Quash warnings for floating-point equality comparisons

GCC and Clang's -Wfloat-equal warning notes that comparing floating-point
values with == or != may be questionable. But the few instances of these in
SolveSpace are defensibly correct (as discussed with Jonathan), so to keep
folks from getting nervous that a CAD application isn't handling its floats
correctly, we define an EXACT() macro inside which the -Wfloat-equal
warning is disabled. This macro will also serve as a source-code
annotation, like a comment but better.

(The warning is only disabled for Clang, alas, because GCC is particular
about where _Pragma() can be used. This isn't so bad, however, because the
warning is much easier to enable on Clang [thanks to -Weverything], whereas
with GCC it has to be requested explicitly.)

graphicswin.cpp

@@ -355,12 +355,12 @@ void GraphicsWindow::ZoomToFit(bool includingInvisibles) {
                          projUp.   ScaledBy(-ym));
   
     // And based on this, we calculate the scale and offset
-    if(dx == 0 && dy == 0) {
+    if(EXACT(dx == 0 && dy == 0)) {
         scale = 5;
     } else {
         double scalex = 1e12, scaley = 1e12;
-        if(dx != 0) scalex = 0.9*width /dx;
-        if(dy != 0) scaley = 0.9*height/dy;
+        if(EXACT(dx != 0)) scalex = 0.9*width /dx;
+        if(EXACT(dy != 0)) scaley = 0.9*height/dy;
         scale = min(scalex, scaley);
 
         scale = min(300, scale);

solvespace.h

@@ -7,6 +7,18 @@
 #ifndef __SOLVESPACE_H
 #define __SOLVESPACE_H
 
+// The few floating-point equality comparisons in SolveSpace have been
+// carefully considered, so we disable the -Wfloat-equal warning for them
+#ifdef __clang__
+#   define EXACT(expr) \
+        (_Pragma("clang diagnostic push") \
+         _Pragma("clang diagnostic ignored \"-Wfloat-equal\"") \
+         (expr) \
+         _Pragma("clang diagnostic pop"))
+#else
+#   define EXACT(expr) (expr)
+#endif
+
 // Debugging functions
 #define oops() do { dbp("oops at line %d, file %s\n", __LINE__, __FILE__); \
                     if(0) *(char *)0 = 1; exit(-1); } while(0)

srf/ratpoly.cpp

@@ -261,7 +261,7 @@ void SBezier::MakePwlInto(SContour *sc, double chordTol) {
     lv.Clear();
 }
 void SBezier::MakePwlInto(List<Vector> *l, double chordTol) {
-    if(chordTol == 0) {
+    if(EXACT(chordTol == 0)) {
         // Use the default chord tolerance.
         chordTol = SS.ChordTolMm();
     }

util.cpp

@@ -479,9 +479,9 @@ bool Vector::Equals(Vector v, double tol) {
 }
 
 bool Vector::EqualsExactly(Vector v) {
-    return (x == v.x) &&
-           (y == v.y) &&
-           (z == v.z);
+    return EXACT(x == v.x &&
+                 y == v.y &&
+                 z == v.z);
 }
 
 Vector Vector::Plus(Vector b) {
@@ -681,7 +681,7 @@ Vector Vector::ScaledBy(double v) {
 
 Vector Vector::WithMagnitude(double v) {
     double m = Magnitude();
-    if(m == 0) {
+    if(EXACT(m == 0)) {
         // We can do a zero vector with zero magnitude, but not any other cases.
         if(fabs(v) > 1e-100) {
             dbp("Vector::WithMagnitude(%g) of zero vector!", v);