style: Split TimingFunction into a separate file to match spec.

TimingFunction is defined in a separate spec (i.e. css-easing), instead
of transform, so we move it into a different file.

Depends on D9310

Differential Revision: https://phabricator.services.mozilla.com/D9311

components/style/values/generics/easing.rs

@@ -0,0 +1,87 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+//! Generic types for CSS Easing Functions.
+//! https://drafts.csswg.org/css-easing/#timing-functions
+
+use values::CSSFloat;
+
+/// A generic easing function.
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToCss)]
+#[value_info(ty = "TIMING_FUNCTION")]
+pub enum TimingFunction<Integer, Number> {
+    /// `linear | ease | ease-in | ease-out | ease-in-out`
+    Keyword(TimingKeyword),
+    /// `cubic-bezier(<number>, <number>, <number>, <number>)`
+    #[allow(missing_docs)]
+    #[css(comma, function)]
+    CubicBezier {
+        x1: Number,
+        y1: Number,
+        x2: Number,
+        y2: Number,
+    },
+    /// `step-start | step-end | steps(<integer>, [ start | end ]?)`
+    #[css(comma, function)]
+    #[value_info(other_values = "step-start,step-end")]
+    Steps(Integer, #[css(skip_if = "is_end")] StepPosition),
+}
+
+#[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+)]
+pub enum TimingKeyword {
+    Linear,
+    Ease,
+    EaseIn,
+    EaseOut,
+    EaseInOut,
+}
+
+#[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
+#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, Parse, PartialEq, ToComputedValue, ToCss)]
+pub enum StepPosition {
+    Start,
+    End,
+}
+
+#[inline]
+fn is_end(position: &StepPosition) -> bool {
+    *position == StepPosition::End
+}
+
+impl<Integer, Number> TimingFunction<Integer, Number> {
+    /// `ease`
+    #[inline]
+    pub fn ease() -> Self {
+        TimingFunction::Keyword(TimingKeyword::Ease)
+    }
+}
+
+impl TimingKeyword {
+    /// Returns the keyword as a quadruplet of Bezier point coordinates
+    /// `(x1, y1, x2, y2)`.
+    #[inline]
+    pub fn to_bezier(self) -> (CSSFloat, CSSFloat, CSSFloat, CSSFloat) {
+        match self {
+            TimingKeyword::Linear => (0., 0., 1., 1.),
+            TimingKeyword::Ease => (0.25, 0.1, 0.25, 1.),
+            TimingKeyword::EaseIn => (0.42, 0., 1., 1.),
+            TimingKeyword::EaseOut => (0., 0., 0.58, 1.),
+            TimingKeyword::EaseInOut => (0.42, 0., 0.58, 1.),
+        }
+    }
+}