Allow for `Catch::Approx` to be used in a `constexpr` contextconstexpr_approx

1 files changed, 67 insertions, 22 deletions

diff --git a/src/catch2/catch_approx.hpp b/src/catch2/catch_approx.hpp
index de4d2ab4..831604bc 100644
--- a/src/catch2/catch_approx.hpp
+++ b/src/catch2/catch_approx.hpp
@@ -11,26 +11,67 @@
 #include <catch2/catch_tostring.hpp>
 
 #include <type_traits>
+#include <limits>
 
 namespace Catch {
 
     class Approx {
     private:
-        bool equalityComparisonImpl(double other) const;
-        // Sets and validates the new margin (margin >= 0)
-        void setMargin(double margin);
+        // Performs equivalent check of std::fabs(lhs - rhs) <= margin
+        // But without the subtraction to allow for INFINITY in comparison
+        constexpr bool marginComparison (double lhs, double rhs, double margin) const {
+            return (lhs + margin >= rhs) && (rhs + margin >= lhs);
+        }
+
+        constexpr double fabs(double value) const {
+            return (value < 0.0) ? -value : value;
+        }
+
+        constexpr bool isinf(double value) const {
+            return value == std::numeric_limits<double>::infinity() || value == -std::numeric_limits<double>::infinity();
+        }
+
+        constexpr bool equalityComparisonImpl(double other) const {
+            // First try with fixed margin, then compute margin based on epsilon, scale and Approx's value
+            // Thanks to Richard Harris for his help refining the scaled margin value
+            return marginComparison(m_value, other, m_margin)
+                || marginComparison(m_value, other, m_epsilon * (m_scale + fabs(isinf(m_value)? 0 : m_value)));
+        }
+
         // Sets and validates the new epsilon (0 < epsilon < 1)
-        void setEpsilon(double epsilon);
+        constexpr void setEpsilon(double epsilon) {
+            if(epsilon < 0)
+                throw std::domain_error("Invalid Approx::epsilon. Approx::epsilon has to be in [0, 1]");
+            m_epsilon = epsilon;
+        }
+
+        // Sets and validates the new margin (margin >= 0)
+        constexpr void setMargin(double margin) {
+            if(margin < 0)
+                throw std::domain_error("Invalid Approx::margin. Approx::Margin has to be non-negative.");
+            m_margin = margin;
+        }
 
     public:
-        explicit Approx ( double value );
+        constexpr inline explicit Approx ( double value )
+        :   m_epsilon( static_cast<double>(std::numeric_limits<float>::epsilon())*100. ),
+            m_margin( 0.0 ),
+            m_scale( 0.0 ),
+            m_value( value )
+        {}
 
-        static Approx custom();
+        static constexpr Approx custom() {
+            return Approx( 0.0 );
+        }
 
-        Approx operator-() const;
+        constexpr Approx operator-() const {
+            auto temp(*this);
+            temp.m_value = -temp.m_value;
+            return temp;
+        }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        Approx operator()( T const& value ) const {
+        constexpr Approx operator()( T const& value ) const {
             Approx approx( static_cast<double>(value) );
             approx.m_epsilon = m_epsilon;
             approx.m_margin = m_margin;
@@ -39,67 +80,67 @@ namespace Catch {
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        explicit Approx( T const& value ): Approx(static_cast<double>(value))
+        constexpr inline explicit Approx( T const& value ): Approx(static_cast<double>(value))
         {}
 
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator == ( const T& lhs, Approx const& rhs ) {
+        friend constexpr bool operator == ( const T& lhs, Approx const& rhs ) {
             auto lhs_v = static_cast<double>(lhs);
             return rhs.equalityComparisonImpl(lhs_v);
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator == ( Approx const& lhs, const T& rhs ) {
+        friend constexpr bool operator == ( Approx const& lhs, const T& rhs ) {
             return operator==( rhs, lhs );
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator != ( T const& lhs, Approx const& rhs ) {
+        friend constexpr bool operator != ( T const& lhs, Approx const& rhs ) {
             return !operator==( lhs, rhs );
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator != ( Approx const& lhs, T const& rhs ) {
+        friend constexpr bool operator != ( Approx const& lhs, T const& rhs ) {
             return !operator==( rhs, lhs );
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator <= ( T const& lhs, Approx const& rhs ) {
+        friend constexpr bool operator <= ( T const& lhs, Approx const& rhs ) {
             return static_cast<double>(lhs) < rhs.m_value || lhs == rhs;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator <= ( Approx const& lhs, T const& rhs ) {
+        friend constexpr bool operator <= ( Approx const& lhs, T const& rhs ) {
             return lhs.m_value < static_cast<double>(rhs) || lhs == rhs;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator >= ( T const& lhs, Approx const& rhs ) {
+        friend constexpr bool operator >= ( T const& lhs, Approx const& rhs ) {
             return static_cast<double>(lhs) > rhs.m_value || lhs == rhs;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        friend bool operator >= ( Approx const& lhs, T const& rhs ) {
+        friend constexpr bool operator >= ( Approx const& lhs, T const& rhs ) {
             return lhs.m_value > static_cast<double>(rhs) || lhs == rhs;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        Approx& epsilon( T const& newEpsilon ) {
+        constexpr Approx& epsilon( T const& newEpsilon ) {
             const auto epsilonAsDouble = static_cast<double>(newEpsilon);
             setEpsilon(epsilonAsDouble);
             return *this;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        Approx& margin( T const& newMargin ) {
+        constexpr Approx& margin( T const& newMargin ) {
             const auto marginAsDouble = static_cast<double>(newMargin);
             setMargin(marginAsDouble);
             return *this;
         }
 
         template <typename T, typename = std::enable_if_t<std::is_constructible<double, T>::value>>
-        Approx& scale( T const& newScale ) {
+        constexpr Approx& scale( T const& newScale ) {
             m_scale = static_cast<double>(newScale);
             return *this;
         }
@@ -114,8 +155,12 @@ namespace Catch {
     };
 
 namespace literals {
-    Approx operator ""_a(long double val);
-    Approx operator ""_a(unsigned long long val);
+    constexpr Approx operator ""_a(long double val) {
+        return Approx(val);
+    }
+    constexpr Approx operator ""_a(unsigned long long val) {
+        return Approx(val);
+    }
 } // end namespace literals
 
 template<>