Fix -Wshift-negative-value error

[henrybaxter]

Running run_tests.sh yielded

/Users/henry/liquidfun/liquidfun/Box2D/Box2D/Particle/b2ParticleSystem.cpp:55:57: error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
                                                    (-1 << xShift));
                                                     ~~ ^

This can be fixed with

diff --git a/liquidfun/Box2D/Box2D/Particle/b2ParticleSystem.cpp b/liquidfun/Box2D/Box2D/Particle/b2ParticleSystem.cpp
index ce1c01e..dbc02c4 100644
--- a/liquidfun/Box2D/Box2D/Particle/b2ParticleSystem.cpp
+++ b/liquidfun/Box2D/Box2D/Particle/b2ParticleSystem.cpp
@@ -52,7 +52,7 @@ static const uint32 yMask = ((1u << yTruncBits) - 1u) << yShift;
 static const uint32 xMask = ~yMask;
 static const uint32 relativeTagRight = 1u << xShift;
 static const uint32 relativeTagBottomLeft = (uint32)((1 << yShift) +
-                                                    (-1 << xShift));
+                                                    -(1L << xShift));

 static const uint32 relativeTagBottomRight = (1u << yShift) + (1u << xShift);

And then the unit tests complete without problem (yes the + - is unnecessary, but I thought it read the cleanest).

[pacifire]

@henrybaxter : Thanks Man it worked.

[pacifire]

@henrybaxter : Could you please help me with this : #70

[subatomicglue]

Just ran into this with the stable tar.gz release also. The fix above worked, thank you!
https://github.com/google/liquidfun/releases liquidfun-1.1.0.tar.gz on xcode 9.2 MacOSX 10.13.2

I additionally had to change the MacOSX SDK from 9 to 13 (latest).