Allow several rules for combining friction/restitution coefficients.

src/dynamics/coefficient_combine_rule.rs

@@ -0,0 +1,34 @@
+use crate::math::Real;
+
+/// Rules used to combine two coefficients.
+///
+/// This is used to determine the effective restitution and
+/// friction coefficients for a contact between two colliders.
+/// Each collider has its combination rule of type
+/// `CoefficientCombineRule`. And the rule
+/// actually used is given by `max(first_combine_rule as usize, second_combine_rule as usize)`.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+pub enum CoefficientCombineRule {
+    /// The two coefficients are averaged.
+    Average = 0,
+    /// The smallest coefficient is chosen.
+    Min,
+    /// The two coefficients are multiplied.
+    Multiply,
+    /// The greatest coefficient is chosen.
+    Max,
+}
+
+impl CoefficientCombineRule {
+    pub(crate) fn combine(coeff1: Real, coeff2: Real, rule_value1: u8, rule_value2: u8) -> Real {
+        let effective_rule = rule_value1.max(rule_value2);
+
+        match effective_rule {
+            0 => (coeff1 + coeff1) / 2.0,
+            1 => coeff1.min(coeff2),
+            2 => coeff1 * coeff2,
+            _ => coeff1.max(coeff2),
+        }
+    }
+}