1 files changed, 70 insertions, 0 deletions
diff --git a/servo/components/style/stylesheets/cascading_at_rule.rs b/servo/components/style/stylesheets/cascading_at_rule.rs
new file mode 100644
index 0000000000..b23b0720fe
--- /dev/null
+++ b/servo/components/style/stylesheets/cascading_at_rule.rs
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Cascading at-rule types and traits
+
+use crate::stylesheets::Origin;
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ToCss};
+
+/// Computes the cascade precedence as according to
+/// <http://dev.w3.org/csswg/css-cascade/#cascade-origin>
+#[inline]
+fn cascade_precendence(origin: Origin, important: bool) -> u8 {
+    match (origin, important) {
+        (Origin::UserAgent, true) => 1,
+        (Origin::User, true) => 2,
+        (Origin::Author, true) => 3,
+        (Origin::Author, false) => 4,
+        (Origin::User, false) => 5,
+        (Origin::UserAgent, false) => 6,
+    }
+}
+
+/// Cascading rule descriptor implementation.
+/// This is only used for at-rules which can cascade. These are @viewport and
+/// @page, although we don't currently implement @page as such.
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToShmem)]
+pub struct DescriptorDeclaration<T> {
+    /// Origin of the declaration
+    pub origin: Origin,
+    /// Declaration value
+    pub descriptor: T,
+    /// Indicates the presence of a !important property.
+    pub important: bool,
+}
+
+impl<T> DescriptorDeclaration<T> {
+    #[allow(missing_docs)]
+    pub fn new(origin: Origin, descriptor: T, important: bool) -> Self {
+        Self {
+            origin,
+            descriptor,
+            important,
+        }
+    }
+    /// Returns true iff self is equal or higher precedence to the other.
+    pub fn higher_or_equal_precendence(&self, other: &Self) -> bool {
+        let self_precedence = cascade_precendence(self.origin, self.important);
+        let other_precedence = cascade_precendence(other.origin, other.important);
+
+        self_precedence <= other_precedence
+    }
+}
+
+impl<T> ToCss for DescriptorDeclaration<T>
+where
+    T: ToCss,
+{
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        self.descriptor.to_css(dest)?;
+        if self.important {
+            dest.write_str(" !important")?;
+        }
+        dest.write_char(';')
+    }
+}