1 files changed, 151 insertions, 0 deletions
diff --git a/servo/components/style/values/animated/font.rs b/servo/components/style/values/animated/font.rs
new file mode 100644
index 0000000000..f890a3b2bd
--- /dev/null
+++ b/servo/components/style/values/animated/font.rs
@@ -0,0 +1,151 @@
+/* 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/. */
+
+//! Animation implementation for various font-related types.
+
+use super::{Animate, Procedure, ToAnimatedZero};
+use crate::values::computed::font::FontVariationSettings;
+use crate::values::computed::Number;
+use crate::values::distance::{ComputeSquaredDistance, SquaredDistance};
+use crate::values::generics::font::{FontSettings as GenericFontSettings, FontTag, VariationValue};
+
+/// <https://drafts.csswg.org/css-fonts-4/#font-variation-settings-def>
+impl Animate for FontVariationSettings {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        FontSettingTagIter::new(self, other)?
+            .map(|r| r.and_then(|(st, ot)| st.animate(&ot, procedure)))
+            .collect::<Result<Vec<ComputedVariationValue>, ()>>()
+            .map(|v| GenericFontSettings(v.into_boxed_slice()))
+    }
+}
+
+impl ComputeSquaredDistance for FontVariationSettings {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        FontSettingTagIter::new(self, other)?
+            .map(|r| r.and_then(|(st, ot)| st.compute_squared_distance(&ot)))
+            .sum()
+    }
+}
+
+impl ToAnimatedZero for FontVariationSettings {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Err(())
+    }
+}
+
+type ComputedVariationValue = VariationValue<Number>;
+
+// FIXME: Could do a rename, this is only used for font variations.
+struct FontSettingTagIterState<'a> {
+    tags: Vec<&'a ComputedVariationValue>,
+    index: usize,
+    prev_tag: FontTag,
+}
+
+impl<'a> FontSettingTagIterState<'a> {
+    fn new(tags: Vec<&'a ComputedVariationValue>) -> FontSettingTagIterState<'a> {
+        FontSettingTagIterState {
+            index: tags.len(),
+            tags,
+            prev_tag: FontTag(0),
+        }
+    }
+}
+
+/// Iterator for font-variation-settings tag lists
+///
+/// [CSS fonts level 4](https://drafts.csswg.org/css-fonts-4/#descdef-font-face-font-variation-settings)
+/// defines the animation of font-variation-settings as follows:
+///
+///   Two declarations of font-feature-settings[sic] can be animated between if
+///   they are "like".  "Like" declarations are ones where the same set of
+///   properties appear (in any order).  Because succesive[sic] duplicate
+///   properties are applied instead of prior duplicate properties, two
+///   declarations can be "like" even if they have differing number of
+///   properties. If two declarations are "like" then animation occurs pairwise
+///   between corresponding values in the declarations.
+///
+/// In other words if we have the following lists:
+///
+///   "wght" 1.4, "wdth" 5, "wght" 2
+///   "wdth" 8, "wght" 4, "wdth" 10
+///
+/// We should animate between:
+///
+///   "wdth" 5, "wght" 2
+///   "wght" 4, "wdth" 10
+///
+/// This iterator supports this by sorting the two lists, then iterating them in
+/// reverse, and skipping entries with repeated tag names. It will return
+/// Some(Err()) if it reaches the end of one list before the other, or if the
+/// tag names do not match.
+///
+/// For the above example, this iterator would return:
+///
+///   Some(Ok("wght" 2, "wght" 4))
+///   Some(Ok("wdth" 5, "wdth" 10))
+///   None
+///
+struct FontSettingTagIter<'a> {
+    a_state: FontSettingTagIterState<'a>,
+    b_state: FontSettingTagIterState<'a>,
+}
+
+impl<'a> FontSettingTagIter<'a> {
+    fn new(
+        a_settings: &'a FontVariationSettings,
+        b_settings: &'a FontVariationSettings,
+    ) -> Result<FontSettingTagIter<'a>, ()> {
+        if a_settings.0.is_empty() || b_settings.0.is_empty() {
+            return Err(());
+        }
+
+        fn as_new_sorted_tags(tags: &[ComputedVariationValue]) -> Vec<&ComputedVariationValue> {
+            use std::iter::FromIterator;
+            let mut sorted_tags = Vec::from_iter(tags.iter());
+            sorted_tags.sort_by_key(|k| k.tag.0);
+            sorted_tags
+        }
+
+        Ok(FontSettingTagIter {
+            a_state: FontSettingTagIterState::new(as_new_sorted_tags(&a_settings.0)),
+            b_state: FontSettingTagIterState::new(as_new_sorted_tags(&b_settings.0)),
+        })
+    }
+
+    fn next_tag(state: &mut FontSettingTagIterState<'a>) -> Option<&'a ComputedVariationValue> {
+        if state.index == 0 {
+            return None;
+        }
+
+        state.index -= 1;
+        let tag = state.tags[state.index];
+        if tag.tag == state.prev_tag {
+            FontSettingTagIter::next_tag(state)
+        } else {
+            state.prev_tag = tag.tag;
+            Some(tag)
+        }
+    }
+}
+
+impl<'a> Iterator for FontSettingTagIter<'a> {
+    type Item = Result<(&'a ComputedVariationValue, &'a ComputedVariationValue), ()>;
+
+    fn next(
+        &mut self,
+    ) -> Option<Result<(&'a ComputedVariationValue, &'a ComputedVariationValue), ()>> {
+        match (
+            FontSettingTagIter::next_tag(&mut self.a_state),
+            FontSettingTagIter::next_tag(&mut self.b_state),
+        ) {
+            (Some(at), Some(bt)) if at.tag == bt.tag => Some(Ok((at, bt))),
+            (None, None) => None,
+            _ => Some(Err(())), // Mismatch number of unique tags or tag names.
+        }
+    }
+}