Move CascadeLevel to its own rule_tree submodule

components/style/rule_tree/level.rs

@@ -0,0 +1,277 @@
+/* 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/. */
+
+#![forbid(unsafe_code)]
+
+use crate::properties::Importance;
+use crate::shared_lock::{SharedRwLockReadGuard, StylesheetGuards};
+use crate::stylesheets::Origin;
+
+/// The cascade level these rules are relevant at, as per[1][2][3].
+///
+/// Presentational hints for SVG and HTML are in the "author-level
+/// zero-specificity" level, that is, right after user rules, and before author
+/// rules.
+///
+/// The order of variants declared here is significant, and must be in
+/// _ascending_ order of precedence.
+///
+/// See also [4] for the Shadow DOM bits. We rely on the invariant that rules
+/// from outside the tree the element is in can't affect the element.
+///
+/// The opposite is not true (i.e., :host and ::slotted) from an "inner" shadow
+/// tree may affect an element connected to the document or an "outer" shadow
+/// tree.
+///
+/// [1]: https://drafts.csswg.org/css-cascade/#cascade-origin
+/// [2]: https://drafts.csswg.org/css-cascade/#preshint
+/// [3]: https://html.spec.whatwg.org/multipage/#presentational-hints
+/// [4]: https://drafts.csswg.org/css-scoping/#shadow-cascading
+#[repr(u8)]
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, PartialEq, PartialOrd)]
+pub enum CascadeLevel {
+    /// Normal User-Agent rules.
+    UANormal,
+    /// User normal rules.
+    UserNormal,
+    /// Presentational hints.
+    PresHints,
+    /// Shadow DOM styles from author styles.
+    AuthorNormal {
+        /// The order in the shadow tree hierarchy. This number is relative to
+        /// the tree of the element, and thus the only invariants that need to
+        /// be preserved is:
+        ///
+        ///  * Zero is the same tree as the element that matched the rule. This
+        ///    is important so that we can optimize style attribute insertions.
+        ///
+        ///  * The levels are ordered in accordance with
+        ///    https://drafts.csswg.org/css-scoping/#shadow-cascading
+        shadow_cascade_order: ShadowCascadeOrder,
+    },
+    /// SVG SMIL animations.
+    SMILOverride,
+    /// CSS animations and script-generated animations.
+    Animations,
+    /// Author-supplied important rules.
+    AuthorImportant {
+        /// The order in the shadow tree hierarchy, inverted, so that PartialOrd
+        /// does the right thing.
+        shadow_cascade_order: ShadowCascadeOrder,
+    },
+    /// User important rules.
+    UserImportant,
+    /// User-agent important rules.
+    UAImportant,
+    /// Transitions
+    Transitions,
+}
+
+impl CascadeLevel {
+    /// Pack this cascade level in a single byte.
+    ///
+    /// We have 10 levels, which we can represent with 4 bits, and then a
+    /// cascade order optionally, which we can clamp to three bits max, and
+    /// represent with a fourth bit for the sign.
+    ///
+    /// So this creates: SOOODDDD
+    ///
+    /// Where `S` is the sign of the order (one if negative, 0 otherwise), `O`
+    /// is the absolute value of the order, and `D`s are the discriminant.
+    #[inline]
+    pub fn to_byte_lossy(&self) -> u8 {
+        let (discriminant, order) = match *self {
+            Self::UANormal => (0, 0),
+            Self::UserNormal => (1, 0),
+            Self::PresHints => (2, 0),
+            Self::AuthorNormal {
+                shadow_cascade_order,
+            } => (3, shadow_cascade_order.0),
+            Self::SMILOverride => (4, 0),
+            Self::Animations => (5, 0),
+            Self::AuthorImportant {
+                shadow_cascade_order,
+            } => (6, shadow_cascade_order.0),
+            Self::UserImportant => (7, 0),
+            Self::UAImportant => (8, 0),
+            Self::Transitions => (9, 0),
+        };
+
+        debug_assert_eq!(discriminant & 0xf, discriminant);
+        if order == 0 {
+            return discriminant;
+        }
+
+        let negative = order < 0;
+        let value = std::cmp::min(order.abs() as u8, 0b111);
+        (negative as u8) << 7 | value << 4 | discriminant
+    }
+
+    /// Convert back from the single-byte representation of the cascade level
+    /// explained above.
