coord: don't use kahan summation for sum(coord)

Our kahan summation algorithm has issues with infinities so we just
avoid using it in coordinates where speed and simplicity are more
important.

coord/ops.hpp

@@ -24,8 +24,10 @@
 #include "../preprocessor.hpp"
 #include "../types/bits.hpp"
 
+#include <algorithm>
 #include <cmath>
 #include <cstdlib>
+#include <iterator>
 
 namespace util {
     ///////////////////////////////////////////////////////////////////////
@@ -741,7 +743,17 @@ namespace util {
     T
     sum (const K<S,T> k)
     {
-        return sum (std::cbegin (k), std::cend (k));
+        // DO NOT USE util::sum(begin, end) from maths.hpp
+        //
+        // It would be nice to use kahan summation from maths.hpp but speed
+        // and simplicity is more important for these fixed sized
+        // coordinates. Infinities tend to crop up using these classes and
+        // they cause a few headaches in the kahan code.
+        //
+        // So, if the user wants kahan summation they can request it
+        // explicitly.
+
+        return std::accumulate (std::cbegin (k), std::cend (k), T{0});
     }
 
 

maths.hpp

@@ -549,8 +549,14 @@ namespace util {
         T c = 0;
 
         for (auto cursor = first; cursor != last; ++cursor) {
+            // Infinities are handled poorly in this implementation. We tend
+            // to produce NaNs because of the subtraction where we compute
+            // `c'. For the time being just panic in this scenario.
+            assert(!std::isinf (*cursor));
+
             T y = *cursor - c;
             T t = sum + y;
+
             c = (t - sum) - y;
             sum = t;
         }

test/coord.cpp

@@ -42,6 +42,16 @@ main (void)
         tap.expect (x == p.x && y == p.y, "structured bindings extract correct data");
     }
 
+    {
+        util::point3f a { 103, 0, 14 };
+        util::point3f b { 104, INFINITY, 15 };
+
+        tap.expect_eq (
+            std::numeric_limits<float>::infinity (),
+            ::util::distance (a, b),
+            "distance with an infinity is infinite"
+        );
+    }
 
     return tap.status ();
 }