From 62fdf0b64cc37c109460dbb6174b79500c091190 Mon Sep 17 00:00:00 2001
From: Oliver Lee <oliverzlee@gmail.com>
Date: Mon, 8 Jan 2024 22:47:14 -0800
Subject: [PATCH] define multivector negation

Change-Id: Ia277a4e73b42baac8295cd1dfc74061a6a150b97
---
 geometry/src/algebra.hpp          | 11 ++++++++++-
 test/algebra_multivector_test.cpp |  7 +++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/geometry/src/algebra.hpp b/geometry/src/algebra.hpp
index 95047c9..89a2406 100644
--- a/geometry/src/algebra.hpp
+++ b/geometry/src/algebra.hpp
@@ -284,7 +284,7 @@ struct algebra
 
     /// construct a multivector from blades
     ///
-    constexpr multivector(Bs... bs) : Bs{bs}... {}
+    constexpr explicit multivector(Bs... bs) : Bs{bs}... {}
 
     /// equality comparison
     ///
@@ -302,6 +302,15 @@ struct algebra
       return not(x == y);
     }
     /// @}
+
+    /// unary negation
+    ///
+    [[nodiscard]]
+    friend constexpr auto
+    operator-(const multivector& x) -> multivector
+    {
+      return multivector{-static_cast<Bs>(x)...};
+    }
   };
 };
 
diff --git a/test/algebra_multivector_test.cpp b/test/algebra_multivector_test.cpp
index 119b004..cbe17e6 100644
--- a/test/algebra_multivector_test.cpp
+++ b/test/algebra_multivector_test.cpp
@@ -51,4 +51,11 @@ auto main() -> int
             eq(multivector(2 * e<0>, 3 * e<1>), 3 * e<1> + 2 * e<0>) and
             eq(multivector(-2 * e<0>, 3 * e<1>), 3 * e<1> - 2 * e<0>));
       };
+
+  "multivector negation"_ctest =  //
+      [] {
+        return expect(
+            eq(e<0> + e<1>, -(-e<0> - e<1>)) and
+            eq(e<0, 1> + e<1, 2>, -(-e<0, 1> - e<1, 2>)));
+      };
 }