1 files changed, 109 insertions, 0 deletions
diff --git a/servo/components/style/values/computed/easing.rs b/servo/components/style/values/computed/easing.rs
new file mode 100644
index 0000000000..d351b3c71d
--- /dev/null
+++ b/servo/components/style/values/computed/easing.rs
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+/* 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/. */
+
+//! Computed types for CSS Easing functions.
+
+use euclid::approxeq::ApproxEq;
+
+use crate::bezier::Bezier;
+use crate::piecewise_linear::PiecewiseLinearFunction;
+use crate::values::computed::{Integer, Number};
+use crate::values::generics::easing::{self, BeforeFlag, StepPosition, TimingKeyword};
+
+/// A computed timing function.
+pub type ComputedTimingFunction = easing::TimingFunction<Integer, Number, PiecewiseLinearFunction>;
+
+/// An alias of the computed timing function.
+pub type TimingFunction = ComputedTimingFunction;
+
+impl ComputedTimingFunction {
+    fn calculate_step_output(
+        steps: i32,
+        pos: StepPosition,
+        progress: f64,
+        before_flag: BeforeFlag,
+    ) -> f64 {
+        // User specified values can cause overflow (bug 1706157). Increments/decrements
+        // should be gravefully handled.
+        let mut current_step = (progress * (steps as f64)).floor() as i32;
+
+        // Increment current step if it is jump-start or start.
+        if pos == StepPosition::Start ||
+            pos == StepPosition::JumpStart ||
+            pos == StepPosition::JumpBoth
+        {
+            current_step = current_step.checked_add(1).unwrap_or(current_step);
+        }
+
+        // If the "before flag" is set and we are at a transition point,
+        // drop back a step
+        if before_flag == BeforeFlag::Set &&
+            (progress * steps as f64).rem_euclid(1.0).approx_eq(&0.0)
+        {
+            current_step = current_step.checked_sub(1).unwrap_or(current_step);
+        }
+
+        // We should not produce a result outside [0, 1] unless we have an
+        // input outside that range. This takes care of steps that would otherwise
+        // occur at boundaries.
+        if progress >= 0.0 && current_step < 0 {
+            current_step = 0;
+        }
+
+        // |jumps| should always be in [1, i32::MAX].
+        let jumps = if pos == StepPosition::JumpBoth {
+            steps.checked_add(1).unwrap_or(steps)
+        } else if pos == StepPosition::JumpNone {
+            steps.checked_sub(1).unwrap_or(steps)
+        } else {
+            steps
+        };
+
+        if progress <= 1.0 && current_step > jumps {
+            current_step = jumps;
+        }
+
+        (current_step as f64) / (jumps as f64)
+    }
+
+    /// The output of the timing function given the progress ratio of this animation.
+    pub fn calculate_output(&self, progress: f64, before_flag: BeforeFlag, epsilon: f64) -> f64 {
+        let progress = match self {
+            TimingFunction::CubicBezier { x1, y1, x2, y2 } => {
+                Bezier::calculate_bezier_output(progress, epsilon, *x1, *y1, *x2, *y2)
+            },
+            TimingFunction::Steps(steps, pos) => {
+                Self::calculate_step_output(*steps, *pos, progress, before_flag)
+            },
+            TimingFunction::LinearFunction(function) => function.at(progress as f32).into(),
+            TimingFunction::Keyword(keyword) => match keyword {
+                TimingKeyword::Linear => progress,
+                TimingKeyword::Ease => {
+                    Bezier::calculate_bezier_output(progress, epsilon, 0.25, 0.1, 0.25, 1.)
+                },
+                TimingKeyword::EaseIn => {
+                    Bezier::calculate_bezier_output(progress, epsilon, 0.42, 0., 1., 1.)
+                },
+                TimingKeyword::EaseOut => {
+                    Bezier::calculate_bezier_output(progress, epsilon, 0., 0., 0.58, 1.)
+                },
+                TimingKeyword::EaseInOut => {
+                    Bezier::calculate_bezier_output(progress, epsilon, 0.42, 0., 0.58, 1.)
+                },
+            },
+        };
+
+        // The output progress value of an easing function is a real number in the range:
+        // [-inf, inf].
+        // https://drafts.csswg.org/css-easing-1/#output-progress-value
+        //
+        // However, we expect to use the finite progress for interpolation and web-animations
+        // https://drafts.csswg.org/css-values-4/#interpolation
+        // https://drafts.csswg.org/web-animations-1/#dom-computedeffecttiming-progress
+        //
+        // So we clamp the infinite progress, per the spec issue:
+        // https://github.com/w3c/csswg-drafts/issues/8344
+        progress.min(f64::MAX).max(f64::MIN)
+    }
+}