Merge pull request #726 from rothmichaels/fix/optimal-inverse-runtime

fix: enable runtime usage of inverse function by using compile-time unit extraction

Author: mpusz

src/core/include/mp-units/math.h

@@ -413,13 +413,15 @@ template<Unit auto To, auto R, typename Rep>
   }
 {
 #if MP_UNITS_API_NATURAL_UNITS
-  if constexpr (!MP_UNITS_ASSOCIATED_UNIT<MP_UNITS_REMOVE_CONST(decltype(To))>)
-    return (representation_values<Rep>::one() * one).force_in(To * q.unit) / q;
-  else
+  if constexpr (!MP_UNITS_ASSOCIATED_UNIT<MP_UNITS_REMOVE_CONST(decltype(To))>) {
+    constexpr Unit auto unit = To * quantity<R, Rep>::unit;
+    return (representation_values<Rep>::one() * one).force_in(unit) / q;
+  } else
 #endif
   {
     constexpr QuantitySpec auto qs = get_quantity_spec(To) * quantity<R, Rep>::quantity_spec;
-    return qs(representation_values<Rep>::one() * one).force_in(To * q.unit) / q;
+    constexpr Unit auto unit = To * quantity<R, Rep>::unit;
+    return qs(representation_values<Rep>::one() * one).force_in(unit) / q;
   }
 }
 

test/runtime/math_test.cpp

@@ -589,4 +589,31 @@ TEST_CASE("math operations", "[math]")
       REQUIRE_THAT(atan2(1. * isq::length[km], 1000. * isq::length[m]), AlmostEquals(45. * angle[deg]));
     }
   }
+
+  SECTION("inverse functions")
+  {
+    SECTION("inverse of time quantity returns frequency")
+    {
+      auto period = 2.0 * isq::time[s];
+      auto frequency = inverse<si::hertz>(period);
+      REQUIRE(frequency == 0.5 * isq::frequency[Hz]);
+    }
+
+    SECTION("inverse works with runtime values")
+    {
+      // Test the specific case that fails with consteval
+      double runtime_value = 3.0;
+      auto period = runtime_value * isq::time[s];
+      auto frequency = inverse<si::hertz>(period);
+      auto expected = (1.0 / 3.0) * isq::frequency[Hz];
+      REQUIRE_THAT(frequency, AlmostEquals(expected));
+    }
+
+    SECTION("inverse with different input units")
+    {
+      auto period_ms = 500.0 * isq::time[ms];
+      auto frequency = inverse<si::hertz>(period_ms);
+      REQUIRE(frequency == 2.0 * isq::frequency[Hz]);
+    }
+  }
 }