Fix broken Eq/Ord consistency for AssetCapacity

docs/release_notes/upcoming.md

@@ -54,6 +54,8 @@ ready to be released, carry out the following steps:
 - Users can now set demand to zero in `demand.csv` ([#871])
 - Fix: sign for levies of type `net` was wrong for inputs ([#969])
 - Fix `--overwrite` option for `save-graphs` command ([#1001])
+- Fix broken `Eq`/`Ord` consistency for `AssetCapacity`: equality is now aligned with
+  `total_cmp` ordering ([#1158])
 
 [#767]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/767
 [#868]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/868
@@ -72,3 +74,4 @@ ready to be released, carry out the following steps:
 [#1021]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/1021
 [#1022]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/1022
 [#1030]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/1030
+[#1158]: https://github.com/EnergySystemsModellingLab/MUSE2/pull/1158

src/asset/capacity.rs

@@ -4,7 +4,7 @@ use std::cmp::Ordering;
 use std::ops::{Add, Sub};
 
 /// Capacity of an asset, which may be continuous or a discrete number of indivisible units
-#[derive(Clone, PartialEq, Copy, Debug)]
+#[derive(Clone, Copy, Debug)]
 pub enum AssetCapacity {
     /// Continuous capacity
     Continuous(Capacity),
@@ -13,6 +13,12 @@ pub enum AssetCapacity {
     Discrete(u32, Capacity),
 }
 
+impl PartialEq for AssetCapacity {
+    fn eq(&self, other: &Self) -> bool {
+        self.cmp(other) == Ordering::Equal
+    }
+}
+
 impl Add for AssetCapacity {
     type Output = Self;
 
@@ -213,4 +219,41 @@ mod tests {
         let got = orig.apply_limit_factor(factor);
         assert_eq!(got, AssetCapacity::Discrete(expected_units, unit_size));
     }
+
+    /// `PartialEq` must be consistent with `Ord`: two values are equal iff `cmp` returns
+    /// `Ordering::Equal`.
+    #[rstest]
+    #[case::continuous_equal(
+        AssetCapacity::Continuous(Capacity(1.0)),
+        AssetCapacity::Continuous(Capacity(1.0)),
+        true
+    )]
+    #[case::continuous_positive_zero_vs_negative_zero(
+        AssetCapacity::Continuous(Capacity(0.0)),
+        AssetCapacity::Continuous(Capacity(-0.0)),
+        false
+    )]
+    #[case::continuous_unequal(
+        AssetCapacity::Continuous(Capacity(1.0)),
+        AssetCapacity::Continuous(Capacity(2.0)),
+        false
+    )]
+    #[case::discrete_equal(
+        AssetCapacity::Discrete(3, Capacity(4.0)),
+        AssetCapacity::Discrete(3, Capacity(4.0)),
+        true
+    )]
+    #[case::discrete_unequal(
+        AssetCapacity::Discrete(2, Capacity(4.0)),
+        AssetCapacity::Discrete(3, Capacity(4.0)),
+        false
+    )]
+    fn eq_consistent_with_ord(
+        #[case] a: AssetCapacity,
+        #[case] b: AssetCapacity,
+        #[case] expected_eq: bool,
+    ) {
+        assert_eq!(a == b, expected_eq);
+        assert_eq!(a.cmp(&b) == Ordering::Equal, expected_eq);
+    }
 }