Write down style guidelines

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+# 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!" is silly*). 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
+
+```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;
+}
+```