Support addition and negation in ratio

src/core/include/units/ratio.h

@@ -60,6 +60,27 @@ struct ratio {
 
   [[nodiscard]] friend constexpr bool operator==(const ratio&, const ratio&) = default;
 
+  [[nodiscard]] friend constexpr ratio operator-(const ratio& r)
+  {
+    return ratio(-r.num, r.den, r.exp);
+  }
+
+  [[nodiscard]] friend constexpr ratio operator+(ratio lhs, ratio rhs)
+  {
+    // First, get the inputs into a common exponent.
+    const auto common_exp = std::min(lhs.exp, rhs.exp);
+    auto commonify = [common_exp](ratio &r) {
+      while (r.exp > common_exp) {
+        r.num *= 10;
+        --r.exp;
+      }
+    };
+    commonify(lhs);
+    commonify(rhs);
+
+    return ratio{lhs.num * rhs.den + lhs.den * rhs.num, lhs.den * rhs.den, common_exp};
+  }
+
   [[nodiscard]] friend constexpr ratio operator*(const ratio& lhs, const ratio& rhs)
   {
     const std::intmax_t gcd1 = std::gcd(lhs.num, rhs.den);

test/unit_test/static/ratio_test.cpp

@@ -40,6 +40,13 @@ static_assert(ratio(4) * ratio(1, 2) == ratio(2));
 static_assert(ratio(1, 8) * ratio(2) == ratio(1, 4));
 static_assert(ratio(1, 2) * ratio(8) == ratio(4));
 
+// ratio negation
+static_assert(-ratio(3, 8) == ratio(-3, 8));
+
+// ratio addition
+static_assert(ratio(1, 2) + ratio(1, 3) == ratio(5, 6));
+static_assert(ratio(1, 3, 2) + ratio(11, 6) == ratio(211, 6)); // 100/3 + 11/6
+
 // multiply with exponents
 static_assert(ratio(1, 8, 2) * ratio(2, 1, 4) == ratio(1, 4, 6));
 static_assert(ratio(1, 2, -4) * ratio(8, 1, 3) == ratio(4, 1, -1));