source_surface(window->cairo, image,
							0,
							(double) window->height/2 / scale - height/2);
				} else {
					double scale = (double) window->height / height;
					cairo_scale(window->cairo, scale, scale);
					cairo_set_source_surface(window->cairo, image,
							(double) window->width/2 / scale - width/2,
							0);
				}
				break;
			}
			case SCALING_MODE_FIT:
			{
				double window_ratio = (double) window->width / window->height;
				double bg_ratio = width / height;

				if (window_ratio > bg_ratio) {
					double scale = (double) window->height / height;
					cairo_scale(window->cairo, scale, scale);
					cairo_set_source_surface(window->cairo, image,
							(double) window->width/2 / scale - width/2,
							0);
				} else {
					double scale = (double) window->width / width;
					cairo_scale(window->cairo, scale, scale);
					cairo_set_source_surface(window->cairo, image,
							0,
							(double) window->height/2 / scale - height/2);
				}
				break;
			}
			case SCALING_MODE_CENTER:
				cairo_set_source_surface(window->cairo, image,
						(double) window->width/2 - width/2,
						(double) window->height/2 - height/2);
				break;
			case SCALING_MODE_TILE:
			{
				cairo_pattern_t *pattern = cairo_pattern_create_for_surface(image);
				cairo_pattern_set_extend(pattern, CAIRO_EXTEND_REPEAT);
				cairo_set_source(window->cairo, pattern);
				break;
			}
			default:
				sway_abort("Scaling mode '%s' not implemented yet!", scaling_mode_str);
			}

			cairo_paint(window->cairo);

			window_render(window);
		}
	}

	while (wl_display_dispatch(registry->display) != -1);

	for (i = 0; i < surfaces->length; ++i) {
		struct window *window = surfaces->items[i];
		window_teardown(window);
	}
	list_free(surfaces);
	registry_teardown(registry);
	return 0;
}
```