style: Use left-to-right indices in the invalidator.

This will make easier to create external invalidations, and also makes reasoning
about the invalidator a bit easier.

components/selectors/matching.rs

@@ -299,11 +299,10 @@ where
 /// Whether a compound selector matched, and whether it was the rightmost
 /// selector inside the complex selector.
 pub enum CompoundSelectorMatchingResult {
+    /// The selector was fully matched.
+    FullyMatched,
     /// The compound selector matched, and the next combinator offset is
     /// `next_combinator_offset`.
-    ///
-    /// If the next combinator offset is zero, it means that it's the rightmost
-    /// selector.
     Matched { next_combinator_offset: usize, },
     /// The selector didn't match.
     NotMatched,
@@ -325,8 +324,9 @@ pub fn matches_compound_selector<E>(
 where
     E: Element
 {
+    debug_assert_ne!(from_offset, 0);
     if cfg!(debug_assertions) {
-        selector.combinator_at(from_offset); // This asserts.
+        selector.combinator_at_parse_order(from_offset - 1); // This asserts.
     }
 
     let mut local_context = LocalMatchingContext {
@@ -334,10 +334,11 @@ where
         matches_hover_and_active_quirk: false,
     };
 
-    for component in selector.iter_raw_parse_order_from(from_offset - 1) {
+    for component in selector.iter_raw_parse_order_from(from_offset) {
         if matches!(*component, Component::Combinator(..)) {
+            debug_assert_ne!(from_offset, 0, "Selector started with a combinator?");
             return CompoundSelectorMatchingResult::Matched {
-                next_combinator_offset: from_offset - 1,
+                next_combinator_offset: from_offset,
             }
         }
 
@@ -350,12 +351,10 @@ where
             return CompoundSelectorMatchingResult::NotMatched;
         }
 
-        from_offset -= 1;
+        from_offset += 1;
     }
 
-    return CompoundSelectorMatchingResult::Matched {
-        next_combinator_offset: 0,
-    }
+    CompoundSelectorMatchingResult::FullyMatched
 }
 
 /// Matches a complex selector.

components/selectors/parser.rs

@@ -440,12 +440,11 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         }
     }
 
-    /// Returns the combinator at index `index` (one-indexed from the right),
+    /// Returns the combinator at index `index` (zero-indexed from the right),
     /// or panics if the component is not a combinator.
-    ///
-    /// FIXME(bholley): Use more intuitive indexing.
-    pub fn combinator_at(&self, index: usize) -> Combinator {
-        match self.0.slice[index - 1] {
+    #[inline]
+    pub fn combinator_at_match_order(&self, index: usize) -> Combinator {
+        match self.0.slice[index] {
             Component::Combinator(c) => c,
             ref other => {
                 panic!("Not a combinator: {:?}, {:?}, index: {}",
@@ -460,16 +459,24 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         self.0.slice.iter()
     }
 
+    /// Returns the combinator at index `index` (zero-indexed from the left),
+    /// or panics if the component is not a combinator.
+    #[inline]
+    pub fn combinator_at_parse_order(&self, index: usize) -> Combinator {
+        match self.0.slice[self.len() - index - 1] {
+            Component::Combinator(c) => c,
+            ref other => {
+                panic!("Not a combinator: {:?}, {:?}, index: {}",
+                       other, self, index)
+            }
+        }
+    }
+
     /// Returns an iterator over the sequence of simple selectors and
-    /// combinators, in parse order (from left to right), _starting_
-    /// 'offset_from_right' entries from the past-the-end sentinel on
-    /// the right. So "0" panics,. "1" iterates nothing, and "len"
-    /// iterates the entire sequence.
-    ///
-    /// FIXME(bholley): This API is rather unintuive, and should really
-    /// be changed to accept an offset from the left. Same for combinator_at.
-    pub fn iter_raw_parse_order_from(&self, offset_from_right: usize) -> Rev<slice::Iter<Component<Impl>>> {
-        self.0.slice[..offset_from_right].iter().rev()
+    /// combinators, in parse order (from left to right), starting from
+    /// `offset`.
+    pub fn iter_raw_parse_order_from(&self, offset: usize) -> Rev<slice::Iter<Component<Impl>>> {
+        self.0.slice[..self.len() - offset].iter().rev()
     }
 
     /// Creates a Selector from a vec of Components, specified in parse order. Used in tests.