Move CSS @import statements to top of combined file

Thanks to jhuriez for pointing this out and for the basis of this patch.

See #39.

classes/casset.php

@@ -107,7 +107,7 @@ class Casset {
 	 */
 	protected static $post_load_callback = null;
 
-	/*
+	/**
 	 * @var function If given, the function to call when we've decided on the name
 	 * for a file, but want to allow the user to tweak it before we write it to the
 	 * page.
@@ -126,6 +126,11 @@ class Casset {
 	*/
 	protected static $css_uri_rewriter = 'absolute';
 
+	/**
+	 * @var Whether to move @import lines to the top
+	 */
+	protected static $move_imports_to_top = true;
+
 	/**
 	 * @var bool Wether we've been initialized.
 	 */
@@ -199,6 +204,8 @@ public static function _init()
 
 		static::$css_uri_rewriter = \Config::get('casset.css_uri_rewriter', static::$css_uri_rewriter);
 
+		static::$move_imports_to_top = \Config::get('casset.move_imports_to_top', static::$move_imports_to_top);
+
 		static::$initialized = true;
 	}
 
@@ -1191,6 +1198,17 @@ protected static function combine($type, $file_group, $minify, $inline)
 					}
 				}
 			}
+			if ($type == 'css' && static::$move_imports_to_top) {
+				// Remove @import lines, and record them
+				$imports = array();
+				$content = preg_replace_callback('/@import.*?;/', function($match) use (&$imports) {
+					$imports[] = $match[0];
+					return '';
+				}, $content);
+				if (count($imports))
+					$content = implode(PHP_EOL, $imports).PHP_EOL.$content;
+			}
+
 			file_put_contents($abs_filepath, $content, LOCK_EX);
 			$mtime = time();
 		}

readme.md

@@ -851,6 +851,14 @@ If you decide to do this, you'll probably want to turn off Casset's rewriting.
 The algorithm is specified using the `css_uri_rewriter` config key.
 This can take values of 'absolute', 'relative', or 'none'.
 
+CSS @import rewriting
+---------------------
+
+CSS `@import` statements need to come at the beginning of the file in which they appear.
+Combining files obviously screws this up, as lines which were at the beginning of a file could now be somewhere in the middle of the combined file.
+If the config key `move_imports_to_top` is true (the default) then casser will take all `@import` lines and move them to the top of the combined file.
+If this is causing problems, you can disable this feature.
+
 Addons
 ------