style: Extract {animated,computed}::Color common parts.

Extract the common parts of `animated::Color` and `computed::Color` out
into `generics::color::Color<T>` that is generic over the type of
RGBA color.

Bug: 1465307
Reviewed-by: xidorn
MozReview-Commit-ID: EymSr7aqnAP

components/style/values/generics/color.rs

@@ -0,0 +1,76 @@
+/* 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 color properties.
+
+/// Ratios representing the contribution of color and currentcolor to
+/// the final color value.
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToAnimatedValue)]
+pub struct ComplexColorRatios {
+    /// Numeric color contribution.
+    pub bg: f32,
+    /// Foreground color, aka currentcolor, contribution.
+    pub fg: f32,
+}
+
+impl ComplexColorRatios {
+    /// Ratios representing a `Numeric` color.
+    pub const NUMERIC: ComplexColorRatios = ComplexColorRatios { bg: 1., fg: 0. };
+    /// Ratios representing the `Foreground` color.
+    pub const FOREGROUND: ComplexColorRatios = ComplexColorRatios { bg: 0., fg: 1. };
+}
+
+/// This enum represents a combined color from a numeric color and
+/// the current foreground color (currentcolor keyword).
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToAnimatedValue)]
+pub enum Color<RGBA> {
+    ///  Numeric RGBA color.
+    Numeric(RGBA),
+
+    /// The current foreground color.
+    Foreground,
+
+    /// A linear combination of numeric color and currentcolor.
+    /// The formula is: `color * ratios.bg + currentcolor * ratios.fg`.
+    Complex(RGBA, ComplexColorRatios),
+}
+
+impl<RGBA> Color<RGBA> {
+    /// Create a color based upon the specified ratios.
+    pub fn with_ratios(color: RGBA, ratios: ComplexColorRatios) -> Self {
+        if ratios == ComplexColorRatios::NUMERIC {
+            Color::Numeric(color)
+        } else if ratios == ComplexColorRatios::FOREGROUND {
+            Color::Foreground
+        } else {
+            Color::Complex(color, ratios)
+        }
+    }
+
+    /// Returns a numeric color representing the given RGBA value.
+    pub fn rgba(color: RGBA) -> Self {
+        Color::Numeric(color)
+    }
+
+    /// Returns a complex color value representing currentcolor.
+    pub fn currentcolor() -> Self {
+        Color::Foreground
+    }
+
+    /// Whether it is a numeric color (no currentcolor component).
+    pub fn is_numeric(&self) -> bool {
+        matches!(*self, Color::Numeric(..))
+    }
+
+    /// Whether it is a currentcolor value (no numeric color component).
+    pub fn is_currentcolor(&self) -> bool {
+        matches!(*self, Color::Foreground)
+    }
+}
+
+impl<RGBA> From<RGBA> for Color<RGBA> {
+    fn from(color: RGBA) -> Self {
+        Self::rgba(color)
+    }
+}

components/style/values/generics/mod.rs

@@ -17,6 +17,7 @@ pub mod basic_shape;
 pub mod border;
 #[path = "box.rs"]
 pub mod box_;
+pub mod color;
 pub mod column;
 pub mod counters;
 pub mod effects;