+    #[inline]
+    pub fn from_byte(b: u8) -> Self {
+        let order = {
+            let abs = ((b & 0b01110000) >> 4) as i8;
+            let negative = b & 0b10000000 != 0;
+            if negative {
+                -abs
+            } else {
+                abs
+            }
+        };
+        let discriminant = b & 0xf;
+        let level = match discriminant {
+            0 => Self::UANormal,
+            1 => Self::UserNormal,
+            2 => Self::PresHints,
+            3 => {
+                return Self::AuthorNormal {
+                    shadow_cascade_order: ShadowCascadeOrder(order),
+                }
+            },
+            4 => Self::SMILOverride,
+            5 => Self::Animations,
+            6 => {
+                return Self::AuthorImportant {
+                    shadow_cascade_order: ShadowCascadeOrder(order),
+                }
+            },
+            7 => Self::UserImportant,
+            8 => Self::UAImportant,
+            9 => Self::Transitions,
+            _ => unreachable!("Didn't expect {} as a discriminant", discriminant),
+        };
+        debug_assert_eq!(order, 0, "Didn't expect an order value for {:?}", level);
+        level
+    }
+
+    /// Select a lock guard for this level
+    pub fn guard<'a>(&self, guards: &'a StylesheetGuards<'a>) -> &'a SharedRwLockReadGuard<'a> {
+        match *self {
+            Self::UANormal | Self::UserNormal | Self::UserImportant | Self::UAImportant => {
+                guards.ua_or_user
+            },
+            _ => guards.author,
+        }
+    }
+
+    /// Returns the cascade level for author important declarations from the
+    /// same tree as the element.
+    #[inline]
+    pub fn same_tree_author_important() -> Self {
+        Self::AuthorImportant {
+            shadow_cascade_order: ShadowCascadeOrder::for_same_tree(),
+        }
+    }
+
+    /// Returns the cascade level for author normal declarations from the same
+    /// tree as the element.
+    #[inline]
+    pub fn same_tree_author_normal() -> Self {
+        Self::AuthorNormal {
+            shadow_cascade_order: ShadowCascadeOrder::for_same_tree(),
+        }
+    }
+
+    /// Returns whether this cascade level represents important rules of some
+    /// sort.
+    #[inline]
+    pub fn is_important(&self) -> bool {
+        match *self {
+            Self::AuthorImportant { .. } | Self::UserImportant | Self::UAImportant => true,
+            _ => false,
+        }
+    }
+
+    /// Returns the importance relevant for this rule. Pretty similar to
+    /// `is_important`.
+    #[inline]
+    pub fn importance(&self) -> Importance {
+        if self.is_important() {
+            Importance::Important
+        } else {
+            Importance::Normal
+        }
+    }
+
+    /// Returns the cascade origin of the rule.
+    #[inline]
+    pub fn origin(&self) -> Origin {
+        match *self {
+            Self::UAImportant | Self::UANormal => Origin::UserAgent,
+            Self::UserImportant | Self::UserNormal => Origin::User,
+            Self::PresHints |
+            Self::AuthorNormal { .. } |
+            Self::AuthorImportant { .. } |
+            Self::SMILOverride |
+            Self::Animations |
+            Self::Transitions => Origin::Author,
+        }
+    }
+
+    /// Returns whether this cascade level represents an animation rules.
+    #[inline]
+    pub fn is_animation(&self) -> bool {
+        match *self {
+            Self::SMILOverride | Self::Animations | Self::Transitions => true,
+            _ => false,
+        }
+    }
+}
+
+/// A counter to track how many shadow root rules deep we are. This is used to
+/// handle:
+///
+/// https://drafts.csswg.org/css-scoping/#shadow-cascading
+///
+/// See the static functions for the meaning of different values.
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, Ord, PartialEq, PartialOrd)]
+pub struct ShadowCascadeOrder(i8);
+
+impl ShadowCascadeOrder {
+    /// A level for the outermost shadow tree (the shadow tree we own, and the
+    /// ones from the slots we're slotted in).
+    #[inline]
+    pub fn for_outermost_shadow_tree() -> Self {
+        Self(-1)
+    }
+
+    /// A level for the element's tree.
+    #[inline]
+    fn for_same_tree() -> Self {
+        Self(0)
+    }
+
+    /// A level for the innermost containing tree (the one closest to the
+    /// element).
+    #[inline]
+    pub fn for_innermost_containing_tree() -> Self {
+        Self(1)
+    }
+
+    /// Decrement the level, moving inwards. We should only move inwards if
+    /// we're traversing slots.
+    #[inline]
+    pub fn dec(&mut self) {
+        debug_assert!(self.0 < 0);
+        self.0 = self.0.saturating_sub(1);
+    }
+
+    /// The level, moving inwards. We should only move inwards if we're
+    /// traversing slots.
+    #[inline]
+    pub fn inc(&mut self) {
+        debug_assert_ne!(self.0, -1);
+        self.0 = self.0.saturating_add(1);
+    }
+}
+
+impl std::ops::Neg for ShadowCascadeOrder {
+    type Output = Self;
+    #[inline]
+    fn neg(self) -> Self {
+        Self(self.0.neg())
+    }
+}