maths: add 'frac' function

Extracts the fractional component from a floating point number. Prefer
to use temporaries that you have on hand over these functions.

coord/ops.hpp

@@ -1368,7 +1368,7 @@ namespace util {
     template <
         typename K,
         typename = std::enable_if_t<
-            is_coord_v<K> && std::is_floating_point<typename K::value_type>::value, void
+            is_coord_v<K> && std::is_floating_point_v<typename K::value_type>
         >
     >
     constexpr auto
@@ -1388,6 +1388,28 @@ namespace util {
     }
 
 
+    ///------------------------------------------------------------------------
+    /// Return the fractional part of a real value.
+    ///
+    /// This is an extraodinarily naive implementation. We avoid doing
+    /// explicit casts here in the hope that floor and sub is more efficient
+    /// (ie, keeps floats as floats in registers).
+    template <
+        typename K,
+        typename = std::enable_if_t<
+            is_coord_v<K> &&
+            std::is_floating_point_v<
+                typename K::value_type
+            >
+        >
+    >
+    constexpr auto
+    frac (const K k)
+    {
+        return k - floor (k);
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// shifts all elements `num' indices to the right, setting the left-most
     /// `num' indices to the value `fill'.

maths.hpp

@@ -451,6 +451,17 @@ namespace util {
     }
 
 
+    template <
+        typename ValueT,
+        typename = std::enable_if_t<std::is_floating_point_v<ValueT>>
+    >
+    ValueT
+    frac (ValueT val)
+    {
+        return val - static_cast<long> (val);
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////////
     // angles, trig
     namespace detail {