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style: linear(...) easing: Implement right-continuous behaviour on discontinuities

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Differential Revision: https://phabricator.services.mozilla.com/D152078
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David Shin 2022-07-18 19:55:08 +00:00 • committed by Martin Robinson
parent 2a1e89468a
commit c296356a8c
1 changed files with 18 additions and 15 deletions
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33
components/style/piecewise_linear.rs
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@ -75,16 +75,24 @@ impl PiecewiseLinearFunction {
next: PiecewiseLinearFunctionEntry, next: PiecewiseLinearFunctionEntry,
asymptote: &PiecewiseLinearFunctionEntry, asymptote: &PiecewiseLinearFunctionEntry,
) -> ValueType { ) -> ValueType {
// Line is vertical, or the two points are identical. Avoid infinite slope by pretending // Short circuit if the x is on prev or next.
// the line is flat. // `next` point is preferred as per spec.
if x.approx_eq(&next.x) {
return next.y;
}
if x.approx_eq(&prev.x) {
return prev.y;
}
// Avoid division by zero.
if prev.x.approx_eq(&next.x) { if prev.x.approx_eq(&next.x) {
return asymptote.y; return next.y;
} }
let slope = (next.y - prev.y) / (next.x - prev.x); let slope = (next.y - prev.y) / (next.x - prev.x);
return slope * (x - asymptote.x) + asymptote.y; return slope * (x - asymptote.x) + asymptote.y;
} }
/// Get the y value of the piecewise linear function given the x value. /// Get the y value of the piecewise linear function given the x value, as per
/// https://drafts.csswg.org/css-easing-2/#linear-easing-function-output
pub fn at(&self, x: ValueType) -> ValueType { pub fn at(&self, x: ValueType) -> ValueType {
if !x.is_finite() { if !x.is_finite() {
return if x > 0.0 { 1.0 } else { 0.0 }; return if x > 0.0 { 1.0 } else { 0.0 };
@ -108,24 +116,19 @@ impl PiecewiseLinearFunction {
} }
let mut rev_iter = self.entries.iter().rev(); let mut rev_iter = self.entries.iter().rev();
let last = rev_iter.next().unwrap(); let last = rev_iter.next().unwrap();
if x > last.x { if x >= last.x {
let second_last = rev_iter.next().unwrap(); let second_last = rev_iter.next().unwrap();
return Self::interpolate(x, *second_last, *last, last); return Self::interpolate(x, *second_last, *last, last);
} }
// Now we know the input sits within the domain explicitly defined by our function. // Now we know the input sits within the domain explicitly defined by our function.
for (prev, next) in self.entries.iter().tuple_windows() { for (point_b, point_a) in self.entries.iter().rev().tuple_windows() {
if x > next.x { // Need to let point A be the _last_ point where its x is less than the input x,
// hence the reverse traversal.
if x < point_a.x {
continue; continue;
} }
// Prefer left hand side value return Self::interpolate(x, *point_a, *point_b, point_a);
if x.approx_eq(&prev.x) {
return prev.y;
}
if x.approx_eq(&next.x) {
return next.y;
}
return Self::interpolate(x, *prev, *next, prev);
} }
unreachable!("Input is supposed to be within the entries' min & max!"); unreachable!("Input is supposed to be within the entries' min & max!");
} }