From 72c2d064097a34ba589b18873df09078af953157 Mon Sep 17 00:00:00 2001
From: Chip Hogg <chogg@aurora.tech>
Date: Mon, 2 Dec 2024 15:41:44 -0500
Subject: [PATCH] Support scaling a unit symbol by a magnitude (#341)

I found this useful in handling some "liters per 100 km" test cases.
---
 au/code/au/unit_symbol.hh      | 3 ++-
 au/code/au/unit_symbol_test.cc | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/au/code/au/unit_symbol.hh b/au/code/au/unit_symbol.hh
index 07fa991e..72fc46b6 100644
--- a/au/code/au/unit_symbol.hh
+++ b/au/code/au/unit_symbol.hh
@@ -30,7 +30,8 @@ namespace au {
 template <typename Unit>
 struct SymbolFor : detail::MakesQuantityFromNumber<SymbolFor, Unit>,
                    detail::ScalesQuantity<SymbolFor, Unit>,
-                   detail::ComposesWith<SymbolFor, Unit, SymbolFor, SymbolFor> {};
+                   detail::ComposesWith<SymbolFor, Unit, SymbolFor, SymbolFor>,
+                   detail::CanScaleByMagnitude<SymbolFor, Unit> {};
 
 //
 // Create a unit symbol using the more fluent APIs that unit slots make possible.  For example:
diff --git a/au/code/au/unit_symbol_test.cc b/au/code/au/unit_symbol_test.cc
index e9b892e0..7e387fd6 100644
--- a/au/code/au/unit_symbol_test.cc
+++ b/au/code/au/unit_symbol_test.cc
@@ -43,4 +43,12 @@ TEST(SymbolFor, ScalesUnitsOfExistingQuantity) {
 
 TEST(SymbolFor, CompatibleWithUnitSlot) { EXPECT_THAT(meters(35u).in(m), SameTypeAndValue(35u)); }
 
+TEST(SymbolFor, CanScaleByMagnitude) {
+    // Yes, the identifier name is a little awkward for these symbols for anonymous scaled units.
+    // But it's still important to have this functionality.
+    constexpr auto u100_m = mag<100>() * m;
+
+    EXPECT_THAT(3.5f / u100_m, SameTypeAndValue(inverse(meters * mag<100>())(3.5f)));
+}
+
 }  // namespace au