swayfx/CONTRIBUTING.md

# Contributing to sway

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&uio=d4) upfront and discuss
your plans.

## Coding Style

Sway is written in C. The style guidelines is [kernel
style](https://www.kernel.org/doc/Documentation/CodingStyle), 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;
}
```